API gravity

[American Petroleum Institute]

(degree API, °API) petroleum processing A scale, expressed in degrees, for the volumic mass of petroleum liquids. Used for liquids less dense than water, it is structured identically with the Baumé scale for such liquids, except that it has its set temperature at 60°F rather than 10° Réaumur (12.5°C, 54.5°F); readings are thus about 1% greater than Baumé.

If S is volumic mass in kg·m^-3, and D is the API degrees, the two relate as follows:

S × (131.5 + D) = 141.5,  hence D = (141.5/S) - 131.5.

Since the volumic mass of water is virtually 1 000 kg·m^-3, the same equations effectively apply if S is interpreted as specific gravity. Practical values of API gravity range from 10° to around 100°, higher values meaning lighter oils, e.g.

Specific gravity	1	0.95	0.9	0.8	0.7
API gravity (°)	10	17.45	25.72	45.38	70.64

The volumic mass of natural crude oils varies by more than 10%, so the mass of a standard barrel of 42 US gal varies likewise. The densities are expressed in API degrees, but are often called gravity. Oil of 40° API has 6.923~ barrels (bbl) to the short ton (7.631~ bbl·t^-1).