The term specific gravity, symbolized sp gr, refers to the ratio of the density of a solid or liquid to the density of water at 4 degrees Celsius. The term can also refer to the ratio of the density of a gas to the density of dry air at standard temperature and pressure, although this specification is less often used. Specific gravity is a dimensionless quantity; that is, it is not expressed in units.
To find the sp gr of a solid or liquid, you must know its density in kilograms per meter cubed (kg/m3) or in grams per centimeter cubed (g/cm3). Then, divide this density by the density of pure water in the same units. If you use kg/m3, divide by 1000. If you use g/cm3, divide by 1 (that is, leave the number alone). It is important to use the same units in the numerator and denominator.
Water has a specific gravity equal to 1. Materials with a specific gravity less than 1 are less dense than water, and will float on the pure liquid; substances with a specific gravity more than 1 are more dense than water, and will sink. An object with a density of 85 kg/m3 has a specific gravity of 0.085, and will float high on the surface of a body of water. An object with a density of 85 g/cm3 has a specific gravity of 85, and will sink rapidly.
To find the specific gravity of a gas, you must know its density in kilograms per meter cubed (kg/m3). Then, divide this density by the density of dry air at standard temperature and pressure. This value is approximately 1.29 kg/m3. Gases with a specific gravity less than 1 will rise in the atmosphere at sea level; gases with a specific gravity greater than 1 will sink and seek regions of low elevation at the earth's surface.