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Units & Measures
The Units & Measures library curates in-depth explainers about the numbers behind everyday life. Dive into the origins of the metric and imperial systems, the evolution of standardisation and the stories of iconic units that shaped science, navigation and trade.
Each guide blends historical insight with modern relevance so you can connect foundational measurement concepts to calculators across CalcSimpler. Use these resources to teach, learn or simply satisfy your curiosity about how we compare, quantify and communicate the world around us.
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Tonne of Carbon Dioxide Equivalent (tCO₂e): Greenhouse Gas Accounting Unit
Convert multi-gas inventories into CO₂e totals, manage uncertainty, and align disclosures with ISO 14064 and GHG Protocol rules.
Climate metricsGlobal Warming Potential (GWP): Radiative Forcing Metric
Interpret IPCC GWP factors, document time-horizon choices, and translate diverse gases into comparable climate impacts.
Built environmentEmbodied Carbon Intensity: Construction Impact Metric
Calculate kg CO₂e per square metre across lifecycle modules and integrate embodied carbon into building procurement decisions.
HVAC performanceSpecific Fan Power (SFP): Ventilation Efficiency Metric
Measure ventilation energy in W per litre per second, diagnose duct pressure losses, and meet code-based SFP limits.
Digital infrastructurePower Usage Effectiveness (PUE): Data Center Efficiency Ratio
Track total facility energy versus IT load, improve cooling strategies, and document data center sustainability progress.
Thermal comfortMean Radiant Temperature (MRT): Thermal Comfort Parameter
Measure MRT, capture surface temperature effects, and integrate the variable into PMV, WBGT, and radiant system design.
Lighting and buildingsLighting Power Density (LPD): Building Energy Benchmark
Benchmark lighting watts per square metre, coordinate controls, and align designs with energy codes and sustainability goals.
Building performanceHeating and Cooling Degree Days: Climate-Energy Indicators
Compute HDD/CDD totals, tune balance-point temperatures, and weather-normalise building energy benchmarks.
HVAC efficiencyHeat Pump Coefficient of Performance (COP) Efficiency Ratio
Understand COP definitions, testing standards, and how climate and refrigerants shape electrification performance.
Cooling performanceSeasonal Energy Efficiency Ratio (SEER): Air Conditioner Rating
Decode SEER test procedures, policy thresholds, and seasonal performance for cooling equipment selection.
Heat stressWet Bulb Globe Temperature (WBGT): Heat-Stress Index
Combine humidity, radiant heat, and wind to manage occupational exposure limits and climate resilience plans.
Public healthSun Protection Factor (SPF): Ultraviolet Shielding Metric
Define SPF ratios, compare global testing standards, and integrate UVA metrics into sun safety programmes.
Atmospheric scienceHectopascal (hPa): Meteorological Pressure Unit
Convert between pascals, millibars, and inches of mercury while following WMO and ICAO reporting practices.
Soil scienceSoil Water Potential (ψ): Energy Status in Megapascals
Translate matric suction measurements into irrigation scheduling, ecology insights, and slope stability checks.
Water qualityTotal Suspended Solids (TSS): mg/L Water Quality Metric
Implement gravimetric solids testing, correlate with turbidity, and document compliance-grade mg/L reporting.
Energy metricsSpecific Fuel Consumption: g/kWh and lb/hp·h Efficiency Metric
Map engine fuel usage across units, build BSFC curves, and connect powertrain choices with energy costs.
Fluid dynamicsPump Specific Speed (Ns): Dimensionless Design Parameter
Classify impellers, apply similarity scaling, and pair pump curves with detention and irrigation planning.
Thermal scienceFourier Number (Fo): Transient Heat Conduction Time Scale
Estimate dimensionless time for transient conduction to judge when lumps, charts, or numerical models apply.
AcousticsHelmholtz Number (He): Acoustic Resonance and Wavelength Scaling
Classify cavities, ducts, and aeroacoustic sources by comparing geometry with acoustic wavelength.
Optical metrologySpecific Detectivity (D*): Photodetector Sensitivity in Jones
Normalise detector sensitivity by area and bandwidth to compare infrared and optical sensors.
Colour scienceCIE ΔE Colour Difference: Quantifying Perceptual Shifts
Apply ΔE formulas (1976, 1994, 2000) to manage colour tolerances in print, textile, and display workflows.
Information scienceEffective Number of Bits (ENOB): ADC Dynamic Range Metric
Convert measured SINAD into practical resolution for high-speed and precision data converters.
Information scienceBan and Deciban: Log-Odds Information Units
Sum Bayesian evidence on a base-10 scale, convert between bans, bits, and nats, and track sequential test thresholds.
Power and energyPoncelet: The Hydraulic Power Unit of 19th-Century Engineering
Translate legacy French power data by converting poncelets to SI watts and understanding historical hydraulic testing.
Temperature scalesRéaumur Temperature Scale: Legacy Degrees Between Freezing and Boiling
Convert Réaumur readings into Celsius, Kelvin, and Fahrenheit while appreciating the scale’s culinary and scientific history.
Nutrition metricsGlycemic Index (GI): Quantifying Carbohydrate Response
Understand GI testing protocols, historical context, and how to pair GI with glycemic load and modern nutrition planning.
Fluid dynamicsSwirl Number (S): Quantifying Rotational Flow in Combustors
Characterise angular-to-axial momentum ratios to design stable flames, cyclones, and other swirl-dominated flows.
Colour measurementMunsell Colour System: Three-Dimensional Specification of Visual Samples
Navigate hue, value, and chroma to manage colour quality across design, manufacturing, soil science, and conservation.
Mechanical propertiesBulk Modulus (K): Resistance to Uniform Compression
Quantify volumetric stiffness, relate it to other elastic constants, and apply K to fluids, solids, and geoscience problems.
Mechanical propertiesShear Modulus (G): Rigidity Under Distortion
Explore shear stiffness, anisotropy, and measurement techniques for torsion, vibration, and rheology applications.
Mechanical propertiesPoisson's Ratio (ν): Lateral Strain Response
Connect lateral contraction to axial strain, understand auxetic materials, and integrate ν into structural and geophysical models.
Mechanical propertiesFracture Toughness (KIC): Critical Stress Intensity
Apply linear-elastic fracture mechanics, damage tolerance principles, and mixed-mode analysis using KIC data.
Thermal propertiesLinear Thermal Expansion Coefficient (α): Dimensional Change
Model temperature-driven strain, measurement methods, and design strategies for managing expansion in structures and devices.
ElectrostaticsDebye Length (λD): Electrostatic Screening Scale
Quantify λD in metres to judge quasi-neutrality, plasma diagnostics, and electrolyte double-layer behaviour.
ElectrostaticsSurface Charge Density (C·m⁻²): Interface Charge Control
Link Gauss’s law, capacitance, and electrostatic safety standards when managing surface charge.
Fluid dynamicsVolumetric Flow Rate (m³·s⁻¹): Fluid Discharge Benchmark
Apply continuity, measurement standards, and instrumentation techniques to volumetric flow problems.
Fluid dynamicsStandard Cubic Metre (Sm³): Reference Gas Volume at Defined Conditions
Normalise gas measurements to standard conditions, convert between Sm³ and scf, and document compressibility corrections.
ThermodynamicsPartial Pressure (Pa): Component Gas Pressure
Combine mole fraction, thermodynamic models, and sensors to report species pressures in gas mixtures.
ThermodynamicsSpecific Enthalpy (kJ·kg⁻¹): Energy per Unit Mass
Use enthalpy balances, property tables, and calorimetry to evaluate energy flows in thermal systems.
Performance metricsSpecific Power (W/kg): Power-to-Mass Performance Metric
Benchmark propulsion, electronics, and energy systems by comparing watts per kilogram across technologies.
RheologyShear Rate (s⁻¹): Diagnosing Flow Regimes in Complex Fluids
Quantify shear-rate spectra, link rheology models to flow regimes, and connect γ̇ monitoring with manufacturing quality controls.
Rarefied gas dynamicsKnudsen Number (Kn): Navigating Continuum Breakdown in Rarefied Flows
Classify rarefied-flow regimes, choose slip or DSMC models, and align vacuum diagnostics with ISO notation.
Surface scienceBond Number (Bo): Surface Tension versus Gravity
Balance capillary and gravitational forces to predict droplet shapes, capillary rise, and offshore loading responses.
Thermal scienceThermal Effusivity (e): Matching Materials at Thermal Interfaces
Use √(kρc) to forecast contact temperatures, select interface materials, and tune thermal comfort solutions.
