International System of Units (SI): The Backbone of Measurement
The International System of Units (SI) is the reference adopted by most countries, laboratories and organisations. Its purpose is to make any measurement comparable regardless of the language, instrument or context used to collect it.
Use this guide to understand the principles behind the SI, see how the seven base units are defined and discover practical tools—like the average speed calculator and the boiling point at altitude calculator —that help you apply the system in real scenarios.
When you move beyond the base quantities, lean on our derived-unit explainers for the newton, joule, and pascal to see how force, energy, and pressure stay coherent across ISO 80000-4 guidance and everyday calculators.
Principles of the modern SI
The 2018 revision marked a turning point: units stopped depending on physical artefacts and began deriving from universal constants. That shift strengthens reproducibility and prevents artefact degradation from jeopardising critical measurements.
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Definitions rooted in constants
Since 2018 every base unit has been linked to fundamental constants such as the speed of light or the Planck constant. That makes SI units reproducible in any accredited laboratory.
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Metrological traceability
Measurements can be traced through calibration chains back to national and international references. This guarantees comparability across industries and countries.
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Dimensional coherence
Equations follow consistent dimensions. The SI clarifies how to derive newtons, pascals or joules from the seven base units without ambiguity.
The seven base units
Each fundamental quantity now has a precise, reproducible definition. From there, you can derive pascals, volts or lumens without losing coherence.
Second (s)
Defined by 9,192,631,770 oscillations corresponding to the transition between two hyperfine levels of the ground state of cesium-133.
Metre (m)
The distance light travels in vacuum in 1/299,792,458 of a second.
Kilogram (kg)
Linked to the Planck constant and realised through Kibble balance experiments.
Ampere (A)
Based on the fixed numerical value of the elementary charge.
Kelvin (K)
Tied to the Boltzmann constant for consistent temperature scales.
Mole (mol)
Relates entities to the Avogadro constant so chemists count particles precisely.
Candela (cd)
Defined via the luminous efficacy of monochromatic radiation of frequency 540×10¹² Hz.
Want to put these ideas into practice? Start with our explainer on what a unit of measurement is and then explore problem-solving tools such as the rule of three calculator to keep calculations consistent when converting between units.
Need full narratives for the SI base units that anchor the rest? Dive into the second, metre, kilogram, ampere, kelvin, mole, and candela articles in the Units & Measures hub for deep dives that include practical calculators and ISO cross-references.
Where the SI shows up every day
The SI is more than a theoretical framework: it underpins educational curricula, industrial processes and digital transformation projects worldwide.
Engineering and manufacturing
Drawings, tolerances and quality checks rely on SI units to ensure components made in different plants fit together.
Data quality and standards
ISO 8000 recommends aligning numeric attributes with SI units to avoid ambiguity when sharing technical catalogues.
Education and outreach
Textbooks and learning platforms use SI conventions so students and teachers share comparable examples.
For a deeper look at how measurement standards support enterprise data strategies, continue with our ISO 80000 quantities & units guide. You can also browse the conversion library whenever you need quick reference tables alongside the calculators mentioned above.