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)