What API gravity means
API gravity is related to specific gravity, which compares a liquid's density with water. Unlike density, the API scale runs in the opposite direction: lighter oils have higher API values. This makes API gravity convenient for classifying crude streams and communicating with refiners and traders.
The standard reference is specific gravity at 60 degrees F for petroleum work. Temperature matters because liquids expand and contract, so field density measurements may need temperature correction before conversion.
- Producers use API gravity to characterize crude streams.
- Refiners use it to understand yield and processing behavior.
- Traders use it as part of quality and pricing discussions.
How to calculate API gravity
The formula is: API gravity = 141.5 / specific gravity at 60 degrees F - 131.5. The reverse formula is: specific gravity = 141.5 / (API gravity + 131.5). Specific gravity is dimensionless.
Example: a crude oil has specific gravity of 0.850 at the reference temperature. API = 141.5 / 0.850 - 131.5 = 35.0 degrees API. Reverse check: SG = 141.5 / (35.0 + 131.5) = 0.850.
API gravity and crude classification
Common industry descriptions classify crude as light, medium, heavy, or extra heavy based partly on API gravity. Boundaries vary by organization and market, but light crude is generally higher API, while heavy crude is lower API and usually more viscous.
API gravity alone does not define crude value. Sulfur content, acidity, metals, wax, viscosity, contaminants, distillation yield, transportation constraints, and refinery configuration can matter as much as density.
- Higher API generally indicates lighter crude.
- Lower API generally indicates heavier crude.
- Quality and price require more data than API alone.
Common conversion mistakes
The most common mistake is using density at field temperature without correction to the reference basis. A warm sample appears less dense than a cold sample, which changes calculated API gravity.
Another mistake is confusing specific gravity with density in kg/m3 or lb/ft3. Convert density to specific gravity relative to water at the proper reference condition before using the API formula.
Frequently asked questions
Does higher API mean heavier oil?
No. Higher API gravity means lower specific gravity and therefore lighter oil.
Can API gravity be negative?
Yes, extremely dense petroleum liquids can have negative API gravity, though this is uncommon in ordinary crude discussions.
Is API gravity enough to value crude oil?
No. Sulfur, acidity, yield, contaminants, location, and refinery demand also affect crude value.
Why is 60 degrees F used?
It is a common petroleum reference temperature so density and specific gravity can be compared consistently.