A density scale made for oil
The oil industry describes crude oil and refined products with API gravity rather than plain density. The American Petroleum Institute adopted the scale in 1921, building on the older Baumé scale used for liquids lighter than water. It runs backwards compared with density: the higher the API gravity, the lighter the oil.
The formulas
°API = 141.5 ÷ SG − 131.5
SG = 141.5 ÷ (°API + 131.5)
where SG is the specific gravity at 60 °F (15.6 °C), the density of the oil divided by the density of water at the same temperature. Density in kg/m³ is SG × 999.0.
Water has a specific gravity of 1.000 and therefore an API gravity of exactly 10. Anything above 10 °API floats on water.
Classification of crude oil
| Class | API gravity | Examples |
|---|---|---|
| Light | above 31.1 | West Texas Intermediate (about 40), Brent (about 38) |
| Medium | 22.3 to 31.1 | Many Middle East blends |
| Heavy | 10 to 22.3 | Mexican Maya (about 22), Venezuelan heavy crudes |
| Extra heavy | below 10 | Canadian bitumen, Orinoco belt |
Light crude yields more high-value products such as gasoline, jet fuel and diesel with simple refining, so it usually sells at a premium. Heavy crude needs more complex refineries with cokers and hydrocrackers.
Example
A crude with an API gravity of 35:
- SG = 141.5 ÷ (35 + 131.5) = 0.8498
- Density = 0.8498 × 999.0 ≈ 849 kg/m³, or 7.09 lb per US gallon
- One barrel (42 US gallons, 158.99 L) weighs about 135 kg (298 lb)
- About 7.4 barrels make one metric ton
Temperature matters
Oil expands as it warms, so its density falls and its API gravity rises. Standard values are given at 60 °F. A hydrometer reading taken at another temperature must be corrected with the ASTM D1250 tables before it is reported. For a quick field check, light crude gains a little under 1 °API for every 10 °F of warming, but use the tables for any commercial measurement.
Other uses
API gravity is also used for refined products: gasoline is around 55 to 65 °API, diesel around 30 to 40, and heavy fuel oil around 10 to 20. Drilling engineers use it to estimate the weight of fluid columns in wells, and traders use it in price adjustments between grades of crude.