The units of fundamental physical quantities are called fundamental units. They are length, mass and time. These units can neither be derived from one another nor can be resolved into any other units. They are independent of one another.

Units of physical quantities can be expressed in terms of fundamental units and such units are called derived units.

Unit of area can be an example for derived unit. If L is the length of square then L x L = L^{2} is its area. Similarly, the volume of a cube is L x L x L = L^{3} cubic area. Units of any physical quantity can be derived from its defining equation.