The density of a substance measures how much substance is in a given volume. The formula for density is mass divided by volume (density = mass/volume). Specific gravity is a ratio of the density of a substance to the density of a reference substance, usually water. Since the density of water is one gram per cubic centimeter, specific gravity is calculated by dividing the density of a substance by one gram per cubic centimeter. Since a number divided by one is itself, the specific gravity of a substance is the density absent the units of measure.

Determine the density of a substance. This may be done by dividing the mass by the volume of the substance or more directly through the use of instruments such as a hydrometer.

Divide the density of the substance by the density of water. Water has a density of one gram per cubic centimeter (1 g/cm3).

The quotient is the specific gravity of the substance.