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 can be used to calculate how many barrels of crude oil can be produced per metric ton. Given that the weight of an oil plays an integral role in establishing its market value, API is an important measure.