Transport phenomenaMass Diffusion Coefficient (D): Quantifying Species Transport
Apply Fickian theory, advanced dispersion models, and measurement techniques to characterise mass transport in fluids and porous media.
MagnetismMagnetic Susceptibility (χ): Dimensionless Response to Applied Fields
Define χ across tensor forms, evaluate measurement methods, and translate magnetic response into circuit, medical, and geophysical design.
Energy metricsVolumetric Energy Density: kWh per Cubic Metre for Storage and Fuels
Convert energy inventories into kWh·m⁻³, compare fuels and storage media, and integrate volumetric metrics into decarbonisation roadmaps.
Energy metricsTonne of Oil Equivalent (toe): Global Energy Accounting Benchmark
Convert diverse fuel streams into toe, align with SI energy units, and support national energy balance reporting.
Energy metricsBarrel of Oil Equivalent (boe): Cross-Fuel Energy Portfolio Unit
Aggregate oil, gas, and power outputs in boe, manage conversion factors, and integrate the unit with carbon reporting.
Energy metricsOctane Rating: Research, Motor, and Anti-Knock Index
Define RON, MON, and AKI testing, trace refinery blending strategies, and connect octane metrics to engine performance and compliance.
Atomic & nuclearThe Curie (Ci): Legacy Standard for Quantifying Radioactivity
Translate curie-based activity measurements, calibration history, and regulatory usage into modern SI practice while linking to decay and dose tools.
Atomic & nuclearNuclear Magneton (μN): Benchmark Unit for Nuclear Magnetic Moments
Define μN from fundamental constants, explore magnetometry techniques, and compare nuclear magnetic data across spectroscopy and metrology.
Fundamental constantsPlanck Mass (mP): Natural Mass Scale Bridging Quantum Theory and Gravity
Use Planck-scale dimensional analysis to connect quantum gravity concepts with cosmology, black hole physics, and natural unit conversions.
Information scienceHartley (Hart): Logarithmic Information Unit in Base-10 Communication Theory
Compare Hartleys with bits, decibels, and nepers to interpret decimal-digit information measures across communications and instrumentation.
Statistical measuresCoefficient of Variation (CV): Relative Dispersion Statistic
Normalise standard deviation by the mean to benchmark variability across laboratories, manufacturing lines, and financial portfolios.
Statistical measuresSkewness Coefficient (γ₁): Quantifying Distribution Asymmetry
Diagnose distribution lean, tail risk, and process shifts by translating third-moment behaviour into actionable shape insights.
Statistical measuresExcess Kurtosis (γ₂): Diagnosing Tail Weight and Peakedness
Measure heavy tails and sharp peaks to safeguard forecasts, quality programmes, and risk models from extreme events.
Statistical measuresRoot Mean Square Error (RMSE): Evaluating Model Accuracy
Translate squared residuals into unit-consistent accuracy benchmarks for forecasting, sensing, and optimisation.
Socioeconomic indicatorsGini Coefficient (G): Measuring Income and Wealth Inequality
Convert Lorenz curves into comparable inequality scores that guide policy, ESG reporting, and community planning.
Economic metricsHerfindahl–Hirschman Index (HHI): Market Concentration Metric
Monitor competition, supplier diversity, and portfolio exposure with the squared-share index used in antitrust review.
AstrophysicsSolar Luminosity (L☉): Stellar Energy Output Reference for Astrophysics
Calibrate stellar comparisons, climate studies, and mission planning using the Sun’s total radiative power as a benchmark.
Atmospheric scienceBrunt–Väisälä Frequency (N): Diagnosing Stratified Stability
Quantify stratified buoyancy oscillations, trace historical observations, and apply N diagnostics to weather, wind energy, and ocean mixing.
Soil scienceVolumetric Water Content (θ): Soil Moisture as a Cubic Metre Ratio
Measure m³·m⁻³ soil moisture, calibrate sensors, and apply θ to irrigation scheduling, hydrology, and climate resilience.
Atmospheric scienceWind Chill Index: Equivalent Temperature Under Moving Air
Relate wind speed, air temperature, and clothing choices to cold-weather safety thresholds using the 2001 NWS standard.
Geophysical fluid dynamicsRossby Number (Ro): Ratio of Inertial to Planetary Vorticity
Diagnose rotational control of weather systems, ocean gyres, and planetary atmospheres using Ro-based scaling analyses.
SpectroscopySpectral Line Equivalent Width (Wλ): Quantifying Line Strength
Integrate absorption and emission features, refine calibration workflows, and deploy Wλ diagnostics across astrophysics and plasma labs.
SpectroscopyMolar Absorptivity (ε): Spectrophotometric Constant
Apply Beer–Lambert coefficients to quantify concentration, optical materials, and biochemical assays with traceable uncertainty.
Fluid dynamicsEddy Dissipation Rate (ε): Tracking Turbulent Energy Cascades
Connect Kolmogorov theory with measurement techniques to manage turbulence in aviation, wind energy, and environmental flows.
Atmospheric scienceGeopotential Height (Zg): Dynamic Altitude for Meteorology
Derive geopotential height via the hypsometric equation and apply Zg to forecasting, climate analysis, and aerospace planning.
Time and frequencyZeptosecond (zs): Mapping Ultrafast Temporal Scales
Explore 10⁻²¹-second intervals, from attosecond laser experiments to quantum metrology and nuclear dynamics.
MeteorologySpecific Humidity (q): Mass of Water Vapour per Kilogram of Air
Translate kg·kg⁻¹ moisture ratios into psychrometric design, weather forecasting, and climate diagnostics.
MeteorologyRelative Humidity (%): Ratio of Water Vapour to Saturation
Use saturation ratios to predict condensation, manage HVAC loads, and support heat-stress assessments.
RelativityGravitational-Wave Strain (h): Dimensionless Spacetime Ripple
Measure ΔL/L with interferometers to unlock astrophysical sources, cosmology, and precision metrology frontiers.
RelativityLorentz Factor (γ): Relativity's Time Dilation and Length Contraction Ratio
Compute γ from velocity, manage relativistic timing corrections, and apply Lorentz transformations to navigation and high-energy systems.
Solar engineeringGlobal Horizontal Irradiance (GHI): Surface Solar Resource
Quantify W·m⁻² sunlight on horizontal planes to plan photovoltaics, climate mitigation, and controlled-environment agriculture.
Transport phenomenaSchmidt Number (Sc): Balancing Momentum and Mass Diffusion
Derive Sc relationships, compare laminar and turbulent regimes, and implement mass-transfer correlations across engineering applications.
Radiation protectionSpecific Absorption Rate (SAR): Radiofrequency Energy Deposition
Understand SAR definitions, standards, and measurement techniques that govern RF safety in consumer, medical, and aerospace systems.
SpectroscopyNMR Chemical Shift (δ): Dimensionless Spectroscopic Reference
Explore chemical shift definitions, referencing conventions, and applications spanning chemistry, materials science, and biomedicine.
Signal analysisEquivalent Noise Bandwidth (ENBW): Quantifying Filter Noise Gain
Translate filter responses into effective noise power widths for metrology, communications, and vibration analysis.
Signal analysisNyquist Frequency: Sampling Limit for Discrete-Time Measurements
Set sampling cadences, prevent aliasing, and coordinate anti-alias filtering across instrumentation and signal processing.
Signal analysisSignal-to-Noise Ratio (SNR): Dimensionless Performance Metric
Quantify measurement quality, relate SNR to decibels, and manage noise budgets across acoustics, imaging, and communications.
Signal analysisBit Error Rate (BER): Communication Reliability Metric
Connect BER definitions, testing methods, and coding strategies to deliver robust wired, wireless, and optical links.
AstrophysicsCosmological Redshift (z): Measuring Universal Expansion
Relate redshift to the cosmic scale factor, survey historical measurements, and apply z to mapping large-scale structure.
HeliophysicsSolar Constant (1361 W·m⁻²): Benchmarking Total Solar Irradiance
Define the solar constant, review spaceborne radiometry, and integrate TSI into climate modelling and solar engineering.
Color scienceCIE ΔE Color Difference Metrics: Quantifying Perceptual Uniformity
Compare ΔE formulas, interpret tolerances, and manage color quality across printing, displays, and emerging XR experiences.
Color sciencePlatinum-Cobalt (APHA) Color Units: Hazen Scale for Water Clarity
Measure faint yellow tint in water, industrial liquids, and pharmaceuticals with platinum-cobalt reference solutions.
Spectral analysisWavenumber (m⁻¹): Spectroscopic Spatial Frequency
Master reciprocal-length notation, convert between wavelength, frequency, and energy, and compare spectroscopic measurement techniques.
