Calculate API gravity from density, specific gravity, or a petroleum product and view equivalent kg/m³, g/cm³, lb/US gal, and lb/ft³.
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API Gravity Formula
The calculator uses the American Petroleum Institute definition, which relates API gravity to specific gravity at 60°F.
API = 141.5 / SG - 131.5
Specific gravity is the ratio of the liquid's density to water's density at 60°F (999.016 kg/m³):
SG = rho_liquid / rho_water
Solving the first equation for SG gives the inverse relationship used when API gravity is the input:
SG = 141.5 / (API + 131.5)
- API — degrees API gravity (°API)
- SG — specific gravity at 60°F, dimensionless
- rho_liquid — density of the petroleum liquid at 60°F
- rho_water — density of water at 60°F, 999.016 kg/m³ (about 8.337 lb/US gal)
The Known value mode takes any one of density (kg/m³, g/cm³, lb/US gal, lb/ft³), specific gravity, or °API and converts it to the other two using the formulas above. The Product lookup mode loads a typical density for a named petroleum product and runs the same conversion so you can see its API gravity and SG without typing a number.
Reference Values and Classification
API gravity is mostly used to classify crude oil. The cutoffs below follow common industry usage.
| Class | °API | SG |
|---|---|---|
| Light crude | > 31.1 | < 0.870 |
| Medium crude | 22.3 – 31.1 | 0.870 – 0.920 |
| Heavy crude | 10 – 22.3 | 0.920 – 1.000 |
| Extra heavy / bitumen | < 10 | > 1.000 |
Typical densities for common petroleum products at 60°F:
| Product | Density (kg/m³) | °API |
|---|---|---|
| Gasoline | 750 | 57.2 |
| Kerosene | 775 | 51.2 |
| Jet fuel | 804 | 44.7 |
| Diesel | 832 | 38.7 |
| Light crude | 825 | 40.1 |
| Heavy crude | 950 | 17.5 |
| Bitumen | 1010 | 8.4 |
Worked Example and FAQ
Example. A crude oil sample has a density of 875 kg/m³ at 60°F. Find its API gravity.
SG = 875 / 999.016 = 0.8759. API = 141.5 / 0.8759 - 131.5 = 161.55 - 131.5 = 30.05 °API. That puts the sample just inside the medium crude range.
Why is 10 °API the dividing line? At 10 °API the formula gives SG = 1.000, so the liquid has the same density as water. Anything above 10 °API floats; anything below sinks.
Does temperature matter? Yes. API gravity is defined at 60°F (15.6°C). Density measured at another temperature must be corrected to 60°F before the formula gives a valid API number. Field measurements use ASTM D1250 tables for that correction.
Can API gravity be negative? The math allows it for any SG above about 1.076, but in practice values below about 4 °API are rare. Bitumen typically falls between 6 and 10 °API.
Why use API instead of SG? API spreads the narrow SG range of crude oils (roughly 0.8 to 1.0) across a wider numeric scale (about 10 to 50), which makes small differences easier to read and price.
