# Nautical mile

A**nautical mile**is a unit of length. It is widely used around the world for maritime and aviation purposes.

The international nautical mile is defined as exactly 1,852 metres. This definition was adopted in 1929 by the International Extraordinary Hydrographic Conference, Monaco. The United States adopted it in 1954. Prior to the adoption of the international nautical mile, the nautical mile used by the US and the UK was 6080 feet, or 1853.184 metres.

A nautical mile is approximately a minute of arc along a great circle of the Earth and was formerly defined so. The earth is not a perfect sphere, so a minute of arc can be less than, or more than, a nautical mile by a few metres.

The approximation to the minute of arc is convenient for air and sea navigation. The length of a minute of arc of latitude is found on any nautical chart and can be taken to represent approximately one nautical mile on that particular chart.

The abbreviation "**nm**" is used. The same abbreviation, "nm", is also used to denote nanometre in the SI though little confusion is generated from this, as the contexts of use are very different. For example, the 'nm' abbreviation is typically seen in listings of aircraft flight range, but listed next to the range in kilometers (km). ([Example] of nm, km together- in performance section)

One knot is a unit of speed defined as one nautical mile per hour. It is therefore 1852 metres per hour exactly.

**See also:** conversion of units