The Knot (kn): Maritime and Aeronautical Unit of Speed
The knot—symbol kn—is an accepted non-SI unit used worldwide to express speed in marine, aviation, and meteorological contexts. Defined exactly as one nautical mile per hour, it links Earth's geometry to timekeeping standards discussed in the metre overview and second explainer. This article documents the knot's origins, practical usage, and conversion strategies so you can deploy it confidently alongside SI velocities when drafting navigational plans, analysing flight data, or briefing decision-makers.
Definition and SI Relationship
The International Bureau of Weights and Measures (BIPM) recognises the knot as an accepted non-SI unit permitted for use with the SI, fixed exactly at 1 nautical mile per hour. Because the nautical mile is defined as 1852 metres, the knot equals 1852 metres per hour. Converting to the SI velocity unit metres per second gives 1 kn = 0.514444... m·s⁻¹ (more precisely 1852/3600 m·s⁻¹). This constant underpins every calculator on the site, including the metres per second to knots tool, ensuring reproducible conversions in software and operational documents.
Expressing knots in terms of kilometres per hour multiplies 0.514444 m·s⁻¹ by 3.6, yielding 1 kn ≈ 1.852 km·h⁻¹. The ratio to miles per hour is 1 kn ≈ 1.15078 mph, a value frequently referenced in aviation manuals when harmonising airspeeds for multinational crews. Maintaining precision in these ratios avoids compounding errors during long transoceanic routes or integrated weather-traffic simulations.
Velocity Components and Notation
Navigators often decompose motion into speed (a scalar in knots) and course or heading (an angle). ISO 80000-3 covers angular notation, complementing the speed unit. Speed over ground (SOG) and speed through water (STW) may both be expressed in knots but differ when currents or winds are present. Recording which speed metric is reported preserves clarity when comparing GPS-derived SOG with Doppler log measurements of STW.
Historical Development
The knot traces to early modern navigation techniques using the chip log or log-line method. Sailors threw a wooden panel (the "log") attached to a rope knotted at uniform intervals into the water. Sandglass timing—often 28 seconds—measured how many knots passed over the rail, directly indicating the vessel's speed in nautical miles per hour. These practices emerged in the 16th and 17th centuries and persisted well into the age of sail.
Standardisation advanced with national navies. In 1830 the British Admiralty adopted a nautical mile based on one minute of arc along a great circle of Earth, and by 1929 the International Hydrographic Bureau fixed the international nautical mile at 1852 metres. The adoption of this metric definition harmonised knot conversions across charts and instruments, aligning marine navigation with the SI while retaining the familiar unit.
Aviation incorporated the knot during the 20th century because air navigation borrowed heavily from maritime practice. The International Civil Aviation Organization (ICAO) formalised knot usage in flight levels, approach plates, and performance manuals, encouraging global pilots to express true airspeed (TAS), calibrated airspeed (CAS), and indicated airspeed (IAS) in knots. Meteorological agencies adopted the same unit for wind speeds to maintain interoperability with aviation weather briefings and maritime forecasts.
Conceptual Foundations and Conversions
Link to Nautical Geometry
The nautical mile's tie to Earth's meridians means the knot conveniently links speed to geographic progress. Sailing one knot along a meridian advances the vessel one minute of latitude per hour, simplifying dead reckoning and paper-chart plotting. Even when electronic navigation dominates, this geometric intuition remains valuable for quick mental estimates and verifying automated systems.
Conversion Workflow
Modern navigation suites continuously convert between knots, metres per second, kilometres per hour, and miles per hour. Reproducing these conversions manually is straightforward: multiply the knot value by 0.514444 to obtain m·s⁻¹, by 1.852 for km·h⁻¹, or by 1.15078 for mph. The metres per second to knots calculator automates this process, while the nautical miles to kilometres converter keeps distance units aligned with SI conventions described in the length explainer.
