Knot (unit)
Unit of speed equal to one nautical mile per hour.
The knot (symbol kn, or kt in aviation) is a unit of speed equal to one nautical mile per hour—exactly 1.852 km/h, or roughly 1.151 mph and 0.514 m/s. It is a non-SI unit used in meteorology, maritime navigation, and air navigation. The International Civil Aviation Organization (ICAO) recommends the symbol kt, while the ISO standard is kn. A vessel moving at one knot along a meridian covers about one minute of geographic latitude in an hour.
By definition, one international knot equals one nautical mile per hour, which is exactly 1,852 meters per hour. That works out to approximately 0.51444 m/s, 1.15078 mph, 20.25372 inches per second, or 1.68781 feet per second. The internationally agreed nautical mile is 1,852 m. The United States adopted this definition in 1954, replacing its earlier US nautical mile of 1,853.248 m. The United Kingdom followed in 1970, switching from the UK Admiralty nautical mile of 6,080 ft (1,853.184 m).
Speeds of vehicles relative to the fluid they move through—such as boat speeds and airspeeds—are measured in knots. The same unit is used for navigational fluids like ocean currents, tidal streams, river currents, and wind speeds. Consequently, speed over the ground (SOG) and velocity made good (VMG) are also given in knots. Since 1979, the ICAO has listed the knot as permitted for temporary use in aviation, though no end date for that temporary period has been agreed upon as of 2024.
The knot’s origin lies in the chip log, used at sea until the mid-19th century. A weighted wooden panel was cast over the stern, and a line paid out as the vessel moved. Knots tied 47 feet 3 inches (14.4018 m) apart passed through a sailor’s fingers while another timed the operation with a 30-second sand-glass (now accepted as 28 seconds). The knot count was used in dead reckoning. This method yields a value of 20.25 inches per second, or 1.85166 km/h—less than 0.02% different from the modern definition.
Though the knot is not an SI unit, it remains essential for nautical and aviation use because the nautical mile relates directly to the latitude/longitude coordinate system. On a standard Mercator projection chart, the horizontal scale varies with latitude—for example, the scale from Florida to Greenland changes by a factor of two. A single graphic scale would be useless. Since a nautical mile is roughly equivalent to a minute of latitude, distances can be measured using dividers and the chart’s latitude scale. Recent British Admiralty charts include a latitude scale down the middle for convenience.
Speed is sometimes incorrectly expressed as “knots per hour,” which would imply acceleration (nautical miles per hour per hour).
In U.S. civil aviation, airworthiness standards originally specified distances in statute miles and speeds in miles per hour. From 1969, these were progressively amended to use nautical miles and knots. Various abbreviations distinguish airspeed measurements: TAS (true airspeed), KIAS (indicated airspeed), CAS (calibrated airspeed), and EAS (equivalent airspeed). Indicated airspeed is close to true airspeed only at sea level in standard conditions. At 36,000 ft (11,000 m), an indicated 300 kn may correspond to a true airspeed of 500 kn.
- symbol
- kn (ISO, IEEE); kt (common, ICAO)
- field
- Meteorology, maritime navigation, air navigation
- origin method
- Chip log with knots at 47 ft 3 in intervals, timed by 30-second or 28-second san
Verified Timeline
Lore & Background
Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the moving vessel and the line allowed to pay out. Knots tied at a distance of 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20+1⁄4 inches per second or 1.85166 kilometres per hour. The difference from the modern definition is less than 0.02%.
Reader's Guide
The knot remains essential for maritime and aviation navigation because the nautical mile is closely related to the latitude/longitude coordinate system. On a standard nautical chart using Mercator projection, the horizontal (East–West) scale varies with latitude. On a chart of the North Atlantic, the scale varies by a factor of two from Florida to Greenland. A single graphic scale, of the sort on many maps, would therefore be useless on such a chart. Since the length of a nautical mile, for practical purposes, is equivalent to about a minute of latitude, a distance in nautical miles on a chart can easily be measured by using dividers and the latitude scales on the sides of the chart. Recent British Admiralty charts have a latitude scale down the middle to make this even easier. The speeds of vehicles relative to the fluids in which they travel (boat speeds and air speeds) can be measured in knots. For consistency, the speeds of navigational fluids (ocean currents, tidal streams, river currents, and wind speeds) are also measured in knots. Since 1979, the International Civil Aviation Organization lists the knot as permitted for temporary use in aviation, but no end date to the temporary period has been agreed as of 2024.
Did You Know?
- The knot is exactly 1.852 km/h, based on the internationally agreed nautical mile of 1852 m.
- The US adopted the international nautical mile definition in 1954; the UK followed in 1970.
- A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.
- The chip log method used knots tied at intervals of 47 feet 3 inches, timed with a 30-second or 28-second sand-glass.
- Since 1979, the International Civil Aviation Organization lists the knot as permitted for temporary use in aviation, with no end date agreed as of 2024.
The Knot Among the Holdouts
The knot is a non-SI unit. Although the unit knot does not fit within the SI system, its retention for nautical and aviation use is important because the length of a nautical mile, upon which the knot is based, is closely related to the longitude/latitude geographic coordinate system. As a result, nautical miles and knots are convenient units to use when navigating an aircraft or ship.
Roots in Customary Tradition
Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was cast over the stern of the moving vessel and the line allowed to pay out. Knots tied at a distance of 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation. The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20+1⁄4 inches per second or 1.85166 kilometres per hour. The difference from the modern definition is less than 0.02%.
The Economics of Keeping the Knot
The source does not discuss the economics of keeping the knot or compare it to metrication in other contexts. This section cannot be written from the source.
Speed, Derivation, and the Knot's Functional Role
The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h (approximately 1.151 mph or 0.514 m/s). 1 international knot = 1 nautical mile per hour (by definition), 1852.000 metres per hour (exactly), 0.51444 metres per second (approximately), 1.15078 miles per hour (approximately), 20.25372 inches per second (approximately), 1.68781 feet per second (approximately). The length of the internationally agreed nautical mile is 1852 m. The US adopted the international definition in 1954, having previously used the US nautical mile (1853.248 m). The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080 ft or 1853.184 m).
Frequently Asked Questions
What symbol do you use for the knot?
The ISO and IEEE standard symbol is "kn," while the ICAO and everyday usage prefer "kt." Both forms are widely recognized in their respective fields.
Why is the knot important for navigation?
The nautical mile at the heart of the knot ties directly to the latitude-and-longitude grid, so one knot of speed corresponds roughly to one minute of arc of latitude per hour. This lets sailors and pilots chart progress across the globe without juggling unit conversions.
How did the knot get its name?
Sailors originally measured speed by letting a chip-log line, pre-marked with tied knots, drift overboard for a timed interval of 30 or 28 seconds via a sand-glass, then counting how many knots passed. The spacing between those knots was set at 47 feet 3 inches so the count matched nautical miles per hour.
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