The knot, unit of speed
One knot is exactly 1.852 kilometers per hour, that is one nautical mile per hour. It is the speed unit of shipping and aviation the world over, and it owes nothing to habit: the nautical mile is one minute of latitude, so a speed in knots reads straight off a chart.
The name comes from an instrument. Until the nineteenth century speed was measured with a chip log: a weighted board thrown overboard, held by a line with knots tied every 47 feet 3 inches, that is 14.4018 meters. The line was paid out for 28 seconds timed by a sandglass, and the knots running through the fingers were counted. The arithmetic lands right: 14.4018 meters in 28 seconds is 1,851.7 meters per hour, within two parts in ten thousand of the modern nautical mile. That mile was fixed at exactly 1,852 meters by the First International Extraordinary Hydrographic Conference in Monaco in 1929; the United States adopted it on 1 July 1954 and the United Kingdom in 1970, dropping its Admiralty mile of 6,080 feet, or 1,853.184 meters.
The knot in other units
| 1 kn | 1.852 km/h |
| 1 kn | 1.15078 mph |
| 1 kn | 0.514444 m/s |
| 1 kn | 1.68781 ft/s |
| 1 kn | 0.000514444 km/s |
| 1 kn | 0.000319661 mi/s |
Frequently asked questions
Is a knot a nautical mile per hour?
Yes, which is why "knots per hour" is a mistake: the hour is already in there. A ship making 12 knots covers 12 nautical miles in an hour, that is 22.2 kilometers.
Why do sea and air not count in km/h?
Because the nautical mile is one minute of latitude. On a chart, sixty miles separate two parallels one degree apart: a navigator reads a distance off the dividers with no conversion. The kilometer bears no relation to the shape of the Earth and would lose that property.
How fast does an airliner fly in knots?
Around 450 to 500 knots in cruise, that is 833 to 926 km/h. High-altitude winds add to or subtract from that: it is why Paris to New York takes an hour longer than the return leg, the jet stream blowing west to east at 100 knots and more.
How many kilometers is 20 knots?
None, and that is not a quibble: the knot is a speed, the kilometer a distance. Without a duration the conversion does not exist. Twenty knots covers 37 kilometers in an hour and 18.5 in half an hour. What the question is after is almost always kilometers per hour: 20 knots is 37.04 km/h.