The formula for relative density is the quotient of the mass of the substance divided by the mass of the reference substance. The relative density can also be determined by finding the ratio of the weights in place of the density.
The relative density formula calculates the density of the substance in question in comparison with a second substance, called the reference substance. Specific gravity is similar to relative density, although it is typically used in measurements where the reference substance is water. As both the ambient temperature and pressure can affect the density of a substance, both measurements must be known for the initial substance and the reference substance. Devices to determine relative density include the pycnometer, hydrometer and the digital density meters.