SI - Defining constants

The International System of Units (SI): Defining constants

The SI is defined in terms of a set of seven defining constants. The complete system of units can be derived from the fixed values of these defining constants, expressed in the units of the SI.

The seven defining constants are:

  • the caesium hyperfine frequency ΔνCs
  • the speed of light in vacuum c
  • the Planck constant h
  • the elementary charge e
  • the Boltzmann constant k
  • the Avogadro constant NA, and
  • the luminous efficacy of a defined visible radiation Kcd

It is by fixing the exact numerical value of each that the unit becomes defined, since the product of the numerical value and the unit must equal the value of the constant.

Their numerial values and the units they define are as follows:

Defining constant Numerical value Unit
ΔνCs 9 192 631 770 Hz
c 299 792 458 m s–1
h 6.626 070 15 x 10–34 J s
e 1.602 176 634 x 10–19 C
k 1.380 649 x 10–23 J K–1
NA 6.022 140 76 x 1023 mol–1
Kcd 683 lm W–1


The numerical values of the seven defining constants have no uncertainty.

The definitions of the base units specify the exact numerical value of each constant when its value is expressed in the corresponding SI unit.

All units of the SI can be written either through a defining constant itself, or through products or quotients of the defining constants.

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