Units
Fundamental SI
Supplementary SI
Other SI
SI Prefixes

Fundamental SI units

There are seven fundamental quantities which are assumed to be mutually independent, on which the International System of Units (Le Systeme International d'Unités) or the SI is founded. These units are listed below:
Base quantity 
Name 
Symbol 
length 
meter 
m 
mass 
kilogram 
kg 
time 
second 
s 
electric current 
ampere 
A 
temperature (thermodynamic) 
kelvin 
K 
amount of substance 
mole 
mol 
luminous intensity 
candela 
cd 
Meter (1983)  It is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second. (This statement also implicitly defines 299792458 ms^{1} as the exact speed of light in vacuum).
The standard Meter was used to be the meter bar made of iridiumplatinum alloy kept at the International Bureau of Weights and Measures (BIPM), Paris.
Kilogram (1901)  It is the unit of mass, equal to the mass of the international prototype of the kilogram. It is a standard artifact, made of platinumiridium alloy kept at the BIPM.
Second (1967)  It is the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium133 atom.
Ampere (1948)  It is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular crosssection, and place one meter apart in vacuum, would produce between these two conductors a force equal to 2 x 10^{7} newton per meter of length.
Kelvin (1967)  It is the fraction 1/273.16 of the thermodynamic temperature of the triplepoint of pure water. In other words, the triplepoint of pure water is defined to be 273.16 K.
Mole (1971)  It is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon12 at rest and at the ground state. The elementary entities may be atoms, molecules, ions, electrons etc.
Candela (1979)  It is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^{12} hertz and that has a radiant intensity of (1/683) watt per steradian.

