| Reference | Knots (kn) | Feet per second (fps) |
|---|---|---|
| Walking pace | 2.69978 kn | 4.55672 fps |
| A 100 m sprint world record pace | 20.3024 kn | 34.2666 fps |
| A motorway speed limit | 70.1944 kn | 118.475 fps |
| A cruising airliner | 485.961 kn | 820.21 fps |
The knot is a unit of speed equal to one nautical mile per hour, or exactly 1.852 kilometres per hour. Its symbol is kn, and the older abbreviation kt remains common. It is the speed unit of shipping and aviation worldwide, used by every nation regardless of what it does on land.
The name is literal. Before mechanical logs, a ship's speed was measured with a chip log: a weighted board on a long line, thrown astern to sit still in the water while the line ran out. The line carried knots at fixed intervals, and a sandglass measured a fixed time, usually twenty-eight seconds. Counting the knots that passed through the hand in that interval gave the speed directly, and the count was recorded in the ship's log, which is where the word for that record comes from.
Its survival is not sentimental. The nautical mile is one minute of arc of latitude, so a speed in knots converts directly into change of latitude per hour on a chart. A vessel making ten knots due north advances ten minutes of latitude every hour, a calculation a navigator can do without a table.
Aviation adopted the same unit for the same reason, and airspeed indicators, air traffic control instructions and flight plans worldwide use knots. Meteorology uses it for wind aloft and in marine forecasts, and the barbs on a weather map's wind symbols each represent ten knots.
Wind at sea is described in the same unit, and the Beaufort scale is essentially a table of knots dressed as descriptions. Force 4, a moderate breeze, is 11 to 16 knots; force 8, a gale, is 34 to 40; and force 12, a hurricane, begins at 64. A shipping forecast that says severe gale force nine is telling a listener that the wind is between 41 and 47 knots without giving a number at all. Aviation keeps the unit for the same practical reason as shipping: an aircraft's speed through the air, its speed over the ground and the wind that separates the two are all quoted in knots, so a navigation triangle can be solved without converting anything.
One knot equals exactly 1.852 kilometres per hour, about 1.15078 miles per hour, or about 0.514444 metres per second.
The foot per second is a unit of speed equal to one foot travelled in one second. Its symbol is fps, sometimes written ft/s. It is the imperial and US customary counterpart of the metre per second, and it appears wherever a short-duration speed is measured in a country that works in feet.
Ballistics is its main home. Muzzle velocities are quoted in feet per second almost universally in English-language sources, so a rifle cartridge at 3000 feet per second or a subsonic pistol round below 1100 are standard descriptions among shooters. The threshold matters because the speed of sound at sea level is about 1125 feet per second, and a bullet that crosses it produces a sonic crack that suppressors cannot remove.
American engineering uses it for anything falling, flowing or rotating over short intervals. Acceleration due to gravity is 32.174 feet per second squared, a number as familiar in American textbooks as 9.81 metres per second squared is elsewhere. Fluid velocities in pipes, conveyor speeds and the descent rates on aircraft instruments follow the same convention, though aviation more often uses feet per minute for vertical speed.
Sport measurement in the United States uses it for pitch and swing speeds where a short flight makes miles per hour feel coarse, and film and animation borrow the abbreviation for a different quantity entirely, frames per second, which is a frequency rather than a speed.
Falling bodies gave the unit its place in the classroom. In imperial terms gravity accelerates an object at 32.174 feet per second every second, so a stone dropped from rest is moving at about 32 feet per second after one second and 64 after two, and the height fallen is sixteen feet in the first second. Those round-ish numbers were the staple of American and British physics teaching for a century, and they survive in rules of thumb: a fall from a first-floor window reaches roughly 25 feet per second, and the terminal velocity of a human body in air is about 175. Each of them describes a hazard in the unit the trade already uses.
One foot per second equals 0.3048 metres per second exactly, about 1.09728 kilometres per hour, or about 0.681818 miles per hour.