Specific gravity (SG) is a dimensionless measure of the density of a substance relative to the density of water at 4 °C (39.2 °F). It is defined as the ratio of the density of a substance to the density of water at 4 °C. The specific gravity of water at 4 °C is 1.000.

Specific gravity = Density of substance / Density of water at 4 °C

Specific gravity is a useful measure of density because it is independent of the units of measurement used. For example, if the density of a substance is 1000 kg/m³, and the density of water at 4 °C is 1000 kg/m³, then the specific gravity of the substance is 1.000. This is true regardless of whether the units of measurement are kg/m³, g/mL, or any other unit of density.

Specific gravity is used in a variety of applications, including:

* Determining the density of liquids and solids

* Identifying the composition of materials

* Measuring the purity of substances

* Designing and testing equipment

Specific gravity is a simple but powerful measure of density that is used in a wide variety of applications.