Fluids include liquids, gases, and mixtures of liquids, solids, and gases. For the purposes of this reference book, the terms fluid and liquid are both used to mean a pure liquid or a liquid mixed with gases or solids that acts essentially like a liquid in a pumping application. Density or specific weight of a fluid is its weight per unit volume, often expressed in units of grams per cubic centimeter or pounds per cubic foot. The density of a fluid changes with temperature.
If weight is 36.36 kg (80 lb), density is 1.28 g/cm3 (80 lb/cu ft).
Specific gravity of a fluid is the ratio of its density to the density of water. Since it is a ratio, it has no units associated with it.
Specific gravity = 80 lb/62.4 lb or, Specific gravity = 1.282