Specific gravity is an index used to measure the density of a liquid. Specific gravity is calculated as the ratio of the density of a liquid to the density of water. So any liquid with a density greater than water has a specific gravity greater than 1. Liquids with a density lower than water, which includes most crude oil grades and petroleum products, will have a specific gravity between 0.0 and 1.0.

In the oil industry, specific gravity is a common quality specification for finished products. However, for crude oil grades, the API gravity index is more commonly used.

Calculation of specific gravity from API gravity:

Specific gravity = 141.5/(API gravity + 131.5)