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 Base units

The SI base units are a choice of seven well-defined units which by convention are regarded as dimensionally independent:

 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. Practical realization Evolution of the definition CCL kilogram, kg The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. Practical realization The name "kilogram" Work at the BIPM CCM 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. Practical realization Secondary representations Work at the BIPM CCTF 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. Practical realization Work at the BIPM CCEM kelvin, K The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Practical realization History CCT mole, mol 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. 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. Practical realization Work at the BIPM CCQM 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. Practical realization CCPR

All other SI units can be derived from these, by multiplying together different powers of the base units.

 Summary On the future revision of the SI Brief history of the SI SI prefixes Base units