Pilots and sailors measure distance in nautical miles and speed in knots because both are tied to the shape of the Earth. A nautical mile was originally one minute of latitude, a sixtieth of a degree, which makes it easy to measure distances on a chart and to navigate by latitude and longitude. A knot is simply one nautical mile per hour.
What a nautical mile is
Since 1929, by international agreement, a nautical mile has been exactly 1,852 meters, about 1.151 ordinary (statute) miles or 6,076 feet. That fixed length is the average length of one minute of latitude; the real length varies slightly over the Earth, because the planet isn't a perfect sphere. The United States adopted the international value in 1954.
The link to latitude is what makes it useful. Sixty nautical miles is one degree of latitude, anywhere in the world. A navigator can measure a distance on a chart with dividers against the latitude scale on its edge, without any conversion at all.
Where the knot comes from
From ships. Sailors measured speed with a chip log: a weighted piece of wood tied to a long line with knots at regular intervals, thrown off the stern. They counted how many knots ran out over the side while a sandglass ran, typically 28 or 30 seconds. With the knots spaced right, the count was the ship's speed in nautical miles an hour. The name stuck long after the rope disappeared.
Why aviation kept them
- Navigation. Aviation inherited its navigation methods, charts and much of its vocabulary from the sea. Nautical miles fit latitude and longitude, and long-distance flying is navigation by latitude and longitude.
- Standardisation. ICAO, the UN's aviation agency, sets knots and nautical miles as the standard for speed and distance, so pilots everywhere share units. Altitude is in feet in most of the world, too, although a few countries use meters.
- Winds. Weather reports for pilots give wind in knots, so a plane's speed and the wind add up directly.
Quick conversions
| One of these | Is |
|---|---|
| Nautical mile | 1.852 km · 1.151 miles · 6,076 feet |
| Knot | 1.852 km/h · 1.151 mph · 0.514 m/s |
| Mile | 0.869 nautical miles |
| Kilometer | 0.540 nautical miles |
For example, London to New York is 2,991 nautical miles on the great circle: 3,442 miles, or 5,540 km.
Airspeed, ground speed and Mach
Pilots juggle several speeds. Indicated airspeed is what the instruments read, which drops below the true airspeed as the air thins with height. True airspeed is how fast the plane moves through the air; add the wind and you get ground speed, which decides when you land. Up high, airliners mostly fly by Mach number, their speed as a fraction of the speed of sound. A typical cruise is around Mach 0.8, or roughly 460 to 490 knots of true airspeed. Our post on why planes cruise at 35,000 feet explains why the speed of sound changes with height.
Convert it yourself
The knots converter handles speeds, with Mach at any altitude, and the nautical miles converter handles distances. In Flight, every focus flight crosses real nautical miles, from your airport to anywhere.