API gravity is a number that tells you how light or heavy a petroleum product is. Lighter fuels like gasoline carry a higher number, heavier ones like fuel oil a lower one.
Every petroleum product has a density, meaning how much a given volume weighs. API gravity is a standard scale built around that density, run by the American Petroleum Institute, which is where the API comes from. It turns density into one easy number you can read off a ticket.
The scale runs opposite to weight: the higher the API gravity number, the lighter the product. Gasoline sits high, diesel in the middle, heavy fuel oil and crude lower down. So the number alone tells you roughly what grade of product you are looking at.
It matters because fuel changes volume with temperature, and density is part of how gallons get corrected to a standard 60 degrees. It also flags a product that is off spec. If a load comes in with a gravity reading that does not match the product it is supposed to be, something is wrong before it ever goes into a tank.
In useThe lab sheet showed an API gravity that read too low for gasoline, so the buyer held the load and called the terminal before unloading.
Where the word comes from
API stands for the American Petroleum Institute, the industry body that set the scale in the early 1900s so the trade would have one agreed measure of how light or heavy a product is.
See also Octane, Gross vs net gallons