# Unit of length

A **unit of length** is a way of measuring length or distance.

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

- metre and units that are developed from it, such as "centimetre" or "kilometre"

But there are a lot of units that do not fit in the SI-System:

- 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)

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

- inch (25.4 mm)
- mil (one thousandth of an inch, one thou)
- foot (0.3048 m, 12 inches)
- yard (0.9144 m)
- (terrestrial) mile (1609.344 m)

In addition, the following are used by mariners:

- fathom (for depth) (1.8288 m)
- nautical mile (1852 m)

Surveyors in the United States continue to use:

Horse racing keeps alive:

- furlong (~201 m)

Astronomical measure uses:

- astronomical unit (AU) (~149 gigametres)
- light year (ly) (~9.46 petametres)
- parsec (pc) (~30.8 petametres), including kiloparsec (kpc) and megaparsec (Mpc)

Physics also uses:

Old units of distance are:

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

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: