API Gravity to Specific Gravity and Density: Formulas and Table
A hydrometer gives you API gravity. The bill of lading, an international supplier, or your weight and balance might want specific gravity, kilograms per cubic meter, or pounds per gallon instead. They're all the same measurement in different units, and converting between them takes one formula. Here are the formulas, a worked example, and a table for the range fuelers actually see.
The Formulas
Everything starts from specific gravity (SG) at 60°F, the fuel's density divided by water's at the same temperature:
From SG, density and weight are one multiplication each, using water's own density at 60°F:
Two things to keep straight. First, all of these need the corrected API gravity, at 60°F. Plug in an uncorrected reading from a warm sample and every number downstream is off. Second, metric paperwork usually quotes density at 15°C, not 60°F. 15°C is 59°F, so the fuel is a little denser there: a few tenths of a kg/m³ higher than the 60°F figure. Use the petroleum tables (or AvGrav) for the 15°C number rather than the formula above.
A Worked Example
Say you've corrected a Jet A sample to 43.79° API at 60°F.
- SG = 141.5 ÷ (43.79 + 131.5) = 141.5 ÷ 175.29 = 0.8072
- Density at 60°F = 0.8072 × 999.016 = 806.4 kg/m³
- Weight = 0.8072 × 8.32774 = 6.722 lb per US gallon
So 5,000 gallons of it weighs about 33,610 pounds. That's the same fuel as on AvGrav's result screen, which shows SG, pounds per gallon, and kg/m³ at 15°C (806.8 for this sample) under every reading.
Conversion Table
API gravity at 60°F and its equivalents, in steps of two degrees. Jet A typically falls between about 36 and 51° API, diesel between 30 and 42, and 100LL and mogas between 55 and 65.
| API | SG 60/60°F | kg/m³ 60°F | kg/L | lb/US gal |
|---|---|---|---|---|
| 30 | 0.8762 | 875.3 | 0.875 | 7.296 |
| 32 | 0.8654 | 864.6 | 0.865 | 7.207 |
| 34 | 0.8550 | 854.1 | 0.854 | 7.120 |
| 36 | 0.8448 | 843.9 | 0.844 | 7.035 |
| 38 | 0.8348 | 834.0 | 0.834 | 6.952 |
| 40 | 0.8251 | 824.3 | 0.824 | 6.871 |
| 42 | 0.8156 | 814.8 | 0.815 | 6.792 |
| 44 | 0.8063 | 805.5 | 0.805 | 6.714 |
| 46 | 0.7972 | 796.4 | 0.796 | 6.639 |
| 48 | 0.7883 | 787.5 | 0.788 | 6.565 |
| 50 | 0.7796 | 778.8 | 0.779 | 6.492 |
| 52 | 0.7711 | 770.4 | 0.770 | 6.422 |
| 54 | 0.7628 | 762.1 | 0.762 | 6.352 |
| 56 | 0.7547 | 753.9 | 0.754 | 6.285 |
| 58 | 0.7467 | 746.0 | 0.746 | 6.218 |
| 60 | 0.7389 | 738.2 | 0.738 | 6.153 |
| 62 | 0.7313 | 730.5 | 0.731 | 6.090 |
| 64 | 0.7238 | 723.1 | 0.723 | 6.027 |
| 66 | 0.7165 | 715.8 | 0.716 | 5.966 |
| 68 | 0.7093 | 708.6 | 0.709 | 5.907 |
| 70 | 0.7022 | 701.5 | 0.702 | 5.848 |
Which Number Goes Where
- API gravity is what US fuel paperwork and the Certificate of Analysis usually quote, and what you compare your reading with.
- Density in kg/m³ at 15°C is the international standard, and what Jet A-1 specifications outside the US are written in.
- Pounds per gallon turns a gallon count into weight, for weight and balance or for comparing a metered uplift against a weight on paper.
- Specific gravity is the bridge between all of them, and what some older forms and fuel slips still ask for.
Doing It on the Ramp
AvGrav does every one of these at once. Enter the observed API and temperature (or a density reading in kg/m³, if that's the hydrometer you have), and the result shows corrected API, SG, pounds per gallon, and kg/m³ at 15°C together. Add a fuel volume and it gives you net gallons at 60°F and the total weight. It works the same on Apple Watch, and it's now $0.99.
AvGrav and this guide are reference and convenience tools, not certified measurement instruments or a substitute for a calibrated hydrometer, thermometer, or your organization's official procedures. Always follow your organization's quality control program and applicable regulations before accepting, rejecting, or releasing any fuel.