< Units of Measurement
Length
Dimension L
Usual Symbol l
Coherent units
System Unit Symbol
SI metre m
CGS centimetre cm
Imperial foot ft


SI units

The base unit of length in the SI system is the metre. This has had several definitions over time:

1889
The International Bureau of Weights and Measures defines the metre as the distance between two marks on a platinum-iridium alloy bar (the International Prototype Metre). This prototype is kept in a vault in Sèvres, near Paris.
1960
The 11th General Conference on Weights and Measures[1] redefines the metre in terms of a specific number of wavelengths of light of a specific frequency in a vacuum. It is defined to be 1 650 763.73 wavelengths of the electromagnetic radiation emitted by an electron transitioning between the levels 2p10 and 5d5 of the krypton-86 (86Kr) atom in a vacuum. The metre is now defined in terms of reproducible physical effects, rather than a synthetic artifact.
1983
The 17th General Conference on Weights and Measures[2] redefines the metre in terms of the distance traversed by light (having a universally constant speed in a vacuum) in a specific time. It is defined as the distance traveled by a beam of light in a vacuum in 1/299 792 458 second. This is the current definition. This has the effect that the speed of light is defined to be exactly 299 792 458 m/s.
Conversion of metric units of length[3]
A B
UnitSymbol pm nm μm mm cm m km
picometre pm  1  10-3 10-6 10-9 10-10 10-12 10-15
ångström† Å 102 10−1 10−4 10−7 10−8 10−10 10−13
nanometre nm 103  1  10−3 10−6 10−7 10−9 10−12
micrometre μm 106 103  1  10−3 10−4 10−6 10−9
millimetre mm 109 106 103  1  10−1 10−3 10−6
centimetre cm 1010 107 104 101  1  10−2 10−5
metre m 1012 109 106 103 102  1  103
kilometre km 1015 1012 109 106 105 103  1 
  • 1 A unit = b B units (b given in the table)
  • † Ångström is accepted for use with, but not defined by the SI system. Its use is officially discouraged by the CIPM.

Imperial units

One international yard is defined as 0.914 4 m

Astronomical units

References

  1. ↑ Conférence Générale des Poids es Mesures, ed (1960). Compte rendu des séances de la 11e Conférence générale des Poids es Mesures. Paris: Gauthier-Villars. pp. 85.
  2. ↑ Conférence Générale des Poids es Mesures, ed (1983). Compte rendu des séances de la 13e Conférence générale des Poids es Mesures.
  3. ↑ After Diem, K., Lentner, C. (ed.), ed (1970). "Physics". Documenta Geigy Scientific Tables (7th ed. ed.). Basle Bold text: J.R. Geigy S.A.. pp. 200.
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