Materials metrologyVickers Hardness Number (HV): Microindentation Testing
Apply Vickers equations, microhardness protocols, and calibration practices to characterise heat treatments, coatings, and advanced materials.
Materials metrologyRockwell Hardness (HR): Depth-Based Indentation Scale
Select Rockwell scales, interpret depth-based readings, and manage heat-treatment quality control with traceable HR data.
Materials metrologyShore Durometer Hardness: Elastomer and Foam Scales
Differentiate Shore A, D, and 00 durometers, control dwell and temperature effects, and translate readings into elastomer design criteria.
AcousticsSound Exposure Level (SEL): Cumulative Noise Dose Metric
Integrate acoustic energy over events, relate SEL to Leq, and document measurement standards for occupational and environmental assessments.
AstronomyApparent Magnitude: Astronomical Brightness Scale
Use logarithmic magnitude systems, calibrate photometry, and communicate brightness across astronomy and outreach contexts.
Radiative scienceAlbedo: Reflectance Coefficient for Planetary Climate
Quantify broadband reflectance, trace measurement history, and apply albedo data to climate modelling and energy engineering.
Imaging metrologyF-Number (f/N): Lens Aperture Ratio for Exposure Control
Relate aperture ratios to exposure, depth of field, and optical calibration across photography, cinematography, and sensing.
Ocean observationPractical Salinity Unit (PSU): Ocean Conductivity Scale
Convert conductivity ratios to PSU, maintain calibration, and apply salinity datasets in oceanography and desalination.
BiosphereLeaf Area Index (LAI): Canopy Density Metric
Measure and model canopy leaf area to support agricultural planning, climate analysis, and ecosystem monitoring.
BiosphereDaily Light Integral (DLI): Photoperiod Energy Budget for Controlled Environment Agriculture
Quantify daily photon budgets, align sensor logging with crop recipes, and balance lighting energy across greenhouses and vertical farms.
BiosphereSpecific Leaf Area (SLA): Area per Unit Mass for Plant Functional Traits
Measure SLA to interpret plant strategies, calibrate crop and ecosystem models, and design climate-resilient planting schemes.
MechanicsInertia vs. Gravitational Forces
Distinguish inertial and gravitational effects, trace historical experiments, and apply the balance to aerospace, civil, and geophysical design.
Fluid dynamicsAdvanced Froude Number Analysis
Extend Froude theory with dimensional analysis, scaling strategies, and instrumentation tips for hydraulic, naval, and planetary studies.
Time & frequencyThe SI Second: Evolution and Realization
Follow the atomic definition of the second, dissemination infrastructure, and applications across navigation, telecom, and science.
Radiological scienceHalf-Life: Exponential Decay Modeling
Model radioactive, pharmacokinetic, and environmental decay with rigorous half-life definitions, measurement techniques, and applications.
ElectromagnetismHenry (H): SI Unit of Electromagnetic Inductance
Translate henry-based definitions into design practices for power electronics, RF systems, and emerging magnetics.
ElectromagnetismInductance Explained: Concepts and Design Principles
Break down self and mutual inductance, modelling strategies, and measurement workflows for electronics and sensing.
Fundamental constantsPlanck Time: Limits of Temporal Measurement
Investigate Planck-scale time, quantum gravity frameworks, and observational prospects for fundamental physics.
AcousticsAcoustic Impedance: Pressure–Velocity Bridge
Connect impedance theory with measurement methods and applications in architecture, ultrasound, and industrial acoustics.
Materials scienceSpecific Surface Area: Interface Density in Materials
Measure and apply specific surface area to catalysis, energy storage, soils, and environmental engineering.
Particle characterisationFeret Diameter: Caliper-Based Particle Size Metric
Extract Feret statistics from images, relate them to sieve gradations, and interpret particle shape effects on flow and packing.
Thermal scienceThermal Diffusivity: Heat Propagation Metric
Quantify thermal response speed with historical context, measurement techniques, and cross-sector applications.
ElectromagnetismComplex Conductivity in the Siemens per Metre
Model dispersive conductivity with complex σ(ω) to support spectroscopy, EMC, and subsurface sensing projects.
Electrical metrologyThe Siemens and the Quantum of Electrical Conductance
Connect macroscopic admittance with quantum conductance, calibration chains, and high-frequency network analysis.
Electrical materialsElectrical Resistivity in the Ohm Metre
Translate Ω·m resistivity into material selection, reliability models, and temperature-dependent performance.
Time & frequencyPrecision Frequency Metrology with the Hertz
Survey atomic clocks, Allan deviation, and timing distribution that keep telecom and navigation services aligned.
ElectromagnetismVacuum Permittivity and Material Response
Explain ε₀, relative permittivity, and dispersion for RF design, photonics, and dielectric materials engineering.
ElectrochemistryMolar Conductivity (S·m²·mol⁻¹)
Characterise ion mobility and concentration effects to optimise electrolytes for batteries, fuel cells, and water quality.
Thin filmsSheet Resistance (Ω/□)
Link resistivity, thickness, and mapping methods to evaluate thin films, transparent conductors, and printed electronics.
Environmental scienceDecisiemens per Metre (dS/m)
Use dS/m salinity benchmarks to guide irrigation planning, hydroponics, and environmental monitoring programmes.
Radiological scienceDecay Constant (s⁻¹): Governing Nuclear Transformation Rates
Model exponential decay, link half-life, activity, and safety cases with a probabilistic constant for every radionuclide.
Radiological protectionSievert (Sv): Dose Equivalent for Spaceflight and High-Altitude Operations
Translate mixed-field radiation environments into sievert-based risk metrics for aviation crews and astronauts.
Electrical metrologyCoulomb (C): Quantum-Accurate Charge Realisations for Energy Systems
Follow quantum electrical standards, coulometry, and current integration techniques that secure charge traceability.
ElectromagnetismVolt per Metre (V·m⁻¹): Field Strength Standards for EMC and 5G Testing
Design, generate, and verify electromagnetic fields that satisfy modern compliance and wireless performance targets.
Medical imagingKerma-Area Product (Gy·cm²): Managing Fluoroscopic Beam Output
Integrate air kerma across beam area to document fluoroscopy energy delivery, quality assurance, and regulation.
Radiation fieldsParticle Fluence (m⁻²): Characterising Radiation Fields by Track Density
Quantify cumulative particle tracks to support shielding, electronics qualification, and reactor surveillance.
RadiometryEnergy Fluence (J·m⁻²): Connecting Irradiation Dose to Material Response
Integrate irradiance over time to control UV disinfection, laser processing, and solar energy harvesting.
Thermophysical propertiesAmagat (amg): Number Density Benchmark for Gas Mixtures
Normalise gas densities to standard conditions for aeronomy, plasma physics, and precision spectroscopy.
Indoor environmental qualityAir Changes per Hour (ACH): Ventilation Performance Metric
Measure indoor air turnover, balance infection control with energy targets, and translate airflow data into ACH values for every room type.
AcousticsSound Transmission Class (STC): Rating Partition Attenuation
Decode ASTM contour fitting, manage flanking paths, and align partition isolation targets with codes and wellness standards.
Risk analysisMicromort: Quantifying Acute Mortality Risk
Translate diverse hazard statistics into one-in-a-million fatality units to support transparent decision analysis and communication.
Energy & resourcesTherm (thm): Legacy Heat Unit in Energy Markets
Convert utility billing therms into SI metrics, reconcile heating values, and align gas data with decarbonisation reporting.
Remote sensingGround Sample Distance (GSD): Resolution Metric for Remote Sensing
Relate sensor geometry, flight altitude, and mission planning to on-the-ground pixel size for satellites and UAVs.
Remote sensingNormalized Difference Vegetation Index (NDVI): Remote Sensing Signal for Vegetation Health
Compare red and near-infrared reflectance, calibrate sensors, and apply NDVI trends to agriculture, ecology, and urban resilience.
Environmental metricsAir Quality Index (AQI): Communicating Pollutant Burden
Convert pollutant concentrations into health-based AQI categories and coordinate indoor responses to outdoor air events.
Environmental metricsWet-Bulb Temperature: Psychrometric Indicator of Moist Air
Use adiabatic saturation insights to manage humidity, heat stress, and evaporative cooling across HVAC, weather, and agricultural applications.
Environmental metricsHeat Index (HI): Apparent Temperature in Humid Climates
Combine temperature and humidity into public-facing heat-risk guidance with Steadman equations, measurement best practices, and mitigation planning.
Environmental metricsDew Point Temperature: Condensation Thresholds for Atmosphere and Process Control
Translate humidity observations into condensation thresholds for meteorology, HVAC, preservation, and industrial quality control.
