What does API gravity measure for crude oil and petroleum products?

API gravity measures the relative density of crude oil and petroleum products compared with water.

The American Petroleum Institute scale expresses how light or heavy a petroleum liquid is. Higher API gravity means a lower density and generally indicates a lighter oil; lower API gravity indicates a denser, heavier oil. The scale is calculated from specific gravity rather than measured as a simple percentage.

Refiners use API gravity alongside sulfur content and other properties to evaluate crude oil. Light crude often yields more high-value transportation fuels through straightforward processing, while heavy crude may require additional conversion equipment such as cokers or hydrocrackers.

API gravity is not the same as octane rating, cetane number, or viscosity. Octane describes gasoline’s resistance to knocking, cetane concerns diesel ignition quality, and viscosity describes resistance to flow. API gravity instead describes density relative to water.