Classification of Unit System

Necessity
All physical quantities are to be measured in terms of standard quantities.

Unit
A unit is defined as a standard or fixed quantity of one kind
used to measure other quantities of the same kind.

Classification
Fundamental units and derived units are the two classifications.

Fundamental units
Units of basic quantities of length, mass and time.

Derived units
Units which are derived from basic units and bear a
constant relationship with the fundamental units.E.g. area,
volume, pressure, force etc.