The International System of Units (SI)

The International System of Units (SI) is the system of units in which:

  • the Planck constant h is 6.626 070 15 × 10‑34 J s
  • the speed of light in vacuum c is 299 792 458 m s-1
  • the unperturbed ground state hyperfine transition frequency of the caesium 133 atom ΔνCs is 9 192 631 770 Hz
  • the elementary charge e is 1.602 176 634 × 10-19 C
  • the Boltzmann constant k is 1.380 649 × 10-23 J K-1
  • the Avogadro constant NA is 6.022 140 76 × 1023 mol-1
  • the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm W-1

where the hertz, joule, coulomb, lumen, and watt, with unit symbols Hz, J, C, lm, and W, respectively, are related to the units second, metre, kilogram, ampere, kelvin, mole, and candela, with unit symbols s, m, kg, A, K, mol, and cd, respectively, according to Hz = s-1, J = kg m2 s-2, C = s A, lm = cd m2 m-2 = cd sr, and W = kg m2 s-3.

These definitions specify the exact numerical value of each constant when its value is expressed in the corresponding SI unit. The defining constants have been chosen such that, when taken together, their units cover all of the units of the SI. In general, there is no one-to-one correspondence between the defining constants and the SI base units, except for the caesium frequency ΔνCs and the Avogadro constant NA.

Prior to 2019, the concept of base units and derived units was used to define the SI. These categories, although not essential in the SI, are maintained in view of their convenience and widespread use.