The standards are defined by operational definitions.
- The SI unit of length is the metre. One metre is defined as the distance light travels in a vacuum in 1/299792458 second. This standard was adopted in 1983, when the speed of light in vacuum was defined to be precisely 299792458 m/s.
- The SI unit of time is the second. One second is defined as the time required for 9192631770 cycles of a hyperfine transition in cesium 133. This definition was adopted in 1967.
- The SI unit of mass is the kilogram. One kilogram is defined to be the mass of a specific cylinder of platinum-iridium alloy, kept at the International Bureau of Weights and Measures (near Paris).
The most widely used system of units in science is the SI units (Système international).