The relative density or apparent density is the numerical relationship obtained by dividing the density of a substance by the density of another that is taken as a reference. Both densities are expressed in the same units and under the same conditions of temperature and pressure, so, as it is a quotient, the relative density is a dimensionless magnitude (without units). It is often expressed by indicating the value followed by expressions such as "times denser than ..." or similar, to indicate the substance that has been used as a reference. The determination of relative density is carried out with the help of certain instruments called densimeters (sometimes hydrometers).

Sometimes it is called specific density, but currently the ISO 80000-1 standard, in line with others such as the IUPAC Green Book, recommends reserving the adjective specific to the quotient with the mass.[1]​.

The relative density is defined as the quotient between the density that is primarily of a substance and that of another substance taken as a reference, resulting in:

where is the relative density, is the absolute density and is the reference density.

The relative density can also be calculated or measured as the ratio between the weights or masses of identical volumes of the test substance and the reference substance: