Base Units

The International System of Units (SI) defines seven units as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:

  • metre for length

  • kilogram for mass

  • second for time

  • ampere for electric current

  • kelvin for temperature

  • candela for luminous intensity

  • mole for the amount of substance.

The SI base quantities form a set of mutually independent dimensions as required by dimensional analysis.

Metre, m


of the distance travelled by light in one second.

Kilogran, kg


Equal to the mass of the international prototype of the kilogram

Second, s


Duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of  caesium 133.

ampere, A

electric current

The current through each of two straight parallel conductors of infinite length, negligible cross-section, 1 metre apart in a vacuum, which would  a force equal tonewton per metre of length

Kelvin, K

Absolute temperature

Equal to the fraction 1/273.16 of the thermodynamic temperature of the triple point of water

Mole, mol

amount of substance

The amount of substance which contains as many elementary entities - atoms, molecules, ions, electrons, other particles, or specified groups of such particles - as atoms in 12g carbon 12

Candela, cd


The luminous intensity of a source that emits monochromatic radiation of frequencyhertz and that has a radiant intensity in that direction of 1/683 watt per steradia (solid angle).