Knot (unit)

Last updated

Airspeed Indicator.svg
An airspeed indicator, which shows speed in knots
General information
Unit system
  • meteorology
  • aviation
  • maritime
Unit of speed
1 kn in ...... is equal to ...
    km/h    1.852
    mph    1.15078
    m/s    0.514444
    ft/s    1.68781

The knot ( /nɒt/ ) 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] The ISO standard symbol for the knot is kn. [2] The same symbol is preferred by the Institute of Electrical and Electronics Engineers (IEEE), while kt is also common, especially in aviation, where it is the form recommended by the International Civil Aviation Organization (ICAO). [3] The knot is a non-SI unit. [4] The knot is used in meteorology, and in maritime and air navigation. A vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.



1 international knot =
1  nautical mile per hour (by definition),
1852.000 metres per hour (exactly), [4]
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). [5] The UK adopted the international nautical mile definition in 1970, having previously used the UK Admiralty nautical mile (6080 ft or 1853.184 m).

Conversions between common units of speed
m/s km/h mph knot ft/s
1 m/s = 13.6000002.236936*1.943844*3.280840*
1 km/h = 0.277778*10.621371*0.539957*0.911344*
1 mph = 0.447041.60934410.868976*1.466667*
1 knot = 0.514444*1.8521.150779*11.687810*
1 ft/s = 0.30481.097280.681818*0.592484*1

(* = approximate values)


The speeds of vessels relative to the fluids in which they travel (boat speeds and air speeds) are measured in knots. For consistency, the speeds of navigational fluids (ocean currents, tidal streams, river currents and wind speeds) are also measured in knots. Thus, speed over the ground (SOG; ground speed (GS) in aircraft) and rate of progress towards a distant point ("velocity made good", VMG) are also given in knots.


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. [6] 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. [7] 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 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%.

Derivation of knots spacing:

, so in seconds that is metres per knot.

Modern use

Graphic scale from a Mercator projection world map, showing the change with latitude World Scale from DMA Series 1150 map.png
Graphic scale from a Mercator projection world map, showing the change with latitude

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.

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. [8]

Speed is sometimes incorrectly expressed as "knots per hour", [9] which would mean "nautical miles per hour per hour" and thus would refer to acceleration.

Aeronautical terms

Prior to 1969, airworthiness standards for civil aircraft in the United States Federal Aviation Regulations specified that distances were to be in statute miles, and speeds in miles per hour. In 1969, these standards were progressively amended to specify that distances were to be in nautical miles, and speeds in knots. [10]

The following abbreviations are used to distinguish between various measurements of airspeed: [11]

The indicated airspeed is close to the true airspeed only at sea level in standard conditions and at low speeds. At 11000 m (36000 ft), an indicated airspeed of 300 kn may correspond to a true airspeed of 500 kn in standard conditions.

See also

Related Research Articles

<span class="mw-page-title-main">Minute and second of arc</span> Units for measuring angles

A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, or complete rotation, one arcminute is 1/21600 of a turn. The nautical mile (nmi) was originally defined as the arc length of a minute of latitude on a spherical Earth, so the actual Earth circumference is very near 21600 nmi. A minute of arc is π/10800 of a radian.

<span class="mw-page-title-main">Mile</span> Unit of length

The mile, sometimes the international mile or statute mile to distinguish it from other miles, is a British imperial unit and United States customary unit of distance; both are based on the older English unit of length equal to 5,280 English feet, or 1,760 yards. The statute mile was standardised between the Commonwealth of Nations and the United States by an international agreement in 1959, when it was formally redefined with respect to SI units as exactly 1,609.344 metres.

A nautical mile is a unit of length used in air, marine, and space navigation, and for the definition of territorial waters. Historically, it was defined as the meridian arc length corresponding to one minute of latitude at the equator, such that Earth's polar circumference is very near to 21,600 nautical miles. Today the international nautical mile is defined as 1,852 metres. The derived unit of speed is the knot, one nautical mile per hour.

<span class="mw-page-title-main">Flight instruments</span> Instruments in an aircrafts cockpit which provide the pilot with crucial information during flight

Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with data about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading and much more other crucial information in flight. They improve safety by allowing the pilot to fly the aircraft in level flight, and make turns, without a reference outside the aircraft such as the horizon. Visual flight rules (VFR) require an airspeed indicator, an altimeter, and a compass or other suitable magnetic direction indicator. Instrument flight rules (IFR) additionally require a gyroscopic pitch-bank, direction and rate of turn indicator, plus a slip-skid indicator, adjustable altimeter, and a clock. Flight into instrument meteorological conditions (IMC) require radio navigation instruments for precise takeoffs and landings.

