# Derived Units: Similarities and differences with fundamental units

### What are Derived Units?

Derived units are the units of derived quantities. They are dependent on fundamental quantities and are expressed from the combination of two or more fundamental units.

As the name implies, derived units are always derived (from fundamental units). For instance, velocity is a quantity whose unit is derived from length and time (both of which are fundamental quantities) as summarized in the paragraph below.

• Length (which could also mean distance) is a fundamental quantity measured in meters (m) while time is another fundamental quantity measured in seconds (s). To this end, the unit of velocity is meter per second (m/s) since it is derived from the fundamental units (length and time) respectively.

Since derived units are always derived from fundamental units, we cannot have 'feet per second' as the unit of velocity. This is because the unit feet isn't the ideal measurement for length (or distance); although it may be used as a unit of measurement in some cases.

Below are examples of some derived quantities and their units:

The diagram below shows some derived quantities, their respective derived unit and the unit's symbol. For instance, the first derived quantity from the table is 'area'. The derived unit for area is meter square (or square meter) whose symbol is m2.

It is important to state that the unit of area (m2) is derived from length. Meanwhile, recall that length is a fundamental unit.

### Similarities between fundamental and derived units

1. Both units are used to express physical quantities.

2. Both are recognized as the Standard International System of Units (SI Units).

### Differences between fundamental and derived units

1. Fundamental units are independent of every other unit including themselves while derived units are dependent on fundamental units.

2. Fundamental units cannot be further broken down while derived units can be broken down.

3. We have seven fundamental units while derived units are much more in number.

4. Fundamental units are uniquely expressed while derived units are expressed in terms of fundamental units.