SI - Base units

The International System of Units (SI): Base units

Base quantity
Base unit
Name Typical symbol   Name Symbol
time t   second s
length l, x, r, etc.   metre m
mass m   kilogram kg
electric current I, i   ampere A
thermodynamic temperature T   kelvin K
amount of substance n   mole mol
luminous intensity Iv   candela cd

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


In the 2018 revision of the SI, the definitions of four of the SI base units – the kilogram, the ampere, the kelvin and the mole – were changed. Their new definitions are based on fixed numerical values of the Planck constant (h), the elementary charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively.

Further, the definitions of all seven base units of the SI are now uniformly expressed using the explicit-constant formulation. Specific mises en pratique have been drawn up to explain the realization of the definitions of each of the base units in a practical way.

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