New search facility:
BIPM Home | Site map | Metrologia | KCDB | JCTLM-DB | Contact us
 > You are here: SI > base units
 SI base units Version française
 Summary Unit of length (metre) Unit of mass (kilogram) Unit of time (second) Unit of electric current (ampere) Unit of thermodynamic temperature (kelvin) Unit of amount of substance (mole) Unit of luminous intensity (candela)
 Direct access
There are seven base units of the SI:

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.
kilogram  kg
The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.
second  s
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
ampere  A
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.
kelvin  K
The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
mole  mol
 1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
candela  cd
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

 Summary Unit of length (metre) Unit of mass (kilogram) Unit of time (second) Unit of electric current (ampere) Unit of thermodynamic temperature (kelvin) Unit of amount of substance (mole) Unit of luminous intensity (candela)