Radiological protectionRem (rem): Converting Legacy Radiation Protection Limits
Translate historical rem records into sieverts, understand quality factors, and maintain regulatory alignment across industries.
Classical mechanicsDyne (dyn): CGS Force Unit in Classical Mechanics
Convert dyne-based mechanics and surface science data into SI units without losing dimensional clarity.
Imperial metrologyKip (kip): Imperial Force Unit for Structural Engineering
Summarise 1,000 lbf load cases, convert to kilonewtons, and coordinate structural design teams working across unit systems.
Imperial metrologySlug: Imperial Mass Unit for Dynamics
Keep F = ma consistent in foot–pound–second systems and bridge slug-based specifications with SI models.
AstrophysicsSolar Mass (M☉): Benchmark for Stellar and Exoplanet Studies
Use the Sun’s mass as a reference for stellar evolution, exoplanet characterisation, and mission design across astrophysics.
Vacuum scienceLangmuir (L): Surface Exposure in Vacuum Science
Quantify vacuum surface exposure, manage contamination budgets, and coordinate pumping strategies with langmuir units.
AcousticsEquivalent Continuous Sound Level (Leq): Energy-Averaged Noise Metrics
Energy-average fluctuating noise, satisfy regulatory reporting, and design human-centric soundscapes with Leq.
Specialised unitsThe Ångström (Å): Sub-Nanometre Lengths in Spectroscopy and Materials Science
Measure atomic-scale distances with confidence by understanding how the ångström bridges crystallography, spectroscopy, and nanotechnology workflows.
AcousticsThe Sabin (sab): Quantifying Sound Absorption Area in Architectural Acoustics
Translate reverberation goals into actionable material choices using the sabin, Wallace Sabine’s foundational unit of absorption area.
Mass metrologyThe Metric Carat (ct): Gemological Mass Standardisation
Trace the 200 mg carat from historic trade roots to today’s precision weighing practices in gemology, dentistry, and advanced materials.
Food & beverageDegree Plato (°P): Extract Concentration for Brewers and Fermenters
Master sucrose-referenced concentration measurements that power modern brewing, distilling, and fermentation analytics.
Energy & resourcesAPI Gravity (°API): Petroleum Density Benchmarking
Classify crude oils, manage custody transfer, and optimise refinery operations using the petroleum industry’s signature density scale.
Materials & manufacturingTex (tex): Linear Mass Density for Fibres and Filaments
Connect fibre fineness, quality control, and Industry 4.0 analytics through the SI-coherent tex unit for linear mass density.
Materials & manufacturingDenier (den): Legacy Textile Linear Mass Density
Translate apparel and technical textile specifications expressed in denier into SI-consistent data for quality control, costing, and sustainability reporting.
Materials & manufacturingDecitex (dtex): SI-Derived Linear Density for Textiles
Specify fibre fineness, ensure measurement traceability, and align apparel and technical textile workflows with SI notation.
Space scienceThe Rayleigh (R): Upper-Atmosphere Column Emission Brightness
Decode auroral and airglow brightness using the rayleigh to support space weather forecasting and astronomical planning.
Space scienceDelta-v (Δv): Velocity Budget for Space Mission Design
Budget m·s⁻¹ requirements for launch, transfer, and landing, linking propulsion performance with mission architecture decisions.
Lighting & imagingThe Mired (micro reciprocal degree): Practical Color Temperature Tuning
Express correlated color temperature shifts linearly with mireds for lighting design, cinematography, and horticulture.
Lighting & imagingCorrelated Color Temperature (CCT): Kelvin Scale for Perceived White Light
Specify kelvin targets, manage chromaticity tolerances, and integrate CCT with photometric and horticultural lighting design.
Fundamental constantsDefining Constants in the Modern SI
Survey the seven defining constants that anchor today’s SI and see how they replaced artefact-based unit definitions across science and engineering.
Base unitsThe SI Base Units: The Seven Defining Quantities of Modern Measurement
Review the 2019 constant-based definitions for the seven SI base quantities and connect them with calculators and conversion tools for daily use.
Base unitsThe Metre (m): The SI Base Unit of Length
Understand the light-speed definition of the metre, its practical realization, and how to keep distance calculations traceable across labs and fieldwork.
Fundamental constantsSpeed of Light in Vacuum: Fixing the Metre
Trace how the exact value of c defines the metre, links electromagnetism and relativity, and underpins ranging, interferometry, and communications.
Base unitsThe Kilogram (kg): The SI Base Unit of Mass
Follow the kilogram’s journey from platinum-iridium artifacts to Planck-constant realizations and apply mass standards to trade and science.
Quantum constantsPlanck Constant and Quantum Electrical Standards
Explore how the fixed Planck constant unites mass and electrical metrology through Kibble balances, Josephson arrays, and quantum Hall devices.
Quantum constantsPlanck Length (ℓₚ): Quantum Gravity’s Fundamental Scale
Derive ℓₚ from ħ, G, and c to frame discussions about spacetime discreteness, black hole thermodynamics, and early-universe physics.
Quantum constantsPlanck Time (tₚ): Quantum Duration Scale
Combine ħ, G, and c to define the 5.39 × 10⁻⁴⁴ s Planck time, then explore its role in early-universe models and quantum gravity research.
Base unitsThe Second (s): The SI Base Unit of Time
Explore atomic timekeeping, relativistic corrections, and dissemination methods that keep global infrastructure synchronized.
Time & frequencyHyperfine Transition Frequency of Cesium-133
Understand how the 9,192,631,770 Hz cesium transition defines the second and powers global navigation, telecom, and timing systems.
Base unitsTime: Definition, Units, Realization, and Use in Science and Engineering
Survey time as a quantity, review SI and accepted units, and see how precise clocks enable navigation, telecom, and data integrity.
Time & frequencyThe Hertz (Hz): SI Unit of Frequency and Cyclic Phenomena
Bridge timekeeping, vibration, and signal processing with hertz-based definitions, measurement techniques, and engineering applications.
Time & frequencyClock Frequency (Hz): Synchronising Digital Systems
Translate oscillator specifications, jitter budgets, and timing analysis into dependable synchronous design practice.
Base unitsMass: Definition, SI Units, Realization, and Use in Science and Engineering
Trace mass from balances to Kibble balances, understand unit usage, and apply best reporting practice across science and industry.
Base unitsLength: Definition, Units, Realization, and Use in Science and Engineering
Examine length as an SI base quantity, follow its historical evolution, and connect measurement practice to calculators and conversions.
Base unitsThe Ampere (A): The SI Base Unit of Electric Current
Understand the charge-counting definition of current, quantum realisations, and how to apply ampere-based measurements to power and electronics.
Quantum constantsElementary Charge: Counting Single Electrons
Learn how the fixed elementary charge defines the ampere, enables quantum current standards, and governs charge quantization from nanoelectronics to power grids.
Electrical quantitiesElectric Current: Definition, History, Concepts, and Applications
Survey charge flow, Maxwell’s equations, measurement science, and engineering uses that keep electric current central to modern infrastructure.
Derived unitsThe Newton (N): SI Unit of Force
Follow how ISO 80000-4 defines force, see why the newton underpins weight, torque, and stress, and discover calculators that keep F = m·a consistent.
MechanicsCoefficient of Friction (μ): Friction Force over Normal Load
Compare static and kinetic coefficients, measurement methods, and design implications for safe, efficient contact mechanics.
Derived unitsThe Newton-Metre (N·m): Torque, Work, and Rotational Energy
Connect torque, work, and rotational energy fundamentals with ISO 80000 terminology and calculators for mechanical design.
MechanicsNewton-Second (N·s): The SI Unit of Impulse
Integrate force over time to connect impact testing, propulsion budgets, and momentum conservation in mechanics.
Base unitsThe Kelvin (K): The SI Base Unit of Thermodynamic Temperature
Follow the Boltzmann-constant definition of temperature, ITS-90 dissemination, and traceable thermometry practices for labs and industry.
Thermodynamic constantsBoltzmann Constant: Energy per Degree of Freedom
Examine how the fixed Boltzmann constant connects microscopic energy with macroscopic temperature to guide thermometry and thermal modelling.
Thermodynamic quantitiesThermodynamic Temperature: Definition, History, Concepts, and Applications
Connect statistical mechanics, ITS-90 practice, and cross-disciplinary applications that rely on precise temperature control.
ThermodynamicsFugacity (f): Effective Thermodynamic Pressure
Use fugacity coefficients to bridge ideal and real fluids, solve phase equilibria, and support energy, chemical, and climate system design.
