Maritime Glossary
Nautical Mile
Also known as: NM, nmi, sea mile
A unit of distance used at sea and in the air, equal to exactly 1,852 metres. It was originally defined as the length of one minute of arc of latitude along a meridian, which ties distance directly to a position on the globe.
Full Definition
The nautical mile is the standard measure of distance in navigation because it maps cleanly onto the coordinate system used to fix a ship's position. One minute of latitude equals one nautical mile, so a navigator reading a chart can step off distance against the latitude scale in the margin without any conversion. Because the Earth is not a perfect sphere, the true length of a minute of latitude varies slightly between the equator and the poles, so in 1929 the international community fixed the international nautical mile at exactly 1,852 metres to give everyone a single, unambiguous value.
The nautical mile is distinct from the statute (land) mile of 1,609 metres, which is why a speed or distance given for a ship or aircraft is always larger than the same figure would be on land. A nautical mile is about 1.15 statute miles or 2,025 yards. Divisions of the nautical mile (the cable, at one tenth of a nautical mile or about 185 metres) are used for short distances such as the range to a buoy or the length of a mooring.
The nautical mile underpins the knot, the unit of speed at sea: one knot is one nautical mile per hour. Together they let a mariner work the classic speed-distance-time triangle directly against chart coordinates. Every modern positioning and navigation system (GPS, ECDIS, radar and the paper chart) reports range and distance in nautical miles, so it remains the working unit of the bridge and the flight deck alike, recognised across every flag and jurisdiction.