Many different units of length 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.
List
| unit | conversation | notes |
| Planck length | 10-35 meters | Any thing that are shorter than this makes no physical sense. |
| yoctometer | 10−24m | |
| zeptometer | 10−21m | A very small unit to measure quarks. |
| attometer | 10−18 m | A larger unit to measure quarks, 1000 zm = 1 am. |
| femtometer | 10−15 m | Measuring nucleus. |
| picometer | 10−12 m | |
| nanometer | 10−9 m | |
| micrometer | 10−6 m | Size of living cells. |
| millimeter | 0.001 m | |
| centimeter | 0.01 m | |
| decimeter | 0.1 m | ten centimeter |
| yard | 0.9144 m | almost a meter |
| meter | 1 m | main unit of measurement |
| decameter | 10 m | large animal |
| hectometer | 100 m | |
| kilometer | 1000 m | average city length |
| miles | 1609 m | 1.609344000000865 km. |
| nautical miles | 1852m | 1.15078 mile. |
| megameter | 106 m | Planets size |
| gigameter | 109 m | Sun is 1.4 gigameters. |
| terameter | 1012 m | larger star like vy canis majoris (3 tm). |
| light day | 26 terameters | 25902068371200 meters |
| centiparsec | 0.30856776 petameters | |
| petameter | 1015 m | nebulaaaaas... |
| deciparsec | 0.1 parsec, 3.0856776 petameters. | |
| light year(ly) | 9.46 petemeters | 460730472580800 meters, woah! alot! |
| parsec | 30.856776 petameters, 3.3 ly. | |
| decaparsec | 308.56776 petameters | |
| exameter | 1018 m | measure larger nebula or galaxy. |
| hectoparsec | 3085.6776 petameters | |
| kiloparsec | 30856.776 petameters | one thousand parsec. |
| zettameter | 1021 m | for larger galaxy |
| megaparsec | 1 million parsec | |
| yottameter | 1024 m | length of clusters and super clusters. |
| gigaparsec | 1 billion parsec | the observable universe is only 14 gigaparsec. |
| teraparsec | 1 trillon parsec | |
| petaparsec | 1 quadrillion parsec | |
| exaparsec | 1 quintillion parsec | |
| zettaparsec | 1 sextillion parsec | |
| yottaparsec | 1 septillion parsec |
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:
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:
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)
- moctar (mc) (1 Moctar is equivalent to ten times the span of the largest galaxy in the universe)
Archaic units
Archaic units of distance include:
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: