What this converter does
This converter turns API gravity into specific gravity and back. Because API is defined as a reciprocal of SG, the relationship is a formula, not a fixed factor — water is 10°API heavier than 40°API light crude. Type a value and read the result instantly.
Crude above 10°API floats on water; lighter (higher-API) crude is more valuable. A 40°API oil has an SG of about 0.825.
The units it covers
API gravity and specific gravity both describe how heavy a crude oil is relative to water.
View all units & their values
| Unit | Symbol | Value | Mainly used |
|---|---|---|---|
| API gravity | °API | 141.5/SG−131.5 | US petroleum industry scale |
| Specific gravity | SG | 141.5/(API+131.5) | Density relative to water at 60°F |
The formula
The two scales are reciprocally related through the API definition, not a fixed factor:
SG = 141.5 ÷ (API + 131.5)Where:
- API = the API gravity in degrees
- SG = specific gravity relative to water at 60°F
- 141.5 / 131.5 = the API scale constants
Worked example
Convert 40°API to specific gravity.
SG = 141.5 ÷ (API + 131.5)141.5 ÷ (40 + 131.5) = 0.825So a 40°API light crude has a specific gravity of about 0.825.
The units in this example
The American Petroleum Institute scale for how heavy or light a crude oil is; higher means lighter and generally more valuable.
- 10°API = 1.000 SG (water)
- 31.1°API ≈ 0.870 SG (medium)
- 40°API ≈ 0.825 SG (light)
- API = 141.5/SG − 131.5
Density of the oil relative to water at 60°F. Below 1.0 floats on water; above 1.0 sinks.
- 1.000 SG = 10°API
- 0.900 SG ≈ 25.7°API
- 0.825 SG ≈ 40°API
- SG = 141.5/(API+131.5)