The International System of Units (SI) is structured based on the seven base units: the meter (m), the kilogram (kg), the second (s), the ampere (A), the kelvin (K), the mole (mol) and the candela (cd). These are clearly defined units for seven dimensionally independent quantities such as length, mass, time, electricity, thermodynamic temperature, amount of substance and luminous intensity. There are other units besides the base units, as shown in the figure below. They are called derived units, which are formed by a combination of the seven base units.

The number on the right shoulder of each base unit indicates its degree of uncertainty (k = 2) .