Calculation of API gravity
API is calculated from a hydrocarbon's specific gravity using this formula:
API = (141.5/Specific Gravity) - 131.5
API gravity of crude oil grades
A crude oil will typically have an API between 15 and 45 degrees. Higher API indicates a lighter (lower density) crude. Lower API indicates a heavier (more dense) crude. Generally, lighter (high API) crudes are more valuable because they yield more high-value light products when run through a refinery.
Light crude is typically in the 35-45 API range, which includes most of the highest valued crudes such as Brent and WTI. Crudes lighter than 45 are typically considered extra-light crude or condensates and are valued lower than light crude because they contain a lot of light ends such as propane and butane. A medium crude is in the 25-35 API range, and a heavy crude is in the 15-25 API range. Anything below 15 API would be considered an extra-heavy crude.