The specific gravity table published by the American Petroleum Institute (API) is a tool for determining the relative density of various types of oil. While it has no units of measurement, an oil's rating is expressed as API degrees. The scale is inversely related to the density of water. Any oil with an API score under 10 degrees sinks in water.
The API table was developed to help engineers estimate shipping capacity based on the per-unit weight of different types of oil. It is also useful for predicting the behavior of oil that's spilled into the ocean. Extra-heavy crude oils – those with a rating under 10 degrees can be expected to sink when they're spilled. Lighter grades spread across the surface and call for a different containment approach.
The API scale runs from 1 to 75, but it was calibrated in such a way that most of the values would cluster between 10 and 70 degrees. The scale is helpful in developing a classification scheme for the different weights of oil. Anything under 22.3 degrees is considered heavy crude oil, with weights under 10 considered extra heavy. Medium oil ranges from 22.3 to 31.1 degrees. Anything higher than 31.1 is light crude.