API gravity (American Petroleum Institute Grade) is a conventional, commonly used in the United States, density unit for crude oils. It is used worldwide for the characterization and as a measure of quality of crude oil.
The API gravity results from the relative density of the crude oil at 60 ° F ( 15 5/ 9 ° C) relative to water by
This is known crude oil 31.1 ° API as "light", between 22.3 ° API and 31.1 ° API as "medium" and 22.3 ° API as "severe". The API gravity is included at the variety name in the rule.