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 U.S. customary units in the United States and metric units 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.   
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." It is approximately equal to . Other units are derived from the metre by adding  prefixes from the table below:
For example, a
kilometre is .
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)
line ( of an inch)
foot (12 inches, 0.3048 m)
yard (3 feet (0.91 m)) (terrestrial)
mile 5,280 feet (1,610 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, or )
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:
Double-decker bus (9.5-11 metres in length)
Football field (generally around 110 metres in length) Thickness of a human hair (around 80 micrometres)
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 .
Donald Fenna (26 October 2002). . Oxford University Press. pp. 130-1. A dictionary of weights, measures, and units 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.
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
"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