| Reference | Knots (kn) | Miles per hour (mph) |
|---|---|---|
| Walking pace | 2.69978 kn | 3.10686 mph |
| A 100 m sprint world record pace | 20.3024 kn | 23.3636 mph |
| A motorway speed limit | 70.1944 kn | 80.7783 mph |
| A cruising airliner | 485.961 kn | 559.234 mph |
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 mile per hour is a unit of speed equal to one mile travelled in one hour. Its symbol is mph. It is the road speed unit of the United States and the United Kingdom, and of a small number of territories connected to them, and it is used almost nowhere else.
Britain's position is the odd one. The country converted its shops, its schools and its industry to metric units over the second half of the twentieth century, but left road signs alone. Distances appear in miles and yards, speed limits in miles per hour, and a plan to change them has been announced and abandoned more than once, largely because replacing every sign in the country is expensive and the political return is small.
The number sixty is what makes the unit intuitive to those who grew up with it. At sixty miles per hour a vehicle covers a mile a minute, which turns journey planning into simple mental arithmetic. Speed limits cluster around convenient multiples: thirty in towns, seventy on British motorways, sixty-five or seventy on most American interstates.
Sport and weather keep it alive beyond the road. Baseball pitch speeds, tennis serves and cricket deliveries are quoted in miles per hour in English-language coverage worldwide, and hurricane wind speeds in the Atlantic basin are reported the same way, with the Saffir-Simpson categories defined by thresholds in miles per hour.
Records set in the unit have become cultural landmarks in their own right. The four-minute mile is a pace of fifteen miles per hour, the sound barrier at sea level lies near 767, and the land speed record has stood above 700 since 1997. Everyday driving figures carry the same weight of familiarity: 30 miles per hour in a built-up area, 70 on a motorway, and the observation that a car doing 60 covers 88 feet in a second, which is the number American driving instructors use to explain following distances. Because the unit rarely appears in physics, almost all of its landmarks come from the road, the track and the runway.
One mile per hour equals about 1.609344 kilometres per hour, about 0.44704 metres per second, or about 0.868976 knots.