API gravity

One of the most important characteristics of crude oil is its density: whether it is light or heavy.

Although it would be possible to quote densities using normal units (such as kg/litre), the oil industry convention is to use API (American Petrochemical Institute) gravity. API gravity is:

(141.5 ÷s) - 131.5

where s is specific gravity at 60 degrees Fahrenheit.

The specific gravity is the ratio of the density of the oil to the density of water.

There is an inverse relationship between API gravity and density; the higher the density the lower the API gravity. Light crudes are generally those with an API gravity over 40. Those with an API gravity below 40 are regarded as heavy.

The density of oil (as measured by API gravity) is one of the key factors used to classify and grade types of crude oil.