Units of Measurement/Length

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

SI unitsEdit

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

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.
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.
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]
Unit Symbol 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 unitsEdit

One international yard is defined as 0.914 4 m

Astronomical unitsEdit

External linksEdit


  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.