<span class="mw-page-title-main">Wind speed</span> Rate at which air moves from high- to low-pressure areas

In meteorology, wind speed, or wind flow speed, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Wind speed is now commonly measured with an anemometer.

<span class="mw-page-title-main">Nautical chart</span> Topographic map of a maritime area and adjacent coastal regions

A nautical chart or hydrographic chart is a graphic representation of a sea region or water body and adjacent coasts or banks. Depending on the scale of the chart, it may show depths of water (bathymetry) and heights of land (topography), natural features of the seabed, details of the coastline, navigational hazards, locations of natural and human-made aids to navigation, information on tides and currents, local details of the Earth's magnetic field, and human-made structures such as harbours, buildings, and bridges. Nautical charts are essential tools for marine navigation; many countries require vessels, especially commercial ships, to carry them. Nautical charting may take the form of charts printed on paper or computerized electronic navigational charts. Recent technologies have made available paper charts which are printed "on demand" with cartographic data that has been downloaded to the commercial printing company as recently as the night before printing. With each daily download, critical data such as Local Notices to Mariners are added to the on-demand chart files so that these charts are up to date at the time of printing.

<span class="mw-page-title-main">Airspeed indicator</span> Flight instrument

The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometres per hour (km/h), knots (kn), miles per hour (MPH) and/or metres per second (m/s). The recommendation by ICAO is to use km/h, however knots is currently the most used unit. The ASI measures the pressure differential between static pressure from the static port, and total pressure from the pitot tube. This difference in pressure is registered with the ASI pointer on the face of the instrument.

<span class="mw-page-title-main">Airspeed</span> Speed of an aircraft relative to the surrounding air

In aviation, airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are:

<span class="mw-page-title-main">True airspeed</span> Speed of an aircraft relative to the air mass through which it is flying

The true airspeed of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Traditionally it is measured using an analogue TAS indicator, but as the Global Positioning System has become available for civilian use, the importance of such air-measuring instruments has decreased. Since indicated, as opposed to true, airspeed is a better indicator of margin above the stall, true airspeed is not used for controlling the aircraft; for these purposes the indicated airspeed – IAS or KIAS – is used. However, since indicated airspeed only shows true speed through the air at standard sea level pressure and temperature, a TAS meter is necessary for navigation purposes at cruising altitude in less dense air. The IAS meter reads very nearly the TAS at lower altitude and at lower speed. On jet airliners the TAS meter is usually hidden at speeds below 200 knots (370 km/h). Neither provides for accurate speed over the ground, since surface winds or winds aloft are not taken into account.

<span class="mw-page-title-main">Indicated airspeed</span> Displayed on the airspeed indicator on an aircraft

Indicated airspeed (IAS) is the airspeed of an aircraft as measured by its pitot-static system and displayed by the airspeed indicator (ASI). This is the pilots' primary airspeed reference.

<span class="mw-page-title-main">Metrication in the United States</span> Adoption of the metric system in the US

Metrication is the process of introducing the International System of Units, also known as SI units or the metric system, to replace a jurisdiction's traditional measuring units. U.S. customary units have been defined in terms of metric units since the 19th century, and the SI has been the "preferred system of weights and measures for United States trade and commerce" since 1975 according to United States law. However, conversion was not mandatory and many industries chose not to convert, and U.S. customary units remain in common use in many industries as well as in governmental use. There is government policy and metric (SI) program to implement and assist with metrification, however there is major social resistance for further metrication.

Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error.

<span class="mw-page-title-main">United Airlines Flight 585</span> 1991 aviation accident in Colorado

United Airlines Flight 585 was a scheduled passenger flight on March 3, 1991, from Denver to Colorado Springs, Colorado, carrying 20 passengers and 5 crew members on board. The plane experienced a rudder hardover while on final approach to runway 35 at Colorado Springs Municipal Airport, causing the plane to roll over and enter an uncontrolled dive. All 25 people on board were killed.

Aircraft maneuvering is referenced to a standard rate turn, also known as a rate one turn (ROT).

<span class="mw-page-title-main">Flight plan</span> Document filed by a pilot or flight dispatcher indicating the aircrafts flight path

Flight plans are documents filed by a pilot or flight dispatcher with the local Air Navigation Service Provider prior to departure which indicate the plane's planned route or flight path. Flight plan format is specified in ICAO Doc 4444. They generally include basic information such as departure and arrival points, estimated time en route, alternate airports in case of bad weather, type of flight, the pilot's information, number of people on board, and information about the aircraft itself. In most countries, flight plans are required for flights under IFR, but may be optional for flying VFR unless crossing international borders. Flight plans are highly recommended, especially when flying over inhospitable areas such as water, as they provide a way of alerting rescuers if the flight is overdue. In the United States and Canada, when an aircraft is crossing the Air Defense Identification Zone (ADIZ), either an IFR or a special type of VFR flight plan called a DVFR flight plan must be filed. For IFR flights, flight plans are used by air traffic control to initiate tracking and routing services. For VFR flights, their only purpose is to provide needed information should search and rescue operations be required, or for use by air traffic control when flying in a "Special Flight Rules Area."