ThermodynamicsJoule per Kelvin (J·K⁻¹): Unit for Entropy and Heat Capacity
Link entropy balances, heat capacity data, and information theory metrics through the SI-coherent joule per kelvin unit.
Thermal propertiesThermal Conductivity (λ or k): Understanding Heat Conduction
Survey Fourier’s law, measurement techniques, and the materials science factors that push λ from insulating foams to diamond-class conductors.
Thermal propertiesThermal Resistance (R-Value): Building Envelope Performance Metric
Decode RSI and imperial R ratings, manage thermal bridges, and connect insulation choices to comfort, resilience, and energy models.
Thermal propertiesThermal Diffusivity (α, m²·s⁻¹): Governing Transient Heat Flow
Link conductivity, density, and heat capacity to model transient thermal response in electronics, buildings, and process equipment.
Fluid propertiesDynamic Viscosity (η): The Pascal-Second in Fluid Mechanics
Trace Newton’s law of viscosity from 19th-century experiments to today’s ISO 80000 guidance and keep Pa·s data aligned with flow calculations.
Fluid propertiesThe Poise (P): CGS Unit of Dynamic Viscosity
Bridge centipoise laboratory data with pascal-second engineering models for rheology, pipelines, and product design.
Fluid propertiesStokes (St): CGS Unit of Kinematic Viscosity
Convert between stokes, centistokes, and m²/s while aligning kinematic viscosity testing with ISO 80000 guidance.
Fluid propertiesThe Darcy (D): Unit of Permeability in Porous Media
Relate Henry Darcy's experiments to modern reservoir, hydrogeology, and filtration design with clear conversions between darcies and square metres.
Porous mediaPorosity (n): Dimensionless Void Fraction in Materials and Soils
Measure pore volume fractions, differentiate effective porosity, and apply n to groundwater, energy storage, and material design.
Thermal propertiesSpecific Heat Capacity (c): Storing Thermal Energy
Examine how Q = m·c·ΔT links ISO 80000-5 definitions, calorimetry practice, and energy system design for heating and cooling.
Thermal propertiesThe Clo (clo): Clothing Insulation Unit for Thermal Comfort
Translate clo ratings into heat balance models, HVAC setpoints, and comfort analytics to manage indoor environments with evidence-based insulation data.
Thermal comfortPredicted Mean Vote (PMV) & Predicted Percentage Dissatisfied (PPD): Thermal Comfort Indices for ISO 7730
Model occupant thermal sensation with Fanger’s heat balance equations, instrumentation guidance, and design strategies for buildings, transport, and specialised facilities.
Thermodynamic unitsDegree Celsius (°C): Definition, History, and Applications
Explore how Celsius readings link daily weather, laboratory work, and ISO 80000-5 thermodynamic conventions with Kelvin conversions.
Thermodynamic unitsThe Rankine (°R): Absolute Temperature on the Fahrenheit Scale
Track the Rankine scale from steam tables to modern HVAC, complete conversions to kelvin, and manage dual-unit thermodynamic analyses with confidence.
Base unitsThe Mole (mol): The SI Base Unit of Amount of Substance
Use the Avogadro-constant definition to link mass, concentration, and stoichiometry while keeping chemical measurements interoperable.
Chemical constantsAvogadro Constant: Exact Entity Counting
Discover how the fixed Avogadro constant links microscopic entities to macroscopic measurements for chemistry, materials, and life sciences.
Chemical quantitiesAmount of Substance: Definition, History, Concepts, and Applications
Extend the mole concept with stoichiometry, analytical methods, and industrial applications that demand traceable chemical measurements.
Atomic & nuclear unitsDalton (Da) / Unified Atomic Mass Unit (u): The Natural Mass Scale for Atoms and Molecules
Anchor atomic and molecular masses to the carbon-12 reference and trace Penning-trap measurements through ISO 80000-10 guidance.
Fundamental propertiesDensity (ρ): Mass per Unit Volume
Trace Archimedes’ legacy through ISO 80000-12 definitions and discover how engineers apply ρ across buoyancy, loads, and process control.
Materials & manufacturingBulk Density (ρb): Mass per Unit Volume of Granular and Porous Materials
Measure loose and compacted ρb, relate it to porosity and moisture, and apply results to soils, powders, and construction materials.
Chemical quantitiesAmount-of-Substance Concentration, c (mol·m⁻³)
Follow ISO 80000-9 guidance on molar concentration, activity corrections, and measurement traceability across chemistry labs.
Chemical quantitiesSolution Concentration (mol·m⁻³ and mol·L⁻¹): Preparing Standard Solutions
Standardise molarity workflows with calibration routines, uncertainty budgets, and regulatory-ready documentation.
Chemical quantitiesParts Per Million, Billion, and Trillion: Ratio Units for Trace Concentrations
Define ppm/ppb/ppt bases, convert between SI-compliant units, and apply trace concentration reporting across environmental and industrial programmes.
Chemical quantitiesMolality, b (mol·kg⁻¹)
Understand mass-based composition reporting, colligative properties, and molality preparation under ISO 80000-9.
Chemical quantitiesMole Fraction (x): Reporting Composition in Multicomponent Systems
Report dimensionless compositions with traceable measurements, partial pressure links, and thermodynamic modelling support.
Chemical propertiesPauling Electronegativity Scale: Chemical Bonding Metric
Derive electronegativity values from bond energies, compare alternative scales, and apply trends to reactivity and materials design.
Chemical quantitiespH (Dimensionless, pH = −log10 aH⁺)
Explore hydrogen-ion activity, electrode calibration, and the logarithmic pH scale inside the ISO 80000 framework.
Water chemistry unitsGerman Degree of Hardness (°dH): Water Quality Benchmarking
Translate °dH into CaCO₃ equivalents, manage scaling risks, and align municipal, industrial, and consumer reporting on water hardness.
Water chemistry unitsLangelier Saturation Index (LSI): Water Stability and Scaling Potential
Balance pH, alkalinity, hardness, and TDS to predict CaCO₃ scaling or corrosion in treatment plants, cooling towers, and pools.
Chemical quantitiesThe Katal (kat): SI Unit of Catalytic Activity
Express enzyme assays and heterogeneous catalyst performance in katals so kinetic models and regulatory dossiers stay in SI lockstep.
Biochemical quantitiesCatalytic Concentration (kat·m⁻³): SI Reporting for Enzyme Activity
Report enzyme assays in katals per litre or cubic metre so laboratory medicine, bioprocessing, and accreditation workflows stay interoperable.
Biochemical quantitiesInternational Unit (IU): Standardizing Biological Potency
Calibrate vitamins, hormones, and biologics against WHO reference materials so potency, dosing, and global trade stay consistent.
Genetics & genomicsThe Centimorgan (cM): Genetic Linkage Distance
Connect recombination frequencies to centimorgan maps, relate linkage analysis with physical genomics, and interpret breeding or medical genetics data with confidence.
Base unitsThe Candela (cd): The SI Base Unit of Luminous Intensity
See how luminous efficacy, photometry, and human vision intersect so lighting calculations stay consistent across standards.
Photometric constantsLuminous Efficacy of 540 THz Radiation
See how the fixed 683 lm·W⁻¹ luminous efficacy defines the candela, connects photometry with radiometry, and guides lighting efficiency.
Photometric quantitiesLuminous Intensity: Definition, History, Concepts, and Applications
Link photometry, vision science, and metrology to design lighting, displays, and sensors with precise candela-level control.
Photometric unitsCandela (cd): The SI Unit of Luminous Intensity
Study the ISO 80000-7 definition with historical context, conceptual foundations, and measurement practices for luminous intensity.
Derived unitsThe Joule (J): SI Unit of Energy
Connect mechanical work, heat, and electricity with the joule so every energy balance aligns with ISO 80000 definitions and SI conversions.
Energy unitsKilowatt-Hour (kWh): Tracking Electric Energy Consumption
Interpret utility bills, smart meter logs, and storage ratings in kilowatt-hours while keeping conversions to joules and greenhouse accounting factors precise.
Electrical quantitiesKilovolt-Ampere (kVA): Apparent Power Rating for AC Systems
Relate voltage, current, and power factor to apparent power so generators, transformers, and UPS fleets stay within design limits.
Energy unitsCalories and Kilocalories: Legacy Heat Units in Modern Energy Accounting
Disentangle calorie variants, convert seamlessly to joules, and keep nutrition, combustion, and sustainability reporting aligned with SI guidance.
Energy unitsThe Tonne of Refrigeration (TR): Cooling Capacity in HVAC Engineering
Relate TR ratings to kilowatts, design chillers and ice-storage systems with consistent power balances, and document HVAC performance across global standards.
