The recommended practical system of units of measurement is the International System of Units (Système International d'Unités), with the international abbreviation SI. The SI is defined by the SI Brochure published by the BIPM.

In a landmark decision, Member States voted on 16 November 2018 to revise the SI, changing the world's definition of the kilogram, the ampere, the kelvin and the mole. This decision, made at the 26th meeting of the General Conference on Weights and Measures (CGPM), means that from 20 May 2019 all SI units are defined in terms of constants that describe the natural world. This will assure the future stability of the SI and open the opportunity for the use of new technologies, including quantum technologies, to implement the definitions.

The seven defining constants of the SI are:

• caesium hyperfine frequency DeltanuCs;
• speed of light in vacuum c;
• Planck constant h;
• Elementary charge e;
• Boltzmann constant k;
• Avogadro constant NA; and
• Luminous efficacy of a defined visible radiation Kcd.

The SI was previously defined in terms of seven base units and derived units defined as products of powers of the base units. The seven base units were chosen for historical reasons, and were, by convention, regarded as dimensionally independent: the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela. This role for the base units continues in the present SI even though the SI itself is now defined in terms of the defining constants above.