A specific gravity scale developed by the American Petroleum Institute (API) for measuring the relative density of various petroleum liquids, expressed in degrees. API gravity is gradated in degrees on a hydrometer instrument and was designed so that most values would fall between 10掳 and 70掳 API gravity. The arbitrary formula used to obtain this effect is: API gravity = (141. 5/SG at 60掳F) - 131. 5, where SG is the specific gravity of the fluid.