API Gravity Formulas
The formula used to obtain the API gravity of petroleum liquids is:
API gravity = (141.5/SG at 60°F) – 131.5
Conversely, the specific gravity of petroleum liquids can be derived from the API gravity value as:
SG at 60°F = 141.5/(API gravity + 131.5)
60°F (or 15.5556°C) is used as the normal value for measurements and further tables give adjustments for temperature
(as determined by ASTM D1298).
Thus, a heavy oil with a specific gravity of 1.0 (i.e., with the same density as pure water at 60°F) would have an API gravity of (141.5/1.0) – 131.5 = 10.0 degrees API.