What this converter does
This converter turns specific gravity into API gravity and back. Lab reports often give SG, which the petroleum industry grades on the API scale. The link is the API formula, not a fixed factor. Type a value and read the result instantly.
An SG of 0.825 is about 40°API light crude; an SG of 1.0 is exactly 10°API.
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 an SG of 0.85 to API gravity.
API = 141.5 ÷ SG − 131.5141.5 ÷ 0.85 − 131.5 = 34.97°APISo an SG of 0.85 is about 35°API — a medium crude.
The units in this example
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
- 0.850 SG ≈ 35°API
- SG = 141.5/(API+131.5)
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)
- 25.7°API ≈ 0.900 SG
- 35°API ≈ 0.850 SG
- API = 141.5/SG − 131.5