A ruler, depicting two customary units of length, the centimetre and the inch
unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use. 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.  
base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of seconds." 1⁄ 299792458 It is approximately equal to . Other units are derived from the metre by adding  prefixes from the table below:
For example, a
kilometre is . A slang term for the kilometre in the US and UK militaries is . klick 
Centimetre-gram-second system of units, the basic unit of length is the centimetre, or of a metre.
Other non-SI units are derived from decimal multiples of the metre.
Diagram of English length units and their integer relations to each other.
The basic unit of length in the imperial and U.S. customary systems is the
yard, defined as exactly by international treaty in 1959. 
Common imperial units and U.S. customary units of length include:
thou or mil ( of an inch) 1⁄ 1000
foot (12 inches, 0.3048 m)
yard (3 feet, 0.9144 m) (terrestrial)
mile (5280 feet, 1609.344 m) (land) league 3 miles (4,800 m)
In addition, the following are used by
fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m) nautical mile (one minute of arc of latitude = )
Aviators use feet for altitude worldwide (except in Russia and China) and nautical miles for distance.
Determination of the rod, using the length of the left foot of 16 randomly chosen people coming from church service
Surveyors in the United States continue to use:
chain (22 yards, or ) rod (also called pole or perch) (quarter of a chain, 5 yards, or ) 1⁄ 2
Astronomical measure uses:
In atomic physics, sub-atomic physics, and cosmology, the preferred unit of length is often related to a chosen fundamental physical constant, or combination thereof. This is often a characteristic radius or wavelength of a particle. Some common
natural units of length are included in this table:
Length, in metres
The classical electron radius
The Compton wavelength of the electron
The reduced Compton wavelength of the electron
The Compton wavelength (or reduced Compton wavelength) of any fundamental particle
Bohr radius of the hydrogen atom ( Atomic unit of length)
The reduced wavelength of hydrogen radiation
Stoney unit of length
Quantum chromodynamics (
QCD) unit of length
Natural units based on the electronvolt
Archaic units of distance include:
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:
Horse racing and other equestrian activities keep alive:
Cardarelli, François (2003). . Springer. Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins ISBN . 9781852336820
^ a b
Hinkelman, Edward G.; Sibylla Putzi (2005). Dictionary Of International Trade: Handbook Of The Global Trade Community. World Trade Press. p. 245. ISBN . 9781885073723
Judson, Lewis Van Hagen (1960). . U.S. Department of Commerce, National Bureau of Standards. pp. 3-4 Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents, Issue,233 . Retrieved 2012.
"17th General Conference on Weights and Measures (1983), Resolution 1" . Retrieved .
"MARINE CORPS JARGON" (PDF). hqmc.marines.mil . Retrieved 2017.
Rod Powers. "How Far is a 'Klick' in the Military?". About.com. Archived from the original on 17 February 2006 . Retrieved 2010.
Donald Fenna (26 October 2002). . Oxford University Press. pp. A dictionary of weights, measures, and units 130-1. ISBN 978-0-19-860522-5 . Retrieved 2012.
Cardarelli 2003, pp. 29–30
Moritz, H. (March 2000). "Geodetic Reference System 1980". Journal of Geodesy. 74 (1): 128-133. Bibcode: 2000JGeod..74..128.. doi: 10.1007/s001900050278. S2CID 195290884.
Battat, J. B. R.; Murphy, T. W.; Adelberger, E. G. (January 2009). "The Apache Point Observatory Lunar Laser-ranging Operation (APOLLO): Two Years of Millimeter-Precision Measurements of the Earth-Moon Range". Astronomical Society of the Pacific. 121 (875): 29-40. Bibcode: 2009PASP..121...29B. doi: . 10.1086/596748 JSTOR 10.1086/596748.
Geoff Brumfiel (14 Sep 2012). "The astronomical unit gets fixed: Earth-Sun distance changes from slippery equation to single number" . Retrieved 2012.
, International Astronomical Union The IAU and astronomical units , retrieved
^ a b c d
Quinn, T.J.; Leschiutta, S.; Tavella, P. (August 2000). . Amsterdam ; Washington, DC: IOS Press, 2001. Proceedings of the International School of Physics "Enrico Fermi". pp. 142-143. Recent advances in metrology and fundamental constants ISBN . 9781586031671
"Compton wavelength over 2 pi". The NIST Reference on Constants, Units, and Uncertainty. NIST . Retrieved 2012.
"Planck length". The NIST Reference on Constants, Units, and Uncertainty. NIST . Retrieved 2012.
Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan. ISBN . 9780312370268