🛢️ API Gravity Calculator
Convert between specific gravity and API gravity, the standard scale used to classify crude oil and petroleum products.
What API gravity measures
API gravity is a scale developed by the American Petroleum Institute to express how heavy or light a petroleum liquid is compared to water. Higher API gravity means a lighter, less dense oil.
The formula
API gravity is calculated from specific gravity (SG, measured at 60°F relative to water) as: API = (141.5 / SG) – 131.5. The reverse conversion is SG = 141.5 / (API + 131.5). Oils with API gravity above 10 are less dense than water and float; crude is generally classed as light (above 31.1°API), medium, or heavy (below 22.3°API).
This calculator is for general reference and educational use.
Last reviewed August 2026