A nautical mile is a unit of measurement defined as 1,852 metres
(6,076.1 ft; 1.1508 mi). Historically, it was defined as one
minute of latitude, which is equivalent to one sixtieth of a degree of
latitude. Today, it is a non-SI unit "accepted for use with the
SI", for its continued use in both air and marine navigation,
and for the definition of territorial waters.
One tenth of a nautical mile is a cable length.
The derived unit of speed is the knot, defined as one nautical mile
per hour. The geographical mile is the length of one minute of
longitude along the Equator, about 1,855 m on the WGS 84
1 Unit symbol
3 See also
There is no internationally agreed symbol.
M is used as the abbreviation for the nautical mile by the
International Hydrographic Organization and by the International
Bureau of Weights and Measures.
NM is used by the International Civil Aviation Organization.
nm (the SI symbol for the nanometre) is used by the U.S. National
Oceanic and Atmospheric Administration.
nmi is used by the Institute of Electrical and Electronics
Engineers and the United States Government Publishing Office.
While using M itself, the International Bureau of Weights and Measures
recognises that NM, Nm and nmi are also in use.
The word mile is from the
Latin word for a thousand paces: mille
Navigation at sea was done by eye until around 1500 when
navigational instruments were developed and cartographers began using
a coordinate system with parallels of latitude and meridians of
longitude. In 1617 the Dutch scientist Willebrord Snell assessed the
circumference of the Earth at 24,630 Roman miles (24,024 statute
miles). Around that time British mathematician
Edmund Gunter improved
navigational tools including a new quadrant to determine latitude at
sea. He reasoned that the lines of latitude could be used as the basis
for a unit of measurement for distance and proposed the nautical mile
as one minute or one-sixtieth (1/60) of one degree of latitude. As one
degree is 1/360 of a circle, one minute of arc is 1/21600 of a circle
(or, in radians, π/10800). These sexagesimal (base 60) units
originated in Babylonian astronomy. Gunter used Snell's circumference
to define a nautical mile as 6,080 feet, the length of one minute of
arc at 48 degrees latitude. Since the earth is not a perfect sphere
but is an oblate spheroid with slightly flattened poles, a minute of
latitude is not constant, but about 1,861 metres at the poles and
1,843 metres at the Equator, with a mean value of 1,852.3 metres
(6,077 ft). Other countries measure the minute of arc at 45
degrees latitude, giving the nautical mile a length of 6076 ft
(approximately 1852 m).
In 1929, the international nautical mile was defined by the First
International Extraordinary Hydrographic Conference in
precisely 1,852 metres.
Imperial units and
United States customary units
United States customary units used a definition of
the nautical mile based on the Clarke (1866) Spheroid. The United
States nautical mile was defined as 6,080.20 feet (1,853.24 m)
based in the
Mendenhall Order foot of 1893. It was abandoned in favour
of the international nautical mile in 1954.
The Imperial nautical mile, often called an Admiralty mile, or more
correctly, an Admiralty measured mile, was defined by its relation to
the Admiralty knot, 6,080 imperial feet per hour, so 1 imperial
nautical mile is about 1,853.181 metres. It was abandoned in 1970
and, legally, references to the obsolete unit are now converted to
Despite the existence of modern precise definitions, in the early 21st
century the old definitions are still in use. The Royal Yachting
Association says in its manual for day skippers: "1 (minute) of
Latitude = 1 sea mile", followed by "For most practical purposes
distance is measured from the latitude scale, assuming that one minute
of latitude equals one nautical mile".
Conversion of units
Orders of magnitude (length)
^ a b c d e f g Göbel, E.; Mills, I.M.; Wallard, Andrew, eds. (2006).
International System of Units
International System of Units (SI) (PDF) (in English from French
original) (8th ed.). Paris: Bureau International des Poids et Mesures.
p. 127. ISBN 92-822-2213-6. Retrieved 2017-06-20. CS1
maint: Unrecognized language (link)
^ "mile unit of measurement". Encyclopædia Britannica. Retrieved
^ "UNITED NATIONS CONVENTION ON THE LAW OF THE SEA". www.un.org.
^ Fenna, Donald (2002), "cable, cable length, cable's length", A
Dictionary of Weights, Measures, and Units, Oxford: Oxford University
Press, pp. 35, ISBN 0-19-860522-6, OCLC 62608533,
retrieved 12 January 2017 . Also "fathom", from the same work
(pp. 88–89, retrieved 12 January 2017).
^ Symbols, Abbreviations and Terms used on Charts (PDF) (6 ed.).
International Hydrographic Organization. 2016. Retrieved
^ "WS SIGMET Quick Reference Guide" (PDF). ICAO. ICAO. Retrieved
^ "Law of the Sea". NOAA. Retrieved 2016-06-09.
^ "APPENDIX A: SYMBOLS AND PREFIXES". IEEE. Retrieved
^ "U.S. Government Printing Office Style Manual". U.S. Government
Printing Office. Retrieved 2016-06-10.
^ a b "Mile, Nautical and Statute – FREE Mile, Nautical and Statute
information Encyclopedia.com: Find Mile, Nautical and Statute
research". www.encyclopedia.com. Retrieved 2016-06-10.
^ "REFINEMENT OF VALUES FOR THE YARD AND THE POUND" (PDF). NOAA.
National Bureau of Standards. Retrieved 2016-06-10.
^ "imperial nautical mile". sizes.com. Retrieved 2016-06-10.
^ "The Units of Measurement Regulations 1995". www.legislation.gov.uk.
^ Hopkinson, Sara (2012). RYA day skipper handbook - sail. Hamble: The
Royal Yachting Association. p. 76.