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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 system

  SI units

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

• metre and its multiples, such as "centimetre" or "kilometre"

  Non-SI units

Non-SI units of length include:

• fermi (fm) (= 1 femtometre in SI units)
• angstrom (Å) (= 100 picometres in SI units)
• micron (= 1 micrometre in SI units)
• Norwegian/Swedish mil (= 10,000 metres)

  Imperial/US units

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

• inch (2.54 cm)
• (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)

  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.

  Surveying

Surveyors in the United States continue to use:

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

  Astronomical

Astronomical measure uses:

• Earth radius (RE) (~6,370 km)
• astronomical unit (AU) (~150 gigametres)
• light year (ly) (~9.46 petametres)
• parsec (pc) (~30.8 petametres), including kiloparsec (kpc) and megaparsec (Mpc)

  Archaic units

Archaic units of distance include:

• cana
• cubit
• Rope
• league
• li (China)
• pace (the "double pace" of about 5 feet used in Ancient Rome)
• verst (Russia)

  Informal units

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)
• Widths 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 keeps alive:

• furlong = ~ 0.125 miles (201 m)
• horse length = ~ 8 feet (2.4 m)

Physics also uses:

• Planck length
• Bohr radius

  See also

• Systems of measurement
• Medieval weights and measures
• English unit
• Orders of magnitude (length)