API gravity is a measure of the density of crude oil as compared to density of water or in other words, it is a measure of specific gravity of crude oil. It is defined as,
API gravity = 141.5 X (density of water at 600F/ density of oil at 600F ) - 131.5
API gravity = (141.5 / specific gravity of oil at 600F) - 131.5
Alternatively if API gravity is known specific gravity can be determined as,
Specific gravity of oil at 600F = 141.5/(API gravity+131.5)
From the given equation it is clear that API gravity of water is 10 0API and API gravity is higher for lighter hydrocarbons. For most crude oil fractions, the API gravity values range from 10 0API to 80 0API, all being lighter than water. For hydrocarbon gases, API gravity ranges from 90 0API and above.
If oil density at 600F (or 15.60C) is known, use EnggCyclopedia's liquid density calculator to evaluate water density at 600F and use the given equations to calculate API gravity.