In the revised SI all units are defined in terms of a set of seven reference constants, to be known as the "defining constants of the SI", namely the caesium hyperfine splitting frequency, the speed of light in vacuum, the Planck constant, the elementary charge (i.e. the charge on a proton), the Boltzmann constant, the Avogadro constant, and the luminous efficacy of a specified monochromatic source.
This results in a simpler and more fundamental definition of the entire SI, and dispenses with the last of the definitions based on a material artefact – the international prototype of the kilogram.
Resolution 1 (2018): On the revision of the International System of Units (SI)
On 16 November 2018 the CGPM at its 26th meeting adopted a revision of the SI in which:
- the unperturbed ground state hyperfine transition frequency of the caesium 133 atom Cs is 9 192 631 770 Hz,
- the speed of light in vacuum c is 299 792 458 m/s,
- the Planck constant h is 6.626 070 15 × 1034 J s,
- the elementary charge e is 1.602 176 634 × 1019 C,
- the Boltzmann constant k is 1.380 649 × 1023 J/K,
- the Avogadro constant NA is 6.022 140 76 × 1023 mol1,
- the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, is 683 lm/W,
- 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 = s1, J = kg m2 s2, C = s A, lm = cd m2 m2 = cd sr, and W = kg m2 s3,
- the numerical values of h, e, k, and NA are based on the most recent CODATA adjustment.
The SI may alternatively be defined by statements that explicitly define seven individual base units: the second, metre, kilogram, ampere, kelvin, mole, and candela. These correspond to the seven base quantities time, length, mass, electric current, thermodynamic temperature, amount of substance, and luminous intensity. All other units are then obtained as products of powers of the seven base units, which involve no numerical factors; these are called coherent derived units.
For full details the reader is referred to the formal texts of the adopted Resolutions: