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Unit of length

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Unit of length

Krypton-86 lamp used for definition of Metre between 1960 and 1983.

Many different units of length, width, height, depth, and distance 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.[1][2][3]


  • Metric system 1
    • SI 1.1
    • Non-SI 1.2
  • Imperial/US 2
  • Marine 3
  • Aviation 4
  • Surveying 5
  • Science 6
    • Astronomy 6.1
    • Physics 6.2
  • Archaic 7
  • Informal 8
  • Other 9
  • See also 10
  • References 11
  • Further reading 12

Metric system


The 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 1/299,792,458 of a second."[4] It is approximately equal to 1.0936 yards. Other units are derived from the metre by adding prefixes from the table below:

Standard prefixes for the SI units of measure
Multiples Prefix name deca hecto kilo mega giga tera peta exa zetta yotta
Prefix symbol da h k M G T P E Z Y
Factor 100 101 102 103 106 109 1012 1015 1018 1021 1024
Fractions Prefix name deci centi milli micro nano pico femto atto zepto yocto
Prefix symbol d c m μ n p f a z y
Factor 100 10−1 10−2 10−3 10−6 10−9 10−12 10−15 10−18 10−21 10−24

For example, a kilometre is 1000 metres.


In the Centimetre–gram–second system of units, the basic unit of length is the centimetre, or 1/100 of a metre.

Non-SI units of length include:


Determination of the rod, using the length of the left foot of 16 randomly chosen people coming from church service.

The basic unit of length in the Imperial and U.S. customary systems is the yard, defined as exactly 0.9144 m by international treaty in 1959.[2][5]

Common Imperial units and U.S. customary units of length include:[6]

  • thou or mil (1/1000 of an inch)
  • line (1/12 of an inch)
  • inch (2.54 cm)
  • foot (12 inches, 0.3048 m)
  • yard (3 ft, 0.9144 m)
  • (terrestrial) mile (5280 ft, 1609.344 m)
  • (land) league (3 miles)


In addition, the following are used by sailors:

  • fathom (for depth; only in non-metric countries) (2 yards = 1.8288 m)
  • nautical mile (one minute of arc of latitude = 1852 m)


Aviators use feet (same as US) for altitude worldwide (except in Russia and China) and nautical miles for distance.


Surveyors in the United States continue to use:

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



Astronomical measure uses:

  • Earth radius (R) (≈6,371 km[7])
  • astronomical unit (AU, au or ua) (defined in 2012 as 149,597,870,700 m[8]) Approximately the distance between the Earth and Sun.
  • light-year (ly) (≈9,460,730,472,580.8 km) The distance that light travels in a vacuum in one Julian year.[9]
  • parsec (pc) (≈30,856,775,814,671.9 km or about 3.26156 ly)
  • Hubble length (13.8 billon light year/306593922 parsec)


  • Atomic unit of length (Bohr radius): a_0 \approx 0.529\;177\;210\;92 \times 10^{-10} \mbox{ m}[10]
  • Natural unit of length: \bar{\lambda}_\text{C} \approx 386.159\;268\;00 \times 10^{-15} \mbox{m}[11]
  • Planck length: \ell_\text{P} \approx 1.616\;199 \times 10^{-35} \mbox{ m}[12]


Archaic units of distance include:

See also English units of length.


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)
  • Thickness 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


Horse racing and other equestrian activities keep alive:

See also


  1. ^ Cardarelli, François (2003). Encyclopaedia of Scientific Units, Weights, and Measures: Their SI Equivalences and Origins. Springer.  
  2. ^ a b Hinkelman, Edward G.; Sibylla Putzi (2005). Dictionary Of International Trade: Handbook Of The Global Trade Community. World Trade Press. p. 245.  
  3. ^ Judson, Lewis Van Hagen (1960). Units of Weight and Measure (United States Customary and Metric): Definitions and Tables of Equivalents, Issue 233. U.S. Department of Commerce, National Bureau of Standards. pp. 3–4. Retrieved 16 October 2012. 
  4. ^ "17th General Conference on Weights and Measures (1983), Resolution 1.". Retrieved 2012-09-19. 
  5. ^ Donald Fenna (26 October 2002). A dictionary of weights, measures, and units. Oxford University Press. pp. 130–1.  
  6. ^ Cardarelli 2003, pp. 29–30
  7. ^ Moritz, H. (March 2000). "Geodetic Reference System 1980". Journal of Geodesy 74 (1): 128–133.  
  8. ^ Geoff Brumfiel (14 Sep 2012). "The astronomical unit gets fixed: Earth–Sun distance changes from slippery equation to single number.". Retrieved 14 Sep 2012. 
  9. ^ The IAU and astronomical units, International Astronomical Union, retrieved 2008-07-05 
  10. ^ "atomic unit of length". The NIST Reference on Constants, Units, and Uncertainty. NIST. Retrieved 15 October 2012. 
  11. ^ "natural unit of length". The NIST Reference on Constants, Units, and Uncertainty. NIST. Retrieved 15 October 2012. 
  12. ^ "Planck length". The NIST Reference on Constants, Units, and Uncertainty.  

Further reading

  • Whitelaw, Ian (2007). A Measure of All Things: The Story of Man and Measurement. Macmillan.  
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