Système international d’unités

The SI Units are the most commonly used units for measuring physical sizes.

https://www.bipm.org/en/measurement-units/prefixes.html https://www.bipm.org/en/measurement-units/

The seven defining constants of the SI are:

• the caesium hyperfine frequency DeltanuCs;
• 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.