A nautical mile is 1,852 metres and a knot is one nautical mile per hour. Here is where each comes from, how to convert them, and how to use them.
A nautical mile is exactly 1,852 metres, or 1.852 km. That is about 1.1508 statute (land) miles, or 6,076 feet. The unit was designed to fit the Earth: one nautical mile is roughly one minute of latitude, which is 1/60 of a degree. Because there are 360 degrees around the planet, a full circle works out to about 21,600 nautical miles, and a sailor can measure distance on a chart by reading it off the latitude scale at the side.
Because the Earth is slightly flattened, one minute of latitude is not exactly constant, ranging from about 1,843 m at the equator to about 1,862 m near the poles. In 1929 an international conference fixed the nautical mile at 1,852 m so everyone would use the same figure. The United States adopted it in 1954 and the United Kingdom in 1970.
| Nautical miles | Kilometres | Statute miles | Feet |
|---|---|---|---|
| 1 | 1.85 | 1.15 | 6,076 |
| 5 | 9.3 | 5.8 | 30,381 |
| 10 | 18.5 | 11.5 | 60,761 |
| 50 | 92.6 | 57.5 | 303,806 |
| 100 | 185.2 | 115.1 | 607,612 |
A knot is a speed of one nautical mile per hour, so there is no "knots per hour". Saying "20 knots" already includes the time. One knot is 1.852 km/h, 1.1508 mph or 0.5144 metres per second.
| Knots | km/h | mph | m/s |
|---|---|---|---|
| 1 | 1.9 | 1.2 | 0.51 |
| 5 | 9.3 | 5.8 | 2.57 |
| 10 | 18.5 | 11.5 | 5.14 |
| 15 | 27.8 | 17.3 | 7.72 |
| 20 | 37.0 | 23.0 | 10.29 |
| 30 | 55.6 | 34.5 | 15.43 |
| 50 | 92.6 | 57.5 | 25.72 |
The name comes from the chip log. Sailors tossed a wooden board on a line into the sea, with knots tied at regular spacing along the line, and counted how many knots ran out in a fixed time on a sand glass. The spacing was chosen so each knot counted equalled one nautical mile per hour.
Nautical miles and knots are the standard in shipping, sailing and aviation. Ships quote speed in knots, and charts, tides and navigation lights use nautical miles. Aircraft use knots for airspeed and wind, and pilots plan routes in nautical miles. Weather services give marine and aviation winds in knots.
The territorial sea runs 12 nautical miles from a country's coast, and the exclusive economic zone extends 200 nautical miles. A single "league" at sea is traditionally 3 nautical miles. Another old unit, the cable, is one tenth of a nautical mile (185.2 m) and is still used for short distances between ships.
| Situation | km/h | mph |
|---|---|---|
| Ferry crossing at 20 knots | 37.0 km/h | 23.0 mph |
| Airliner cruising at 450 knots | 833 km/h | 518 mph |
| Wind of 25 knots | 46.3 km/h | 28.8 mph |
| Sailing yacht at 6 knots | 11.1 km/h | 6.9 mph |
Crossing time. Time equals distance divided by speed. A ship covering 3,000 nautical miles at 15 knots takes 200 hours, which is about 8.3 days. In land units, 3,000 nautical miles is 5,556 km.
Arrival time. A boat 45 nautical miles from harbour at 9 knots arrives in 5 hours. If the current is against you at 2 knots, the speed over the ground is 7 knots and the trip takes about 6.4 hours.
Fuel. A powerboat burning 10 litres per hour at 20 knots uses 0.5 litres per nautical mile. A 120 nautical mile passage takes 6 hours and about 60 litres, before any reserve.
The Beaufort scale describes wind force in knots. Wind strengths announced in knots often sound lower than they feel in mph or km/h, so convert before you decide to go out.
| Force | Description | Knots |
|---|---|---|
| 6 | Strong breeze | 22 to 27 |
| 8 | Gale | 34 to 40 |
| 10 | Storm | 48 to 55 |
| 12 | Hurricane force | 64 and above |
A statute mile is 1,609.344 m and a nautical mile is 1,852 m, so a nautical mile is 15 percent longer. A speed of 100 mph is only 87 knots, and a distance of 100 statute miles is 87 nautical miles. In aviation, ground distance on a US sectional chart can use either, so check the legend before computing.
For related units, see imperial vs metric and US vs UK weights and measures.