API Gravity
Standard measure of how light or heavy a crude oil is relative to water.
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Example
SG=0.85: API = 166.5-131.5 = 35°
How it works
API gravity is a petroleum industry scale that compares the density of crude oil to water using specific gravity at 60°F. The constant 141.5 and subtraction by 131.5 are chosen so that water has an API gravity of 10°, with lighter oils giving values above 10 and heavier oils giving values below 10. Because it is based on specific gravity, the formula converts a ratio of densities into a more convenient number for ranking crudes by how easily they tend to flow and be processed.
When to use it
Use this formula when you know a crude oil’s specific gravity and want to classify it as light, medium, or heavy. It is commonly used in petroleum production, refining, and trading to compare oils with one another.
Worked example, step by step
Given SG = 0.92 API = 141.5 / SG - 131.5 API = 141.5 / 0.92 - 131.5 API = 153.8043 - 131.5 API = 22.3043 API ≈ 22.3°
Common mistakes to avoid
- Using density instead of specific gravity, even though the formula requires SG at 60°F
- Forgetting that the result is in degrees API, not degrees Celsius or Fahrenheit
- Reversing the calculation and subtracting 131.5 before dividing by specific gravity
Did You Know?
Crude above 31° API is 'light'; below 22° is 'heavy' and harder (pricier) to refine.
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