True, Ground, and Air Speeds
Navigators distinguish between true speed relative to the medium (water or air) and ground-referenced speed. For ships, STW results from dividing distance travelled through water by time, often measured via electromagnetic or Doppler logs. For aircraft, TAS derives from pitot-static sensors corrected for compressibility and temperature. Ground speed (GS) or SOG uses GPS or inertial navigation to account for currents or winds. Reporting each quantity in knots maintains compatibility with international flight rules and shipping conventions, but analysts often cross-check results in m·s⁻¹ when integrating with physical models governed by SI units.
Measurement Technologies
Contemporary vessels rarely use log-lines, yet the principle—distance over time—remains. Ships employ Doppler velocity logs emitting acoustic beams; the frequency shift upon reflection from the seafloor or suspended particles yields STW in knots. Electromagnetic logs measure the voltage induced in seawater by the ship's movement through Earth's magnetic field, another direct proxy for knots.
Aircraft rely on pitot tubes feeding air data computers, which compute IAS and then CAS and TAS in knots after applying position and compressibility corrections. Inertial measurement units and GPS receivers provide groundspeed. Meteorological radars and LIDAR systems capture wind speed in knots for storm tracking, keeping outputs aligned with aviation weather briefings.
Integrated bridge systems and flight management systems display knots while storing high-resolution data in SI units. This dual representation ensures compatibility with regulatory reporting requirements that often mandate SI metrics, while preserving operational familiarity for crews trained in nautical units.
Calibration and Traceability
Calibration laboratories verify speed sensors using tow tanks, wind tunnels, or moving-platform rigs with traceable distance and time references. The second and metre underlie these calibrations, ensuring that when the instrument reports 15 kn it corresponds to exactly 7.7167 m·s⁻¹ within stated uncertainties. Documenting uncertainty budgets and environmental corrections keeps the knot compatible with SI-based modelling and safety analyses.
Applications Across Domains
Marine Navigation and Logistics
Shipping companies plan voyages by balancing knots against fuel consumption curves, known as speed–power relationships. Reducing speed by one or two knots can lower fuel burn significantly, influencing emissions strategies aligned with International Maritime Organization (IMO) regulations. Harbour pilots use knots to coordinate vessel traffic services, aligning speed orders with tidal currents and berth availability.
Aviation Operations
Air traffic control issues speed instructions in knots to maintain separation on approach and departure. Airlines monitor climb and cruise speeds in knots to evaluate aircraft performance relative to flight plans. Because the knot ties back to nautical miles, pilots can quickly estimate time-to-go by dividing remaining distance by current groundspeed. Performance manuals include tables of stall, approach, and manoeuvring speeds in knots, aiding quick recall under workload.
Meteorology and Climate Science
Weather services report wind speeds, tropical cyclone intensity, and jet stream analyses in knots to maintain compatibility with aviation weather briefings. Forecasters convert these values into metres per second when feeding numerical weather prediction models, demonstrating the unit's coexistence with SI calculations. Oceanographers likewise describe surface currents in knots, cross-validating with satellite altimetry and drifting buoy data stored in SI units.
Search and Rescue Planning
Search and rescue (SAR) coordinators express drift rates, leeway, and asset speeds in knots to estimate search areas over time. Integrating these values with scheduling tools such as the business-day planner helps logistics teams align resource availability with rapid response windows.
Why the Knot Remains Important
The knot persists because it embeds navigational intuition: a vessel or aircraft covering one minute of latitude per hour. Retaining the unit supports legacy documentation, international regulation, and human factors while still aligning with SI definitions. Precise conversions, supported by calculators and rigorous reporting, prevent errors when mixing units across multidisciplinary teams.
When documenting operations, pair knot-based summaries with SI references—state both kn and m·s⁻¹, cite the 1852-metre definition of the nautical mile, and reference related articles on timekeeping and length measurement. This dual framing helps regulators, engineers, and analysts interpret knot-based data sets with confidence while maintaining compliance with SI-aligned reporting frameworks.