Why is the value of a substance's specific gravity always the same numerically as its density?

Unlike density, which has units of mass per volume, specific gravity is a pure number, i. e., it has no associated unit of measure. If the densities of the substance of interest and the reference substance are known in the same units (e. g., both in g/cm3 or lb/ft3), then the specific gravity of the substance is equal to its density divided by that of the reference substance. Similarly, if the specific gravity of a substance is known and the density of the reference substance is known in some particular units, then the density of the substance of interest, in those units, is equal to the product of its specific gravity and the density of the reference substance.

The most widely used reference substance for determining the specific gravities of solids and liquids is water. Because the density of water is very nearly 1 g/cm3, the density of any substance in g/cm3 is nearly the same numerically as its specific gravity relative to water. In the English system of units the density of water is about 62.4 lb/ft3, so the near equality between specific gravity and density is not preserved in this system. Specific gravities of gases are often given with dry air as the reference substance. Because the densities of all substances vary with temperature and pressure, the temperature and (particularly for gases) the pressure for both the reference substance and the substance of interest are often included when precise values of specific gravities are given.