Engineering lesson
Gas gravity API gravity and specific gravity
Gas gravity is relative to a specified gas reference, commonly air. API gravity transforms liquid specific gravity at its reference condition. The two scales describe different properties.
By PetroCalcHub Team · Updated October 24, 2026 · Free to read and use
Use the liquid definition correctly
API = 141.5/SG60 − 131.5 on the standard petroleum convention. Higher API corresponds to lower liquid specific gravity.
The reference temperature and water basis matter. An uncorrected density measured at another temperature is not automatically the SG60 needed by the relation.
Keep gas gravity separate
Gas gravity commonly compares gas molecular weight with air molecular weight on a consistent ideal reference. It is not an API value and does not carry liquid density units.
A gas-gravity-based pseudocritical correlation introduces additional empirical assumptions. Converting a gravity definition does not itself determine a real-gas Z factor.
Use a reverse check
Recover SG60 from 141.5/(API + 131.5) and compare with the input. This catches swapped formulas or a percentage entered as a fraction.
The explorer varies liquid SG only. It does not model temperature correction or infer composition.
Explore the example
Replayed with the same equation record and calculation engine as /calculators/api-gravity. Original synthetic inputs; engine outputs are saved in the downloadable record.
Select one of 5 recorded scenarios. Intermediate states are not interpolated.
Specific Gravity of Oil Phase (dimensionless) = 0.85. API (dimensionless): 34.9705882.
- API (dimensionless)
Worked example
API Gravity
Fixed controls: . SG_o is varied as shown.
Relation and unit basis
SG_o: Specific Gravity of Oil Phase (dimensionless).
1. Define the baseline
Specific Gravity of Oil Phase (dimensionless) = 0.85
Fixed controls: . SG_o is varied as shown.
2. Select the relation
SG_o: Specific Gravity of Oil Phase (dimensionless).
3. Substitute the values
SG_o = 0.85 → API = 34.9705882 dimensionless
Retain intermediate precision. The displayed values are rounded only after evaluation.
4. Read the result
API (dimensionless) = 34.9705882
Higher liquid specific gravity corresponds to lower API on the specified reference basis.
Result and interpretation
API (dimensionless) = 34.9705882
Higher liquid specific gravity corresponds to lower API on the specified reference basis.
The inverse API–SG curve is a definition at the stated reference, not a fluid-composition correlation.
Exact scenario data
Replayed with the same equation record and calculation engine as /calculators/api-gravity. Original synthetic inputs; engine outputs are saved in the downloadable record.
| Specific Gravity of Oil Phase (dimensionless) | API (dimensionless) |
|---|---|
| 0.7 | 70.64286 |
| 0.8 | 45.375 |
| 0.85 | 34.97059 |
| 0.9 | 25.72222 |
| 1 | 10 |
Scroll the table to see all columns. The highlighted row is the worked baseline. Downloads retain full numerical precision.
Test your understanding
Work through the question before revealing its answer. Use the stated assumptions and include units in every numerical step.
1. What API gravity corresponds to SG60 = 1?
Show the worked answer
10° API. API gravity zero is therefore not the same as a density of water.
2. Reproduce the baseline API (dimensionless) without rounding intermediate values.
Show the worked answer
SG_o = 0.85 → API = 34.9705882 dimensionless. API (dimensionless) = 34.9705882. Higher liquid specific gravity corresponds to lower API on the specified reference basis.
3. Compare the first and last recorded scenarios. Which input changes?
Show the worked answer
Specific Gravity of Oil Phase (dimensionless) changes from 0.7 to 1. Other controls remain as stated in the premise. API (dimensionless) changes from 70.6428571 to 10.
4. What would be needed to apply this example to a different physical system?
Show the worked answer
Replace the synthetic controls with compatible documented inputs and check the relation's assumptions. SG_o: Specific Gravity of Oil Phase (dimensionless). Higher liquid specific gravity corresponds to lower API on the specified reference basis.
Before using the result
- Keep gas and liquid gravity fields separate.
- Record the liquid reference condition.
- Reverse-check API to SG.
Higher liquid specific gravity corresponds to lower API on the specified reference basis.
Continue learning
Start with the broader workflow guide · Related method guide
Sources and method
The explanation and synthetic examples are original PetroCalcHub teaching material. The references below support the definitions and model context. They do not supply field validation for the synthetic scenarios.