Light and heavy crude oils are usually classied in two ways.

They can be classied by relative density, which in China is the 

ratio of the mass of oil at 20 °C to that of pure water at 4 °C under 1 

standard atmospheric pressure. In general, crude oil with a relative 

density of 0.9-1.0 is classied as heavy crude, and crude oil with a 

relative density below 0.9 is classied as light crude.

Light and heavy crude oils can also be classied by the American 

Petroleum Institute (API) gravity, which is calculated as: API gravity 

= (141.5 / relative density of crude oil at 60 °F) – 131.5, where the 

relative density of crude oil at 60 °F is the ratio of the crude oil’s 

mass to that of a unit volume of pure water at 60 °F (15.6 °C) under 

101,325 Pa of ambient pressure.

By international convention: ultra-light crude has an API gravity ≥ 

50, light crude 35-50, medium crude 35-26, and heavy crude 26-

10. However, this grading system is not a universal standard, as 

dierent countries and companies often take other factors into 

consideration, such as applicable pricing benchmarks.

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