The mass of a substance contained in unit volume is its density (D).
D = m/V
Measuring of densities of substances is easier if we compare them with the density of some other substance of know density. Water is used for this purpose. The ratio of the density of the substance to that of water is called the Specific Gravity (SG) of the substance.
SG = Dsubstance / Dwater
The density of water is 1000 kg/m3