
The SI unit of length is the metre (m):
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Unit of length (metre)


The 1889 definition of the metre, based on the international prototype of platinumiridium, was replaced by the 11th CGPM (1960) using a definition based on the wavelength of krypton 86 radiation. This change was adopted in order to improve the accuracy with which the definition of the metre could be realized, the realization being achieved using an interferometer with a travelling microscope to measure the optical path difference as the fringes were counted. In turn, this was replaced in 1983 by the 17th CGPM (1983, Resolution 1) that specified the current definition, as follows:
The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
It follows that the speed of light in vacuum is exactly 299 792 458 metres per second, c_{0} = 299 792 458 m/s.
The original international prototype of the metre, which was sanctioned by the 1st CGPM in 1889, is still kept at the BIPM under conditions specified in 1889.


The symbol, c_{0} (or sometimes simply c), is the conventional symbol for the speed of light in vacuum.


Decisions relating to the base units of the SI

Length:

1st CGPM, 1889 
sanction of the prototype metre 

7th CGPM, 1927 
definition and use of the prototype metre 

11th CGPM, 1960 
redefinition of the metre in terms of krypton 86 radiation 

15th CGPM, 1975 
recommends value for the speed of light 

17th CGPM, 1983 
redefinition of the metre using the speed of light,
realization of the definition of the metre 

CIPM, 2002 
specifies the rules for the practical realization of the definition of the metre 

CIPM, 2003 
revision of the list of recommended radiations 

CIPM, 2005 
revision of the list of recommended radiations 

CIPM, 2007 
revision of the list of recommended radiations 

23rd CGPM, 2007 
revision of the mise en pratique of the definition of the metre and development of new optical frequency standards 

CIPM, 2009 
updates to the list of standard frequencies 

24th CGPM, 2011 
on the possible future revision of the International System of Units, the SI 

24th CGPM, 2011 
revision of the mise en pratique of the definition of the metre and development of new optical frequency standards 

CIPM, 2013 
updates to the list of standard frequencies 



