Specific gravity is the ratio of the mass of a body to the mass of an equal volume of water or gas at the same temperature. It is used to measure the total amount of dissolved material in a solution.
The standard for comparison of liquids is pure water at four degrees Celsius. The standard for gases is air at normal temperature and pressure.
The temperature of measurement should be specified and controlled in all specific gravity measurements because the specific gravity of a solution is affected by temperature. Standards of specific gravity are set by analysing standard mixes at 27 degrees Celsius (80.6 Fahrenheit).
Specific gravity has more recently been called relative density.