A nautical mile or sea mile is a unit of length. It is a non-SI unit used by special interest groups such as navigators in the shipping and aviation industries. It is commonly used in international law and treaties, especially regarding the limits of territorial waters. It developed from the geographical mile. Since the nautical mile is roughly equal to one minute of angle at the equator, the length of the equator is roughly 21,600 nautical miles (40 000 km).

Definition
There is no official international standard symbol for the unit nautical mile. The symbols M, NM, nm, and nmi are commonly used in some areas.
The International Bureau of Weights and Measures (BIPM) in its International System of Units (SI) brochure lists the nautical mile in the table of units "currently accepted" for use with SI using M as a symbol, mentioning in a footnote other symbols, and stating that as yet there is no internationally agreed symbol.
The abbreviation nm can be confused with the official SI symbol for the nanometre. Similarly the SI symbol N m is a Newton metre.
There are several national unit symbols in use (for example, mpk (meripeninkulma, "sea league") in Finnish. In German, sm is used for Seemeile, "sea mile", though this abbreviation generates confusion in translations to English with the term for the other type of mile, the statute mile. In Icelandic, M is used for sjómíla, "sea mile". The People's Republic of China uses n mile as the national standard symbol with no s added for plural.

Unit symbol
One nautical mile converts to:

1852 metres (exact)
1.150779 mile (statute) [1] (exact: 57875/50292 miles)
2025.372 yard (exact: 2315000/1143 yards)
6076.1155 feet (exact: 2315000/381 feet)
1012.6859 fathoms (exact: 1157500/1143 fathoms)
10 common-definition cables (exact, as one common definition of "cable")
10.126859 "ordinary" (100-fathom) cables (exact: 11575/1143 ordinary cables)
12.152231 US Navy (120-fathom) cables (exact: 9260/762 US Navy cables)
0.998383 equatorial arc minutes = traditional geographical miles (approx.)
0.9998834 mean meridian arc minutes = mean historical nautical miles (approx.) Conversions to other units
The nautical mile was historically defined as a minute of arc along a meridian of the Earth, making a meridian exactly 180×60 = 10 800 historical nautical miles. It can therefore be used for approximate measures on a meridian as change of latitude on a nautical chart. The originally intended definition of the metre as 10
The British definition of the nautical mile originally related to the length on the surface of the Earth just south of Great Britain. It was not specified according to a calibrated measurement of the Earth, but chosen as exactly 800 feet longer than a statute mile, namely 6080 feet. For disambiguation, this is sometimes called the "admiralty mile" after the British Admiralty. The precise definition of the foot varied slightly around the world until the international yard, always equal to exactly three feet, was standardized at exactly 0.9144 m in 1959, making the admiralty mile exactly 1853.184 m. The Royal Hydrographic Office of the United Kingdom converted to the international definition in 1970.
As a simpler approximation, designers of radar systems for ballistic and cruise missiles for use by the United States Navy in the 1950s would take 6000 feet (1829 m) as their equivalent of a nautical mile. In the past, some ship-borne computer systems developed for the Royal Navy also used the "data mile" of 6000 feet, and the more unusual "foot*", equivalent to about nine inches, defined as 6000/8192 feet (223 mm).

See also