Knot (kn)

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.

Knot conversion table