Density of a substance is defined as its mass per unit volume.
The SI unit of density is kg/m3.
Relative Density
Relative density of a substance is defined as the ratio of its density to that of water at 4°C.
Relative density can also be defined as the ratio between the mass of the substance and the mass of an equal volume of water at 4°C.
Density of water at 4°C = 1 g/cm3 = 1000 kg/m3.
Since the relative density is a ratio of similar quantities, it has no unit.
