# What is the difference between density and relative density?

May 23, 2016

#### Explanation:

The density is mass divided by volume and hence it has a unit - say in SI units it is $\frac{k g}{m} ^ 3$ and in FPS units it is $\frac{l b}{f t} ^ 3$.

For example density of water in SI units is $1000 \frac{k g}{m} ^ 3$ and in FPS unit it is 62.42796(lb)/(ft^3.

For iron it is $7874 \frac{k g}{m} ^ 3$ or 491.55776(lb)/(ft^3.

Relative density on the contrary is the ratio of the density of certain matter with that of water. Hence, it does not have a unit, but is just a number. As such, it's value will not change when we change units, as both densities are calculated as ratio.

It is obvious that for water it is just $1$ and for iron it will always be $7.874$ , whatever units one uses.