# Unit of length

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Many different units of length, width, height, depth, and distance have been used around 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.[1][2][3]

## Metric system

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### SI

{{#invoke:main|main}} {{#invoke:see also|seealso}} The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second."[4] It is approximately equal to 1.0936 yards. Other units are derived from the metre by adding prefixes from the table below:

For example, a kilometre is 1000 metres.

### Non-SI

In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or 1/100 of a metre.

Non-SI units of length include:

## Imperial/US

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The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly 0.9144 m by international treaty in 1959.[2][5]

Common Imperial units and U.S. customary units of length include:[6]

• thou or mil (1/1000 of an inch)
• line (1/12 of an inch)
• inch (2.54 cm)
• foot (12 inches, 0.3048 m)
• yard (3 ft, 0.9144 m)
• (terrestrial) mile (5280 ft, 1609.344 m)
• (land) league (3 miles)

## Marine

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)

## Aviation

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

## Surveying

Surveyors in the United States continue to use:

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

## Science

### Astronomy

{{#invoke:main|main}} Astronomical measure uses:

## Archaic

{{#invoke:main|main}} Archaic units of distance include:

## Informal

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 metres in length)
• Football field (generally around 110 metres, depending on the country)
• Thickness of a human hair (around 80 micrometres)
• A beard-second is a unit created as a teaching concept. It is the distance that a beard grows in a second (about 5 nanometres)
• Smoot, a jocular unit of length created as part of an MIT fraternity prank

## Other

Horse racing and other equestrian activities keep alive:

## References

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