Definition
A standardized scale established by the American Petroleum Institute used to express the relative density (specific gravity) of petroleum liquids such as aviation fuels and oils. API gravity is calculated from specific gravity and is expressed in degrees: the higher the API number, the lighter (less dense) the liquid. Pure water has an API gravity of 10°; liquids lighter than water have API gravities greater than 10°.
Plain English
A number that tells you how heavy or light a fuel or oil is compared to water. A higher API number means a lighter liquid. It is a standard way the petroleum industry compares fluids.
Context Anchor
Seen in fuel, oil, and maintenance references where petroleum products are being identified or compared.
Derivation
Named after the American Petroleum Institute, the U.S. body that sets standards for the petroleum industry. The scale was created so refiners and users worldwide could describe and compare petroleum liquids using one agreed measurement.
Why Pilots Care
Fuel weight per gallon varies with API gravity, directly affecting takeoff distance, climb performance, and center-of-gravity calculations.
Grounding Statement
If two fuel samples fill the same size container, the one with the lower API number weighs more.
Intuition Check
API here does not mean a computer software interface. It means American Petroleum Institute, and the scale describes liquid density, not fuel quality by itself.
Example Sentence 1
The fuel specification listed an API gravity of 58°, indicating a relatively light petroleum product typical of aviation gasoline.
Example Sentence 2
Using the API scale value, the pilot converted fuel volume to weight for the weight-and-balance sheet.