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Many different units of length have been used across the world. The main units in modern use are U.S. customary units in the United States and the Metric system elsewhere. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.

Metric systemEdit

Main article: Metric system

SI unitsEdit

Main article: International System of Units

Common units of length in the International System of Units (SI) are:

Non-SI unitsEdit

Non-SI units of length include:

Imperial/US unitsEdit

Main article: Imperial unit
Main article: U.S. customary units

Common Imperial units and U.S. customary units of length include:

  • inch (2.54 cm)
  • mil (one thousandth of an inch, one thou)
  • foot (12 inches, 0.3048 m)
  • yard (3 ft, 0.9144 m)
  • (terrestrial) mile (5280 ft, 1609.344 m)


In addition, the following are used by mariners:

  • fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m)
  • nautical mile (one minute of arc of latitude = 1852 m)


Aviators use feet (same as US) for altitude worldwide except in Russia and China.


Surveyors in the United States continue to use:

  • chain (~20.1m)
  • rod (also called pole or perch) (~5 m)


Main article: Astronomical system of units

Astronomical measure uses:

Archaic unitsEdit

Main article: Ancient weights and measures

Archaic units of distance include:

Informal unitsEdit

In everyday conversation, and in informal literature, it is common to see lengths measured in units of objects of which everyone knows the approximate width. Common examples are:

  • Double-decker bus (9.5–10.9 meters in length)
  • Football field (generally around 110 meters, depending on the country)
  • Widths of a human hair (around 80 micrometers)
  • A beard-second is a unit created as a teaching concept. It is the distance that a beard grows in a second (about 5 nanometers)
  • Smoot, a jocular unit of length created as part of an MIT fraternity prank


Horse racing and other equestrian activities keeps alive:

Physics also uses:

See alsoEdit

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