In any system of units, the units for some physical quantities must be specified through a measurement process. These measurements are the base quantities of the system, and their units are the base units of the system. The algebraic combinations of the base values can then be used to express all other physical quantities. Each of these physical quantities is then referred to as a derived quantity, with each unit being referred to as a derived unit.

The International Organization for Standardization recommends using seven base quantities, which form the International System of Quantities (ISQ). All the other physical quantities can be expressed as combinations of these seven base physical quantities. For example, in geometry, the concept of area is always calculated as the product of two lengths. Thus, area is a derived quantity that can be expressed in terms of SI base units using square meters. Meanwhile, density is defined as mass divided by volume, which is expressed as kilograms per cubic meter (kg/m³) in terms of SI base units. In summary, all physical quantities can be derived from the seven base quantities, and the units of all the physical quantities can be derived from the seven SI base units.

This text is adapted from Openstax University Physics Volume 1, Section 1.2 Units and Standards.