Ships and aircraft measure speed in knots, and their distances in nautical miles. Both units come from the practical problems of navigating by the stars and the sea, long before satellites.
The nautical mile: a minute of latitude
Navigators locate themselves by latitude and longitude, measured in degrees and minutes of arc. If a mile matches one minute of latitude, a distance on the chart can be read straight off the latitude scale at the chart’s edge, with no conversion. That was the original nautical mile: one sixtieth of a degree of latitude.
Because Earth is not a perfect sphere, a minute of latitude varies slightly from the equator to the poles, and different countries used slightly different values. In 1929 an international hydrographic conference adopted a single international nautical mile of exactly 1,852 meters. The United States adopted it in 1954 and the United Kingdom in 1970.
The knot: counting knots on a rope
At sea there is nothing to measure speed against. From the 1500s sailors used a chip log: a wooden board weighted to float upright, tied to a long line wound on a reel. Thrown over the stern, the board stayed roughly still in the water while the ship moved away, pulling line off the reel.
Knots were tied in the line at regular intervals, and a sailor counted how many ran out while a small sandglass emptied. The spacing and the glass were chosen so that the count gave the speed in nautical miles per hour. Hence the unit: one knot is one nautical mile per hour. Saying “knots per hour” is a common mistake; the “per hour” is already included.
Why knots are still used
Charts, navigation and air traffic control all work in latitude and longitude, so nautical miles and knots remain the natural units for ships and aircraft worldwide. Wind speeds in marine and aviation forecasts are given in knots too.
Converting knots
- 1 knot = 1.852 km/h exactly.
- 1 knot ≈ 1.1508 mph, so knots × 1.15 ≈ mph.
- 1 knot ≈ 0.5144 m/s, roughly half a meter per second.
- A container ship cruising at 20 knots is moving at about 23 mph (37 km/h).
Try it in the speed converter: enter 20 knots and watch the car on the road hold the same speed as the ship. For land distances in miles and kilometers, use the length converter.
Try it
- Length & distance converter — mm to miles, drawn on a dual ruler
- Speed converter — mph and km/h, felt on a moving road