# Why is density a derived unit?

Mar 1, 2017

Density is a derived unit because it is defined in terms of other units.

#### Explanation:

Density is measured in $k g \cdot {m}^{-} 2$. This means that it is derived from fundamental units which are the kilogram and metre.

The kilogram is defined as the mass of a platinum-iridium cylinder which is kept in France. This is the only fundamental unit which is still defined by an artefact.

The second is defined at the time take for $9 , 192 , 631 , 770$ transitions of a caesium 133 atom. The speed of light is defined to be exactly $299 , 792 , 458 m {s}^{-} 1$. Now the metre is defined to be the distance light travels in $\frac{1}{299 , 792 , 458}$ seconds.

hence the kilogram and the metre are SI units and density is a derived unit from these units.