API gravity is an arbitrary scale expressing the density of petroleum products. Simply put, it is the “specific gravity” of oil, which is the ratio of the oil’s density to the water’s density. It is calculated with:
API gravity = (141.5/Specific Gravity) – 131.5
The results are expeessed in ‘degrees’, with values over 10 being lighter than water. This scale, created by the American Petroleum Institute, is now the standard for grading crude oils. The optimum crude oil is 40-45 in degrees, having the highest pricing in the market.