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A nautical mile is exactly 1,852 metres: 1.852 kilometres, about 1.15 statute miles or 6,076 feet. The figure is not arbitrary — the nautical mile was born measuring the Earth, and it corresponds to roughly one minute of arc of a meridian. The drawing above shows that 1,852 m radius over the Strait of Dover; drag the map to your own coast to see it to scale.
That makes the nautical mile 15 % longer than the statute mile you use on land (1,609.344 m). So when a ship or an aircraft talks about "miles", they are not the same miles as the ones on a road sign.
Because the unit was not built around the metre, but around the planet. A circle has 360 degrees and each degree is split into 60 minutes of arc: 21,600 minutes in all. Give each minute of a meridian one nautical mile and a full meridian comes to 21,600 nautical miles, that is 40,003 kilometres — while the real meridian measures about 40,008 km. A remarkably good approximation.
That choice has a huge practical advantage at sea: on a nautical chart, one minute of latitude is exactly one nautical mile. You pick up the distance with dividers, take it to the latitude scale in the margin and read it off directly. And a full degree of latitude is 60 nautical miles, or 111.12 km.
Until 1929 there was no single nautical mile: every country used its own. Britain sailed with the Admiralty mile of 6,080 feet (1,853.184 m), and the United States worked with a slightly different value. The First International Extraordinary Hydrographic Conference, held in Monaco in 1929, settled it by fixing the international nautical mile at exactly 1,852 metres. The United States adopted the definition in 1954 and the United Kingdom in 1970, retiring its Admiralty mile.
Why 1,852 and not the "true" value? Because there is no single true value: the Earth is not a perfect sphere but an ellipsoid flattened at the poles, so a minute of latitude runs from about 1,843 m at the equator to some 1,862 m near the poles. The 1,852 metres are the round compromise that stuck.
A knot is simply one nautical mile per hour: 1.852 km/h, about 1.15 mph or 0.514 metres per second (NOAA). Watch out for the classic mistake: there is no such thing as "knots per hour", because the hour is already baked into the knot.
The name comes from how speed was measured before electronics: the chip log. A wooden board tied to a line was thrown off the stern; the line carried knots every 47 feet 3 inches (about 14.4 m) and the crew counted how many ran out while a 28-second sandglass emptied. The knots counted were, directly, the nautical miles per hour.
| Speed | Equals | Typical example |
|---|---|---|
| 1 knot | 1.85 km/h (1.15 mph) | a very slow stroll |
| 6 knots | 11.1 km/h (6.9 mph) | a cruising sailboat |
| 20 knots | 37 km/h (23 mph) | a container ship |
| 30 knots | 55.6 km/h (35 mph) | a speedboat or a frigate |
| 480 knots | 889 km/h (552 mph) | an airliner at cruise |
| Unit | In metres | In nautical miles |
|---|---|---|
| Kilometre | 1,000 m | 0.54 nmi |
| Statute mile | 1,609.344 m | 0.87 nmi |
| Nautical mile | 1,852 m | 1 nmi |
The other way round: one nautical mile is 1.15 statute miles and 6,076 feet. It is abbreviated M at sea, NM in aviation and nmi elsewhere.
Click any distance to see it drawn to scale above:
| Nautical miles | Distance |
|---|---|
| 1 nautical mile | 1,852 m (1.852 km · 1.15 mi) |
| 2 nautical miles | 3.7 km (2.3 mi) |
| 5 nautical miles | 9.26 km (5.75 mi) |
| 10 nautical miles | 18.52 km (11.5 mi) |
| 12 nautical miles (territorial waters) | 22.2 km (13.8 mi) |
| 100 nautical miles | 185.2 km (115 mi) |
| 200 nautical miles (exclusive economic zone) | 370.4 km (230 mi) |
Beyond ships and aircraft, the nautical mile is the unit that draws maritime borders. The law of the sea (the United Nations Convention on the Law of the Sea) measures from the coast 12 nautical miles of territorial waters (about 22.2 km), 24 for the contiguous zone and 200 nautical miles of exclusive economic zone (370.4 km) — hence the famous "200-mile limit" of fishing grounds. For scale, the Strait of Dover under the map above is about 18 nautical miles across at its narrowest, so those 12 miles of territorial waters nearly meet in the middle.
If you want to measure an actual passage rather than a radius, trace it point by point with the measure-a-distance tool: it gives you the length of whatever route you draw on the map.
Want to keep playing with distances? See how far 10,000 steps is or open the distances tool and draw your own. And if areas are more your thing, there is the Hectareometer.