Length  

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:
 1889
 The International Bureau of Weights and Measures defines the metre as the distance between two marks on a platinumiridium 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 2p_{10} and 5d_{5} of the krypton86 (^{86}Kr) 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.
A  B  

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  10^{2}  10^{−1}  10^{−4}  10^{−7}  10^{−8}  10^{−10}  10^{−13}  
nanometre  nm  10^{3}  ^{ }1^{ }  10^{−3}  10^{−6}  10^{−7}  10^{−9}  10^{−12}  
micrometre  μm  10^{6}  10^{3}  ^{ }1^{ }  10^{−3}  10^{−4}  10^{−6}  10^{−9}  
millimetre  mm  10^{9}  10^{6}  10^{3}  ^{ }1^{ }  10^{−1}  10^{−3}  10^{−6}  
centimetre  cm  10^{10}  10^{7}  10^{4}  10^{1}  ^{ }1^{ }  10^{−2}  10^{−5}  
metre  m  10^{12}  10^{9}  10^{6}  10^{3}  10^{2}  ^{ }1^{ }  10^{3}  
kilometre  km  10^{15}  10^{12}  10^{9}  10^{6}  10^{5}  10^{3}  ^{ }1^{ }  

Imperial unitsEdit
One international yard is defined as 0.914 4 m
Astronomical unitsEdit
External linksEdit
ReferencesEdit
 ↑ Conférence Générale des Poids es Mesures, ed (1960). Compte rendu des séances de la 11^{e} Conférence générale des Poids es Mesures. Paris: GauthierVillars. pp. 85.
 ↑ Conférence Générale des Poids es Mesures, ed (1983). Compte rendu des séances de la 13^{e} Conférence générale des Poids es Mesures.
 ↑ 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.