API gravity is named for which U.S. oil and gas trade association?

API gravity is named for the American Petroleum Institute.

API gravity is a scale used by the petroleum industry to describe how heavy or light a crude oil or petroleum liquid is relative to water. It is calculated from specific gravity at 60°F using the formula: degrees API = 141.5 divided by specific gravity, minus 131.5.

The scale runs in the opposite direction from ordinary density measurements. A higher API number means a lighter petroleum liquid, while a lower number means a heavier one. Water has an API gravity of about 10 degrees; liquids above that value are less dense than water and generally float on it.

API gravity is not a measure of quality by itself. Refiners also care about sulfur content, acidity, metals and the product yields a crude can produce. A common mix-up is treating API as the American Gas Association or as an engineering society: in this context, API refers specifically to the American Petroleum Institute, the U.S. oil and gas trade association.