API gravity

The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water. If its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. API gravity is thus a measure of the relative density of a petroleum liquid and the density of water, but it is used to compare the relative densities of petroleum liquids. For example, if one petroleum liquid floats on another and is therefore less dense, it has a greater API gravity. Although mathematically API gravity has no units (see the formula below), it is nevertheless referred to as being in “degrees”. API gravity is graduated in degrees on a hydrometer instrument and was designed so that most values would fall between 10 and 70 API gravity degrees.
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The gravity, (weight per unit volume) of Crude Oil or other related liquids as measured by a system recommended by the American Petroleum Institute. API Gravity is expressed in degrees, a specific gravity of 1.00 is equivalent to 10 API. API Gravity is related to Specific Gravity (SG) by the following formula: API Gravity = ( 141.5 / Specific Gravity ) - 131.5