SI Base Units

The base units of the International System of Units, the units on which every other SI unit is built. Each is tied to one base quantity: the second to time, the metre to length, the kilogram to mass, the ampere to electric current, the kelvin to thermodynamic temperature, the mole to amount of substance and the candela to luminous intensity. Since 2019 each has been defined by fixing the exact numerical value of a defining constant of nature. A unit belongs here when the International Bureau of Weights and Measures lists it as a base unit of the SI. It excludes derived units, which are formed by combining base units, and units outside the SI such as the foot or the pound.