Radiation quantitiesThe Gray (Gy): SI Unit of Absorbed Dose for Radiation Metrology
Track how one joule per kilogram underpins radiation protection, therapy planning, and dosimetry workflows across aerospace and healthcare.
Radiation dosimetryDose-Volume Histogram Metrics (Gy): Mapping Absorbed Dose Distributions
Transform three-dimensional dose data into actionable DVH statistics so radiotherapy teams balance coverage, sparing, and compliance.
Radiation dosimetryExposure (C·kg⁻¹): Ionisation-Based Reference Quantity for Beam Calibration
Connect historical roentgen-era ionisation measurements with modern kerma and dose calibrations in diagnostic imaging.
Radiation quantitiesThe Sievert (Sv): SI Unit of Equivalent Dose and Radiological Risk
Convert absorbed dose to biological effect with sievert-based weighting factors that keep safety cases and mission planning aligned.
Atomic & nuclear unitsThe Becquerel (Bq): SI Unit of Radioactivity
Track decay kinetics, calibrate detectors, and communicate activity measurements that underpin nuclear medicine, safeguards, and environmental surveillance.
Radiation quantitiesSpecific Activity (Bq·kg⁻¹): Assessing Radioactivity Concentration
Normalise radionuclide measurements by mass so environmental surveys, food safety tests, and safeguards remain comparable worldwide.
Material characterisationSpecific Surface Area (m²·kg⁻¹): Characterising Porous Media
Measure BET, porosimetry, and adsorption data to connect pore architecture with catalytic, energy, and environmental performance.
Atomic & nuclear unitsElectronvolt (eV): The Natural Energy Unit of Atomic, Nuclear, and Particle Physics.
Use eV-scale insights for spectroscopy, accelerator physics, and detector calibration while keeping conversions to SI joules exact.
Atomic & nuclear unitsBarn (b): The Canonical Unit of Nuclear Reaction Cross-Section
Quantify interaction probabilities across reactors, beamlines, and astrophysics with barns and their submultiples under ISO 80000-10.
Radiation interactionMass Attenuation Coefficient (m²·kg⁻¹): Photon Interaction Scaling
Model photon absorption and scattering per unit mass so shielding, imaging, and remote-sensing analyses stay accurate across energies.
Nuclear engineeringSeparative Work Unit (SWU): Uranium Enrichment Metric
Quantify enrichment effort with the value function, optimise cascades, and align safeguards reporting with fuel cycle planning.
Derived unitsThe Volt (V): Unit of Electric Potential Difference
Trace voltage definitions from Josephson standards to RMS measurements so circuit design, power quality, and instrumentation stay coherent.
Quantum constantsJosephson Constant: Quantum Voltage Realization
Learn how programmable Josephson arrays convert frequency to voltage with exact proportionality, anchoring traceable voltage standards.
Derived unitsThe Ohm (Ω): Unit of Electrical Resistance
See how quantum Hall standards, impedance models, and material properties keep resistance measurements comparable across labs and fieldwork.
Quantum constantsVon Klitzing Constant and the Quantum Hall Resistance
Explore how quantum Hall devices realize exact resistance using the von Klitzing constant and support precision electrical metrology.
Electrical quantitiesThe Siemens (S): SI Unit of Electrical Conductance and Admittance
Invert ohms, master admittance, and link quantum standards to circuit design so conductance data stays dependable from lab to field.
Electrical quantitiesThe Siemens per Metre (S/m): SI Unit of Electrical Conductivity
Tie microscopic transport models to bulk conductivity measurements so materials, circuits, and geophysics share consistent S/m data.
Electrical quantitiesThe Farad (F): SI Unit of Capacitance
Translate geometry, materials, and frequency response into dependable capacitance values for electronics, power, and sensing systems.
Electrical quantitiesMutual Capacitance Matrices (F): Modelling Multiconductor Networks
Extract and validate capacitance matrices so cables, PCBs, and sensors deliver predictable crosstalk, timing, and EMC performance.
Electrical quantitiesCoulomb (C): The SI Unit of Electric Charge
Count charge with confidence by pairing the ampere’s quantum definition with coulomb-based electrostatics, electrochemistry, and energy balances.
Electrical quantitiesAmpere-Hour (Ah): Practical Unit of Stored Electric Charge
Translate battery capacity ratings into coulombs, watt-hours, and state-of-charge models that align with IEC and SAE performance standards.
ElectromagnetismThe Henry (H): SI Unit of Inductance
Design coils, transformers, and resonant converters with inductance values anchored to SI traceability and ISO 80000 terminology.
ElectromagnetismHenry per Metre (H·m⁻¹): Quantifying Magnetic Permeability
Characterise magnetic materials, shielding, and inductive components using permeability expressed in henry per metre.
ElectromagnetismThe Weber (Wb): SI Unit of Magnetic Flux
Relate Faraday’s law, flux linkage, and calibration methods so magnetic measurements stay coherent from lab instruments to power grids.
Derived unitsThe Tesla (T): Unit of Magnetic Flux Density
Link magnetic flux density to induced voltage, torque, and imaging performance so electromagnetic analyses follow ISO 80000-6.
ElectromagnetismElectric Field Strength (V·m⁻¹): Measuring and Modelling Field Intensity
Quantify electric fields across power, RF, and EMC domains so compliance, safety, and design workflows share traceable V·m⁻¹ data.
ElectromagnetismThe Gauss (G): CGS Unit of Magnetic Flux Density
Translate historical magnetism data expressed in gauss into tesla-based workflows without losing traceability.
ElectromagnetismThe Maxwell (Mx): CGS Unit of Magnetic Flux
Relate legacy maxwell measurements to webers, link flux quantification with Faraday’s law, and harmonise magnetic system documentation with SI practice.
Dimensionless unitsThe Radian (rad): the Natural Unit of Plane Angle
Keep calculus, robotics, and signal analysis coherent with ISO-endorsed radian notation and practical realization tips.
Dimensionless unitsPlane Angle and the Radian
Trace ISO 80000-3 definitions, measurement practices, and software safeguards that keep radians indispensable in engineering.
Dimensionless unitsThe Steradian (sr): The Geometry of Direction
Understand solid angle so photometry, antennas, and heat-transfer view factors share the same directional language.
Photometric unitsLumen (lm): The SI Unit of Luminous Flux
Connect luminous flux, energy efficiency, and photometric measurement so lighting designs stay traceable to ISO 80000-7.
Photometric unitsColor Rendering Index (CRI): Evaluating Lighting Quality
Interpret Ra and R9 scores, compare spectra, and specify lighting that balances colour fidelity with efficiency and wellbeing.
Photometric unitsCandela per Square Metre (cd/m²): SI Unit of Luminance
Link luminance standards, display calibration, and lighting safety codes with traceable cd/m² measurements.
Photometric unitsTalbot (T): Luminous Energy Unit
Integrate luminous flux over time to manage exposure histories for lighting design, conservation, and human-centric applications.
Radiometry & opticsThe Watt per Square Metre (W/m²): SI Unit of Irradiance
Quantify radiant power density for climate science, photovoltaics, lighting, and thermal design with SI-traceable W/m² data.
Radiometry & opticsSpectral Radiance (W·m⁻²·sr⁻¹·nm⁻¹): Characterising Optical Sources
Characterise radiometric source output across direction and wavelength so remote sensing, display metrology, and calibration labs align.
Dimensionless unitsSolid Angle and the Steradian
Explore the ISO 80000-3 steradian, radiometry links, and instrumentation that keep directional quantities traceable.
Dimensionless unitsThe Decibel (dB): Logarithmic Quantities, and Ratio Levels
Master logarithmic power and field ratios with ISO 80000 guidance for acoustics, RF, and optical systems.
Electrical quantitiesdBm: Absolute Power Levels in Logarithmic Form
Reference 1 mW precisely so RF, audio, and optical link budgets can convert between logarithmic dBm levels and linear watts without ambiguity.
Dimensionless unitsThe Neper (Np): Natural Logarithmic Unit for Ratio Levels
Convert exponential gain, attenuation, and damping into additive measures that align perfectly with calculus-based SI formulations.
Derived unitsThe Pascal (Pa): SI Unit of Pressure
Learn how force per area becomes pascals, why kPa and MPa dominate engineering, and how ISO 80000-4 keeps pressure and stress coherent.
Derived unitsBar (bar): Engineering Unit of Pressure
Compare bar readings with pascals and atmospheres to keep hydraulic, meteorological, and oceanographic data interoperable.
Pressure unitsPound-Force per Square Inch (psi): Imperial Pressure in Engineering Practice
Convert legacy psi gauges into SI, understand gauge versus absolute pressure, and keep hydraulic, aerospace, and maintenance documentation aligned.
