Difference Between Fundamental Unit and Derived Unit

Specifying only a number to quantify a physical quantity is not sufficient; in fact it is absolutely incorrect and misleading, unless the number is presented along with corresponding unit. Every measurable physical quantity has certain unit. Although different standards (including international, national and local) follow different units for same physical quantity, one must be proportional to another by a constant factor. Now among all such units used in different standards, there exists only seven independent units and others are combinations of these seven units. Such seven units are called Fundamental Units (or Base Units); whereas all other units which are obtained from fundamental units are called Derived Units. Similarities and differences between fundamental unit and derived unit are discussed in the following sections.

Similarities between fundamental unit and derived unit

  • Both fundamental unit and derived unit are used to express physical quantities.
  • Both are parts of International System of Units (SI) and are standardized.

Differences between fundamental unit and derived unit

Fundamental Unit Derived Unit
Fundamental units are all those units which are independent of any other unit (including themselves). Derived units are all those units which are obtained by multiplying and/or dividing one or more fundamental units with or without introducing any other numerical factor.
Fundamental units cannot be broken down into elementary level; in fact, these are elementary unites. Derived units can be broken down to its elementary level (composed of fundamental units).
Fundamental units cannot be expressed in terms of derived units. Derived units can be expressed in terms of fundamental units.
Only seven fundamental units exist in SI system. A large number of derived units are available.
Examples of fundamental units along with physical properties are:

  • Length (Meter, m)
  • Mass (Kilogram, kg)
  • Time (Second, s)
  • Temperature (Kelvin, K)
  • Amount of substance (Mole, mol)
  • Electric current (Ampere, A)
  • Luminous intensity (Candela, cd)
Examples of derived units:

  • Velocity (m/s)
  • Acceleration (m2/s)
  • Momentum (kg-m/s)
  • Force (N)
  • Density (kg/m3)
  • Heat (J)
  • Energy (J)
  • Power (W), etc.