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

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:
Planck length
Bohr radius

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:
Football field (generally around 110 metres, depending on the country)
Widths of a human hair (around 80 micrometres)