| Reference | Knots (kn) | Kilometers per hour (km/h) |
|---|---|---|
| Walking pace | 2.69978 kn | 5 km/h |
| A 100 m sprint world record pace | 20.3024 kn | 37.6 km/h |
| A motorway speed limit | 70.1944 kn | 130 km/h |
| A cruising airliner | 485.961 kn | 900 km/h |
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 kilometre per hour is a unit of speed equal to one kilometre travelled in one hour. Its symbol is km/h, and the forms kph and kmph appear informally although neither is standard. It is the speed unit almost every country in the world uses on its roads.
It is not an SI unit, since the SI prescribes the metre per second, but it is accepted for use alongside the SI and is far better suited to its purpose. Road speeds in kilometres per hour land in a comfortable range of two-digit and low three-digit whole numbers, from a thirty limit in a residential street to a hundred and thirty on a motorway, which is exactly the property a number on a sign needs.
The United States and the United Kingdom are the significant exceptions, both signing roads in miles per hour. The United Kingdom is the more unusual case, since it is otherwise largely metric: distances on road signs are in miles and yards while almost everything else sold in the country is metric, an inconsistency that has survived several attempts at reform.
Vehicle speedometers in most markets show kilometres per hour prominently with miles per hour in a smaller inner scale, a design that dates from cross-border driving in Europe and North America. Rail speeds, cycling averages and weather reports of wind speed in many countries all use the unit, and it is the default in almost every navigation application outside the two exceptions.
Road safety research has given the unit a set of figures that repeat in every campaign. Stopping distance grows with the square of speed, so a car travelling at 50 kilometres per hour needs roughly twice the braking distance of one at 35 and four times the energy has to be dissipated. The probability that a pedestrian struck by a car survives falls steeply between 30 and 50 kilometres per hour, which is why urban limits across Europe have been moving to 30. The same arithmetic explains why a small reduction in a motorway limit produces a large reduction in fuel used, since air resistance also rises with the square of speed.
One kilometre per hour equals about 0.277778 metres per second, about 0.621371 miles per hour, or about 0.539957 knots.