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API Gravity refers to the American Petroleum Institute gravity, a specific gravity scale developed by the American Petroleum Institute (API). It is used to measure the relative density of various petroleum liquids, expressed in degrees. API Gravity is the measure of how heavy or light petroleum liquid is compared to water. The formula used to obtain this effect is: API gravity = (141.5/SG at 60°F) - 131.5, where SG is the specific gravity of the fluid.