What Is a Nautical Mile?
A nautical mile (symbols NM, nmi or M) is a unit of length used in marine and air navigation, equal to exactly 1,852 meters. That value was agreed at the International Extraordinary Hydrographic Conference in Monaco in 1929, the meeting that led to today’s International Hydrographic Organization. The United States adopted the international nautical mile on July 1, 1954, and the United Kingdom replaced its own Admiralty mile of 6,080 feet in 1970.
The unit comes from the geometry of the globe. Navigators divide each degree of latitude into 60 minutes, and one minute of latitude along a meridian is about one nautical mile. Because the Earth bulges at the equator, a minute of latitude actually varies from about 1,843 m near the equator to about 1,862 m near the poles; 1,852 m is a convenient value in between. The nautical mile is not an SI unit, but NIST Special Publication 811 lists its exact value of 1,852 m.
What Is a Mile?
To sailors, the ordinary mile is the “statute mile” or “land mile”, named because an English statute of 1593 fixed it at 8 furlongs, or 5,280 feet. Its roots go back further, to the Roman mille passus of a thousand paces. In 1959 the International Yard and Pound Agreement defined it as exactly 1,609.344 m, which is the mile on every US and UK road sign.
Unlike the nautical mile, the statute mile has nothing to do with the shape of the Earth. It was built up from feet and furlongs, units of human and agricultural scale, which is why the two “miles” differ by about 15%.
Nautical vs Statute Miles
Ships and aircraft use nautical miles because they pair with latitude and longitude. On a paper chart, a navigator can measure a distance with dividers and read it straight off the latitude scale at the side, one minute per nautical mile. Speeds follow the same unit: one knot is one nautical mile per hour, or 1.151 mph. Longitude scales cannot be used for distance, because meridians converge and a minute of longitude shrinks toward the poles.
Many legal boundaries at sea are defined in nautical miles, so converting them shows how far they reach in familiar units.
| Distance | Nautical Miles (NM) | Miles (mi) |
|---|---|---|
| One minute of latitude (approx.) | 1 | 1.150779 |
| Old marine league (cannon-shot rule) | 3 | 3.452338 |
| Territorial sea limit | 12 | 13.80935 |
| Contiguous zone limit | 24 | 27.61871 |
| Exclusive economic zone limit | 200 | 230.1559 |
| Earth's circumference along a meridian (approx.) | 21,600 | 24,856.84 |
The circumference figure shows the original idea neatly: 360 degrees × 60 minutes = 21,600 minutes of arc, so a trip around the planet through both poles is about 21,600 nautical miles, or roughly 24,857 statute miles. The measured meridian circumference of about 40,008 km is within a few nautical miles of that.
The knot has a nautical history of its own. From the sixteenth century, sailors measured speed with a chip log: a weighted board on a line with knots tied at regular intervals. They threw it overboard and counted how many knots ran out while a sandglass emptied, usually in about 28 seconds, with the knots spaced so that the count equaled the speed in nautical miles per hour. The word stuck long after GPS replaced the log line.
Aviation uses nautical miles for route distances and visibility and knots for airspeed, though altitude is in feet. Mixing units is a classic source of planning errors: a 300 NM leg is 345 statute miles, and fuel planned for 300 statute miles would leave the aircraft about 13% short. Long-haul routes show the gap even more clearly: a transatlantic flight of roughly 3,000 nautical miles is about 3,452 statute miles, about 450 miles more than the nautical figure suggests to someone thinking in land miles.
How Many Nautical Miles in a Mile?
One statute mile is 0.868976 nautical miles, so to convert miles to nautical miles you divide by 1.150779 or multiply by 0.868976. A quick rule of thumb is to take off about 13%: 100 statute miles is 86.9 NM. In kilometers, one nautical mile is exactly 1.852 km.