Derived unitsStandard Atmosphere (atm): Reference Pressure for Science and Aviation
Anchor thermodynamics, weather forecasting, and flight operations to the 101 325 Pa standard atmosphere reference.
FoundationsUnits of Measurement
Read the complete story of how measurement units emerged, evolved into the SI, gained ISO guidance and power today’s tools.
FoundationsCalculation: Definition, History, Key Concepts, and Real-World Applications
Trace how algorithms, ISO standards, and everyday problem solving keep calculations coherent from ancient tools to modern automation.
FoundationsEvolution of Calculation Tools: From Abacus to AI Automation
Follow the milestones that turned counting stones into silicon chips and discover the calculators that carry each tradition forward.
FoundationsCalculation Standards and Notation: Keeping Results Consistent
Learn how shared symbols, typography rules, and unit conventions keep every calculator, report, and simulation in sync.
Applied guidesPractical Applications of Calculation in Science, Finance, and Daily Life
Connect theoretical principles to real-world problem solving with curated examples linked to CalcSimpler tools and standards guides.
FoundationsWhat is a Unit of Measurement?
Define units, trace their origins and see why consistent measurement underpins every field of study.
Global standardsISO 80000-1: General Principles for Quantities and Units
Understand how Part 1 establishes the terminology, typography and base quantities that keep every ISO 80000 chapter aligned.
Global standardsISO 80000-2: Mathematical Signs and Symbols
Learn the notation rules for algebra, calculus and vectors so your calculations match the typography used across ISO 80000.
Global standardsISO 80000-3: Quantities and Units of Space and Time
Dive into the spatial, temporal and kinematic quantities that connect distance, duration and angular motion across the ISO framework.
Global standardsISO 80000-4: Quantities and Units of Mechanics
Study how force, torque, momentum and energy definitions interlock so structural loads and machines stay aligned with SI units.
Global standardsISO 80000-5: Quantities and Units of Thermodynamics
Master the kelvin scale, energy balances, and entropy definitions that power heating, cooling, and energy efficiency studies.
Global standardsISO 80000-6: Quantities and Units of Electromagnetism
Explore the electrical and magnetic field definitions that keep circuitry, power systems, and EMC reports aligned with SI units.
Global standardsISO 80000-7: Quantities and Units of Light and Radiation
Unpack how photometric and radiometric measures tie light measurements to the candela, lumen, lux, and watt for consistent lighting analyses.
Photometric unitsLux (lx): The SI Unit of Illuminance
Apply lux definitions, measurement techniques, and application guidance when auditing lighting performance and safety.
Photometric unitsFoot-Candle (fc): Imperial Illuminance Unit
Convert between foot-candles and lux, trace imperial lighting history, and document illuminance surveys with dual-unit clarity.
Global standardsISO 80000-8: Quantities and Units of Acoustics
Learn how ISO 80000-8 formalises sound pressure, intensity, power and exposure so environmental and product noise studies speak the same language.
AcousticsSound Pressure Level (Lp): Definition, Measurement, and Applications
Follow ISO 80000-8 guidance on SPL references, instrumentation, and reporting so environmental and product assessments align.
AcousticsSound Power Level (Lw): Definition, Methods, and Use Cases
Understand how declared sound power levels are derived, compared, and applied to product labeling and noise control decisions.
AcousticsSound Intensity and Intensity Level (Li): Directional Acoustics Explained
Explore intensity probes, plane-wave relations, and diagnostic workflows that connect vector energy flow to sound power results.
AcousticsThe Sone: Psychoacoustic Loudness Unit
Translate equal-loudness contours, ISO 532 loudness models, and perception-driven design targets into actionable sones.
AcousticsPhon: Psychoacoustic Loudness Level Measurement
Use equal-loudness contours to relate decibel readings to perceived loudness for audio design, regulation, and acoustic comfort assessments.
AcousticsLoudness Units relative to Full Scale (LUFS): Programme Audio Normalisation
Implement ITU-R BS.1770 loudness metering, align broadcast and streaming targets, and document LUFS-based compliance workflows.
AcousticsSavart: Logarithmic Pitch Unit for Frequency Ratios
Convert frequency ratios into savarts, compare with cents, and apply logarithmic intervals in acoustics and music technology.
Astronomical unitsAstronomical Unit (au): Measuring Distances Across the Solar System
Plan planetary missions and solar irradiance models using the exact 149 597 870 700 m astronomical unit definition.
Astronomical unitsLight-Year: Distance Traveled by Light in One Julian Year
Convert cosmic scales into intuitive light-year distances while preserving links to SI-based parallax and redshift data.
Astronomical unitsParsec (pc): Parallax-Based Astronomical Distance Unit
Translate parallax angles into parsecs to build the cosmic distance ladder from nearby stars to distant galaxies.
Astronomical unitsThe Jansky (Jy): Spectral Flux Density Unit in Radio Astronomy
Connect radio-astronomy source catalogues with SI spectral flux density for surveys, calibration, and transient science.
Astronomical unitsSolar Flux Unit (sfu): Solar Radio Activity Index
Monitor 10.7 cm flux in sfu, manage calibration and adjusted values, and apply radio indices to space weather forecasting.
Spatial unitsArea and the Square Metre (m²) under ISO 80000-3
See how ISO 80000-3 ties surface and volume calculations to m² and m³ with practical guidance for surveying, manufacturing, and process control.
Area and land managementHectare (ha): Metric Unit for Land Area
Convert large land parcels into hectares and integrate surveying, GIS, and resource planning data with SI area standards.
Volume unitsThe Litre (L): Accepted Non-SI Unit of Volume
Follow the litre’s geometric definition, historical reforms, and modern applications so lab work and trade documentation stay coherent.
Global standardsISO 80000-9: Quantities and Units of Physical Chemistry
Map moles, concentrations, molecular constants and catalytic activity into SI-coherent terms for reproducible lab records and process models.
Global standardsISO 80000-10: Quantities and Units of Atomic and Nuclear Physics
Trace half-lives, decay constants and radiation dose metrics back to their SI roots so safeguards, medical and research teams share consistent data.
Global standardsISO 80000-11: Quantities and Units of Characteristic Numbers
Translate Reynolds, Mach, Prandtl and other similarity ratios into ISO terminology so scale models, simulations and plant data stay comparable.
Characteristic numbersReynolds Number (Re): Inertial–Viscous Similarity in Fluid Mechanics
Trace the definition, history, and regime guidance for Reynolds number so laminar, transitional, and turbulent analyses remain ISO 80000 compliant.
Characteristic numbersMach Number (Ma): Compressibility and Wave Phenomena in Gas Dynamics
Map subsonic, transonic, and hypersonic regimes with ISO 80000-11 conventions for shock, nozzle, and aerodynamic studies.
SeismologyMoment Magnitude (Mw): Quantifying Earthquake Size
Derive Mw from seismic moment, integrate waveform and geodetic data, and translate magnitudes into seismic hazard planning.
Dynamics and gravimetryGal (Galileo): CGS Unit of Acceleration
Capture microgal gravity changes, seismic peak ground acceleration, and structural vibrations with the CGS gal linked back to SI metres per second squared.
Dynamics and gravimetryg-Force (g): Acceleration Relative to Standard Gravity
Express inertial loads as multiples of g₀ to manage human tolerance, vehicle design, and structural qualification.
Kinematics & navigationThe Knot (kn): Maritime and Aeronautical Unit of Speed
Carry nautical-mile-per-hour speed reporting from its log-line origins into GPS-age aviation and marine navigation workflows.
Kinematics & navigationNautical Mile (nmi): Maritime and Aeronautical Distance Standard
Tie latitude and longitude directly to distance, convert between nautical miles and SI units, and manage regulatory reporting across air and sea.
KinematicsMetre per Second (m·s⁻¹): SI Unit of Speed and Velocity
Anchor transport analytics, CFD studies, and instrumentation workflows with the SI-coherent metre per second velocity unit.
Kinematics & navigationThe Fathom (ftm): Nautical Depth Measure
Trace sounding line traditions into modern sonar, convert fathoms to metres effortlessly, and keep hydrographic surveys interoperable across unit systems.
Maritime regulationGross Register Tonnage (GRT): Measuring Vessel Internal Volume for Maritime Regulation
Calculate enclosed ship volume in register tons, understand Moorsom-to-IMO reforms, and apply tonnage data to dues, safety codes, and fleet planning.
Transport metricsTonne-Kilometre (tkm): Freight Transport Performance Metric
Multiply cargo mass by distance to benchmark logistics productivity, modal energy intensity, and carbon reporting across supply chains.
