Specific gravity (S.G.) is a measure of the relative density of a substance as compared to thedensity of water at a standard temperature. Physicists use 39.2°F (4°C) as the standard, butengineers ordinarily use 60°F. In the International System of Units (SI Units), the density ofwater is 1.00 g/cm3 at the standard temperature. Therefore, the specific gravity (which isdimensionless) for a liquid has the same numerical value as its density in units of g/cm3. Sincethe density of a fluid varies with temperature, specific gravities must be determined and specifiedat particular temperatures.