**Density **is the mass per unit volume of a substance. The symbol used for density is p (Greek letter rho) and its units are kg/m^{3}

where m is the mass in kg, V is the volume in m3 and p is the density in kg/m3

Some **typical values of densities **include:

The **relative density **of a substance is the ratio of the density of the substance to the density of water, i.e.

Relative density has no units, since it is the ratio of two similar quantities. Typical values of relative densities can be determined from above (since water has a density of 1000 kg/m3), and include:

The relative density of a liquid may be measured using a **hydrometer.**

**For example, **the relative density of a piece of steel of density 7850 kg/m3 , given that the density of water is 1000 kg/m3 , is given by: