To convert from specific gravity to API, divide 141.5 by the specific gravity, then subtract 131.5. The specific gravity that should be used is that from oil at sixty degrees Fahrenheit. The 141.5 comes from the old Baum? scale the United States used as a standard of measure for any liquid with a density less than water. Although API values don't have units, they are typically stated in degrees.
Specific gravity is a ratio of the density of one substance to the density of another reference substance, which is usually water. API gravity itself is a specific-gravity measurement used by the oil industry to compare an oil's density to that of water. The calculation is designed to ensure consistency in measurement and is used to describe oil as light, medium, heavy or extra heavy. Light oils have an API greater than 31.1, medium oils have an API between 22.3 and 31.1, heavy oils have an API less than 22.3 and extra heavy oils have an API of less than 10.0. An oil's API rating is especially important because the weight of the oil is the largest determinant of its market value. Light oils have higher value because they contain more hydrocarbons that can be converted into gasoline.