<span class="mw-page-title-main">Instrument approach</span> Aircraft landing procedure

In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landing, or to a point from which a landing may be made visually. These approaches are approved in the European Union by EASA and the respective country authorities and in the United States by the FAA or the United States Department of Defense for the military. The ICAO defines an instrument approach as, "a series of predetermined maneuvers by reference to flight instruments with specific protection from obstacles from the initial approach fix, or where applicable, from the beginning of a defined arrival route to a point from which a landing can be completed and thereafter, if landing is not completed, to a position at which holding or en route obstacle clearance criteria apply."

<span class="mw-page-title-main">Rate of climb</span> Aircraft vertical velocity during flight

In aeronautics, the rate of climb (RoC) is an aircraft's vertical speed, that is the positive or negative rate of altitude change with respect to time. In most ICAO member countries, even in otherwise metric countries, this is usually expressed in feet per minute (ft/min); elsewhere, it is commonly expressed in metres per second (m/s). The RoC in an aircraft is indicated with a vertical speed indicator (VSI) or instantaneous vertical speed indicator (IVSI).

<span class="mw-page-title-main">Flight planning</span>

Flight planning is the process of producing a flight plan to describe a proposed aircraft flight. It involves two safety-critical aspects: fuel calculation, to ensure that the aircraft can safely reach the destination, and compliance with air traffic control requirements, to minimise the risk of midair collision. In addition, flight planners normally wish to minimise flight cost through the appropriate choice of route, height, and speed, and by loading the minimum necessary fuel on board. Air Traffic Services (ATS) use the completed flight plan for separation of aircraft in air traffic management services, including tracking and finding lost aircraft, during search and rescue (SAR) missions.

<span class="mw-page-title-main">Machmeter</span> Flight instrument

A Machmeter is an aircraft pitot-static system flight instrument that shows the ratio of the true airspeed to the speed of sound, a dimensionless quantity called Mach number. This is shown on a Machmeter as a decimal fraction. An aircraft flying at the speed of sound is flying at a Mach number of one, expressed as Mach 1.

<span class="mw-page-title-main">Kilometres per hour</span> Unit of speed

The kilometre per hour is a unit of speed, expressing the number of kilometres travelled in one hour.


  1. Bartlett, Tim (July 2008) [2003]. RYA Navigation Handbook. Southampton: Royal Yachting Association.
  2. "ISO 80000-3:2006". International Organization for Standardization . Retrieved 20 July 2013.
  3. International Standards and Recommended Practices, Annex 5 to the Convention on International Civil Aviation, "Units of measurement to be Used in Air and Ground Operations", ICAO, 4th Edition, July 1979.
  4. 1 2 "Non-SI units accepted for use with the SI, and units based on fundamental constants". SI brochure (8th ed.). International Bureau of Weights and Measures. The knot is defined as one nautical mile per hour. There is no internationally agreed symbol, but the symbol kn is commonly used.
  5. Louis E. Barbow and Lewie V. Judson (1976). "Appendix 4 The international nautical mile" (PDF). Weights and Measures Standards of the United States, A brief history. NIST Physics Laboratory. Archived from the original (PDF) on 26 September 2007. Retrieved 2 August 2007.
  6. Jacob Abbott (1858). Rollo on the Atlantic. DeWolfe, Fiske, & Co., Publishers. Retrieved 30 November 2011.
  7. Kemp, Peter, ed. (1976). The Oxford Companion to Ships and the Sea . Oxford University Press. p.  454. ISBN   0-19-282084-2.
  8. E.g. BA Chart 73, Puerto de Huelva and Approaches, 2002
  9. Wilson, Alastair (22 July 2009). "'Knots an hour'". The Kipling Society . Retrieved 1 December 2012. Since the 1890s or thereabouts, it has been drummed into the young seaman that a knot is a unit of speed, namely, one nautical mile per hour; and that consequently only the uneducated speak of "knots per hour" or "knots an hour". It was therefore inevitable that Kipling's frequent use of this expression should grieve a number of seafaring readers, as the pages of the Kipling Journal testify.
  10. For example, Part 23 of the Federal Aviation Regulations, amendment 23–7, 14 September 1969
  11. "Abbreviations and symbols".