The ratio of the density of a solid or liquid to the density of water is called the specific gravity or relative density. It is a dimensionless quantity and is often used to compare the densities of different substances to the density of water, which is 1 g/cm³ or 1 kg/L at 4°C.

For example, if the density of a substance is 2 g/cm³, then its specific gravity or relative density is 2 since it is twice as dense as water. Similarly, if the density of a substance is 0.5 g/cm³, then its specific gravity is 0.5 since it is half as dense as water.

Specific gravity is a useful property for identifying substances and for determining their purity. For example, the specific gravity of gold is 19.3 g/cm³, which means that it is 19.3 times denser than water. This property can be used to distinguish real gold from counterfeit gold, which may have a different specific gravity.

Specific gravity can also be used to calculate the density of a substance if the density of water is known. For example, if the specific gravity of a substance is 2.5 and the density of water is 1 g/cm³, then the density of the substance is 2.5 g/cm³.