Convert API Gravity to Density with the Correct Reference

Separate API gravity from density

API gravity is a petroleum-industry scale based on specific gravity. It is conventionally reported in degrees API, while density has units such as kg/m³. The numerical API reading is therefore not a density that can simply be relabeled. The U.S. Energy Information Administration gives the relationship °API = 141.5/SG − 131.5, with specific gravity referenced to the petroleum liquid and water at 60°F. The temperature reference is part of the definition. Keep it attached to the value when copying a laboratory result into a spreadsheet or a calculation.

Rearrange the equation first

Add 131.5 to the API value and invert the ratio to obtain SG = 141.5/(°API + 131.5). For a worked reading of 40°API, the denominator is 171.5 and SG ≈ 0.8250728863. In the local Physics Calculator, enter 141.5/(40+131.5). The result is a dimensionless ratio, not kg/m³. Parentheses are essential: 141.5/40+131.5 is a different expression. The calculator is checking the arithmetic you supply; it has no dedicated API-gravity field or automatic reference-temperature conversion.

Apply the stated water reference density

To recover density at the same reference condition, use ρ_liquid = SG × ρ_water,60°F. Suppose a worked calculation adopts a rounded reference density of 999.0 kg/m³ for water. With the preceding 40°API value, the calculation is 141.5/(40+131.5)*999.0 ≈ 824.2478 kg/m³. The 999.0 figure is an explicitly rounded example input, not a precision reference certificate. Use the reference required by your data source or measurement method for actual reporting, and carry its uncertainty and temperature basis with the calculated liquid density.

Check the direction and a second value

A higher API number gives a smaller specific gravity because it increases the denominator. At 45°API, SG = 141.5/176.5 ≈ 0.8016997167. Using the same illustrative 999.0 kg/m³ reference gives approximately 800.8980 kg/m³, less than the 40°API example. This is a helpful direction check against accidental inversion. At 10°API, SG is exactly 1 in the defining arithmetic, so liquid and reference-water densities are equal under that convention. It does not mean that the density is exactly 1000 kg/m³ independent of temperature.

Verify an inverse calculation

If a sample's specific gravity on the required 60°F/60°F basis is 0.85, its API gravity is 141.5/0.85 − 131.5 ≈ 34.970588. Substituting that unrounded result back into 141.5/(°API+131.5) recovers 0.85. If only a density is supplied, first divide it by the appropriate reference-water density to obtain SG; do not place a kg/m³ density directly in the API equation. A reported SG of 0.85 with an unstated temperature basis is missing information for an exact comparison with this convention.

Keep temperature correction separate

The formula above converts scales at their stated reference conditions. It does not correct an observed petroleum volume or density from a storage temperature to 60°F. That is a separate property or measurement calculation requiring an applicable procedure and material information. Do not silently use a water reference at 4°C, a petroleum reading at 20°C and an API formula defined at 60°F as if all three were interchangeable. The mathematical denominator must also remain positive for a positive SG; an invalid or extreme input needs investigation, not a negative material-density interpretation.

Report a reproducible result

Record the supplied API value, reference convention, adopted water density, arithmetic and final density unit. For example, label the worked answer as approximately 824.25 kg/m³ using 40°API and the stated rounded 999.0 kg/m³ water reference. Convert kg/m³ to g/cm³ by dividing by 1000, giving approximately 0.82425 g/cm³ in this example. Do not imply that five displayed decimals establish measurement accuracy. A rounded source API reading and a rounded reference density limit the meaningful precision of the final result.

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