API gravity is the scale developed by the American Petroleum Institute to measure how heavy or light petroleum liquids are.
The scale expresses petroleum density relative to water through a formula based on specific gravity. Higher API-gravity values indicate lighter crude oil, while lower values indicate heavier crude. Crude oil with API gravity above 10 is less dense than water and generally floats on it.
API gravity does not directly measure quality. Refiners also consider sulfur content, acidity, metals, wax, viscosity, and the proportions of gasoline, diesel, jet fuel, and other products that a crude can yield. A light crude may be commercially valuable, but its value still depends on the refinery and market.
The term is sometimes mistaken for a temperature or pressure rating. It is instead a density-related petroleum measurement, commonly reported alongside sulfur content when crude oils are described and priced.