Characteristic numbersNusselt Number (Nu): Convective Enhancement of Heat Transfer
Understand Nu definitions, canonical correlations, and measurement practices that tie convection performance to ISO 80000 standards.
Characteristic numbersPrandtl Number (Pr): Linking Momentum and Thermal Diffusion
Pair viscosity and thermal conductivity data to compute Pr with confidence and apply similarity solutions across convection problems.
Characteristic numbersPéclet Number (Pe): Convection–Diffusion Similarity Parameter
Balance advective and diffusive transport using Péclet numbers so heat exchangers, reactors, and environmental models scale reliably from lab to field.
Characteristic numbersDamköhler Number (Da): Reaction–Transport Competition
Compare reaction rates with transport times to diagnose whether reactors, catalysts, and environmental systems are kinetics- or diffusion-limited.
Characteristic numbersBiot Number (Bi): Conduction–Convection Coupling in Heat Transfer
Explain how Bi compares internal conduction and external convection to validate lumped or spatial thermal models across materials and geometries.
Characteristic numbersGrashof Number (Gr): Buoyancy-Driven Flow Instability
Gauge when buoyancy overcomes viscous damping in natural convection so passive cooling, building envelopes, and geophysical models stay accurate.
Characteristic numbersRayleigh Number (Ra): Natural Convection Thresholds
Predict onset and intensity of buoyancy-driven convection by combining Grashof and Prandtl numbers for enclosures, electronics, and climate studies.
Characteristic numbersLewis Number (Le): Thermal-to-Mass Diffusivity Ratio
Relate heat and mass transport rates to apply Chilton–Colburn analogies in combustion, drying, HVAC, and additive manufacturing workflows.
Global standardsISO 80000-12: Quantities and Units of Solid State Physics
Navigate lattice parameters, elastic constants, transport coefficients and magnetic descriptors with SI-aligned notation for condensed matter work.
Information and computingByte (B): Fundamental Unit of Digital Information
Clarify eight-bit bytes, binary prefixes, and storage conventions so software, networking, and cloud teams share consistent data metrics.
Information and computingData Transfer Rate (bit/s and byte/s): Throughput in Digital Systems
Differentiate bit and byte rates, protocol overhead, and benchmarking methods for networks, storage, and streaming platforms.
Information and computingZettabyte (ZB): Benchmarking Global Data Volumes
Translate worldwide data creation into planning metrics for storage, bandwidth, and sustainability at 10²¹-byte scales.
Information and computingBaud (Bd): Symbol Rate in Digital Communications
Distinguish symbol rate from bit rate, modulation order, and instrumentation requirements for modern high-speed links.
Global standardsISO 80000-13: Quantities and Units of Information Science and Technology
Clarify bits, binary prefixes, data rates and reliability metrics so software, networking and analytics teams speak the same measurement language.
Global standardsISO 80000: Quantities and Units (13 Main Parts)
Jump straight into part-by-part tables that map ISO 80000 quantities to their symbols and SI-aligned units.
Global standardsInternational System of Units (SI)
Review the seven base units, the constants that define them and the tools that keep everyday calculations coherent.
Global standardsISO 80000 Quantities & Units
Follow the volumes that codify symbols, prefixes and derived units so engineers, scientists and buyers speak the same measurement language.
ElectromagnetismDebye (D): CGS Unit of Electric Dipole Moment
Connect molecular polarity measurements with SI coulomb-metre notation, spectroscopy calibration, and dielectric modelling best practices.
Fluid dynamicsSverdrup (Sv): Ocean Volume Transport Unit
Translate basin-scale circulation into actionable transport estimates, heat budget calculations, and climate assessments using the sverdrup.
Environmental monitoringNephelometric Turbidity Unit (NTU): Quantifying Water Clarity
Understand formazin-based calibration, optical physics, and regulatory applications that keep turbidity reporting accurate and comparable.
Water qualityBiochemical Oxygen Demand (BOD₅): Benchmarking Aquatic Biodegradation
Define the five-day BOD test, manage seeding and kinetics, and connect oxygen-demand data to treatment plant optimisation and watershed protection.
Water qualityChemical Oxygen Demand (COD): Measuring Oxidisable Load
Apply dichromate digestion protocols, interpret COD/BOD relationships, and deploy rapid proxies for industrial and municipal water management.
Water qualityTotal Organic Carbon (TOC): Tracking Dissolved Carbon Loads
Master combustion and persulfate TOC analysers, comply with drinking-water and ultrapure standards, and align carbon metrics with sustainability goals.
Water qualityDissolved Oxygen (DO): mg·L⁻¹ Indicator of Aquatic Health
Deploy optical and electrochemical DO sensors, model reaeration and sag curves, and maintain oxygen resilience across ecosystems and treatment assets.
Medical imagingHounsfield Unit (HU): Computed Tomography Attenuation Scale
Review HU definitions, calibration workflows, and clinical applications that link CT attenuation to quantitative diagnosis and therapy planning.
Food scienceScoville Heat Unit (SHU): Measuring Capsaicin Pungency
Trace sensory origins, chromatographic methods, and culinary, regulatory, and agricultural uses of the Scoville scale.
RadiometryLangley (Ly): Historical Unit of Solar Irradiance
Convert legacy Langley datasets into SI units while exploring instrumentation, uncertainty, and solar energy applications.
Characteristic numbersFroude Number (Fr): Wave-Making and Hydrodynamic Similarity
Balance inertia and gravity effects in free-surface flows to scale hydraulic models, ship tests, and environmental simulations confidently.
Atmospheric scienceDobson Unit (DU): Column Density of Atmospheric Ozone
Connect ozone column measurements, remote-sensing techniques, and policy reporting with clear DU-to-SI conversions.
Biophysical unitsSvedberg (S): Sedimentation Coefficients in Molecular Biophysics
Quantify macromolecular sedimentation, understand ultracentrifugation history, and connect Svedberg analyses to viscosity, density, and diffusion data.
Pressure unitsTorr: Practical Vacuum Pressure Benchmark
Translate between torr, pascal, and atmosphere to operate vacuum systems, calibrate gauges, and manage outgassing across semiconductor and laboratory workflows.
Materials propertiesBrinell Hardness Number (HBW): Indentation Testing for Structural Metals
Derive HBW values, control metallurgical quality, and link hardness to strength, welding, and lifecycle integrity decisions across heavy industry.
GeophysicsEötvös (E): Gravity Gradient Measurement in Geodesy
Measure gravity gradients in eötvös units, interpret torsion balance and satellite data, and integrate geophysical surveys for exploration and monitoring.
Kinematic quantitiesJerk (m·s⁻³): Third Derivative of Position in Motion Control
Design jerk-limited trajectories that protect equipment, improve comfort, and coordinate robotics, transport, and biomechanics with SI kinematics.
MeteorologyBeaufort Scale: Empirical Wind Force Grading for Marine Operations
Decode Beaufort numbers, relate qualitative wind observations to speed ranges, and apply the scale to navigation, offshore safety, and climate studies.
Optical unitsDioptre (D): Optical Power in Vision Science and Photonics
Convert focal lengths to dioptres, manage ophthalmic prescriptions, and coordinate optical system design across clinical and engineering teams.
Vision scienceETDRS Letter Score: Standardized Visual Acuity Measurement
Implement logMAR-based acuity charts, control lighting conditions, and link visual performance to refractive corrections.
TelecommunicationsErlang (E): Telecommunications Traffic Intensity
Model call volumes and concurrency with erlangs to dimension networks, staff contact centres, and align service quality with modern digital infrastructure.
Characteristic numbersGalilei Number (Ga): Characterising Particle Settling Dynamics
Balance gravity, buoyancy, and viscosity to classify settling regimes, fluidised beds, and slurry handling performance with Galilei-based diagnostics.
Geophysical fluid dynamicsRichardson Number (Ri): Stratified Shear Stability Metric
Gauge when density stratification suppresses or allows turbulence, supporting weather, ocean, and environmental forecasting.
Interfacial transportMarangoni Number (Ma): Thermocapillary Convection Driver
Predict surface-tension-driven flow in welding, crystal growth, and microfluidics by combining thermal gradients with viscosity and diffusivity data.
Geophysical fluid dynamicsEkman Number (Ek): Viscous–Coriolis Balance in Rotating Flows
Diagnose rotating boundary layers, Ekman pumping, and planetary core dynamics by comparing viscosity with Coriolis effects.
AcousticsLoudness Units Relative to Full Scale (LUFS): Broadcast and Streaming Loudness Standard
Align programme loudness with global delivery standards using LUFS and LU metering, psychoacoustic weighting, and compliance workflows.