Fuel Density Calculator: Find kg/L & API Gravity

This fuel density calculator works out the density of a fuel sample in kilograms per liter (kg/L) and pounds per US gallon (lb/US gal) from its API gravity, or converts between density and API gravity directly. You enter the API gravity value (or density) and the calculator returns the corresponding density (or API gravity) at 60°F (15.6°C), the standard reference temperature for petroleum products. The result helps you convert fuel volumes to mass for weight calculations, cargo loading, and fuel consumption monitoring.

Formula

The calculator uses the standard API gravity definition from the American Petroleum Institute:

API gravity = (141.5 / SG) – 131.5

where SG is the specific gravity of the fuel at 60°F (15.6°C) relative to water at the same temperature. Rearranged to find density:

Density (kg/L) = 141.5 / (API + 131.5)

Since the density of water at 60°F is 0.9990 kg/L, the specific gravity is numerically almost identical to density in kg/L. To convert to pounds per US gallon, multiply by 8.3454 (the weight of one US gallon of water in pounds).

Worked Example

Using the default API gravity of 35°API:

• SG = 141.5 / (35 + 131.5) = 141.5 / 166.5 = 0.84985
• Density in kg/L = 0.84985 kg/L (rounded to 0.8498)
• Density in lb/US gal = 0.84985 × 8.3454 = 7.0923 lb/gal

The calculator displays 0.8498 kg/L and 7.0923 lb/gal for 35°API, matching the steps above.

Why Fuel Density Matters

Fuel is bought and sold by volume (liters or gallons) but its energy content and mass depend on density. A heavier fuel contains more mass per liter, so it delivers more energy per liter. For aviation, marine, and trucking operations, knowing density lets you:

• Convert fuel volume to mass for weight-and-balance calculations.
• Compare fuel purchases from different suppliers.
• Monitor fuel quality – density changes indicate contamination or blending.
• Calculate fuel consumption in mass per distance, which is more accurate than volume per distance.

How to Use the Calculator

Enter the API gravity of your fuel (typically 30–45 for diesel, 50–60 for gasoline, 35–45 for jet fuel). The calculator instantly shows the density in kg/L and lb/US gal. If you have a density value instead, you can convert it to API gravity using the formula: API = (141.5 / SG) – 131.5. For example, a diesel with density 0.85 kg/L has SG 0.85, so API = (141.5 / 0.85) – 131.5 = 35.0°API.

Common Mistakes and Misconceptions

• Using the wrong reference temperature: API gravity is defined at 60°F. If your sample is at a different temperature, you must correct it first. The calculator assumes 60°F.
• Confusing specific gravity with density: Specific gravity is a ratio (no units), density has units. They are numerically close in kg/L but not identical.
• Mixing US and imperial gallons: A US gallon is 3.785 liters; an imperial gallon is 4.546 liters. The lb/gal output here uses US gallons.
• Rounding too early: Round only the final result, not intermediate steps, to avoid errors.

Limits of the Formula

The API gravity formula is empirical and works best for petroleum liquids in the range of 10°API (heavy crude) to 70°API (condensates). It assumes a clean, single-phase liquid at 60°F. It does not apply to:

• Emulsions or fuels with water content
• Biofuels like ethanol or biodiesel (they have different density relationships)
• Liquefied gases (LPG, LNG) – they use different standards
• Very viscous or waxy fuels that may not be homogeneous at 60°F

Always measure density with a hydrometer or digital density meter for official transactions.

Practical Applications

In aviation, fuel density is used to convert fuel volume to mass for load sheets. In shipping, bunker fuel density determines the mass of fuel delivered. In road transport, fuel density affects fuel economy calculations – a truck burning 30 liters per 100 km with a fuel density of 0.84 kg/L actually consumes 25.2 kg per 100 km. The calculator helps you make such conversions quickly.

Frequently Asked Questions

What is the density of diesel fuel?

Typical diesel density ranges from 0.82 to 0.86 kg/L at 15°C, corresponding to API gravity of about 33–41. Winter diesel may be lighter (lower density) to prevent gelling.

How do I convert density to API gravity?

Use the formula API = (141.5 / SG) – 131.5, where SG is the density in kg/L divided by 0.9990 (water density at 60°F). For example, a fuel with density 0.85 kg/L has SG 0.851, so API = (141.5 / 0.851) – 131.5 = 34.8°API.

Does temperature affect fuel density?

Yes. Fuel expands when heated, so density decreases. The API gravity standard is at 60°F. For accurate work, measure the temperature and apply a correction factor (e.g., ASTM D1250) to convert to 60°F.

Why is API gravity used instead of density?

API gravity is a historical scale used in the petroleum industry because it correlates with fuel quality and refining characteristics. Higher API means lighter fuel. It is the standard in the US and many international markets.

Can I use this calculator for gasoline?

Yes, gasoline typically has API gravity of 55–60, which corresponds to density of about 0.72–0.74 kg/L. The formula works for all petroleum liquids in the valid range.

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in api | API gravity | °API | 35 | 0
out density_kgL | Density | kg/L | 141.5 / (api + 131.5) | 4
out density_lbgal | Density | lb/US gal | (141.5 / (api + 131.5)) * 8.3454 | 4
out sg | Specific gravity | - | 141.5 / (api + 131.5) | 4