The specific gravity of a substance is a comparison between the mass or density of a substance as compared to an equal volume of water. The base line for specific gravity is taken from the properties of water and is given the value of 1.
Specific gravity =
Mass of a substance
Mass of an equal volume of water
For example: A cubic meter of iron has a mass of 7901 Kg, while a cubic meter of water has a mass of 1000 Kg. Therefore if you divide the mass of the iron by the mass of an equal volume of water you will find the specific gravity of iron is 7.901
In this case, the specific gravity (the ratio of the density of iron to the density of water) indicates that a cubic meter of iron weighs about 8 times as much as a cubic meter of water. Note that there are no units of measurement. Specific gravity simply states how many times heavier a solid or liquid sample is, from water.
Example 2 : The specific gravity of a certain oil is 0.8, find the mass of 1 litre of oil.
The mass of 1 litre of water is 1 Kg, so the mass of the oil would be 0.8 Kg.