Aircraft Fueling Calculator

Aircraft performance calculations — particularly weight and balance — require fuel weight, not volume. Fuel volume varies with temperature because liquid fuel expands and contracts: a US gallon of 100LL at 35°C weighs approximately 5.91 lb, while the same volume at 0°C weighs approximately 6.08 lb — a difference of nearly 3%. This is significant because at high fuel loads the difference can amount to 30–50 lb, which directly shifts the centre of gravity. Most aircraft fuel gauges read in volume (gallons or litres) not in pounds or kilograms, so pilots must convert. Fuel trucks dispense by volume at the fuelling temperature, not at the standard 15°C reference. For precise weight and balance calculations, especially on hot days or with large fuel loads, a temperature correction to the nominal density is important.

100LL avgas (100 Low Lead) has a standard density of 6.01 lb per US gallon at 15°C (59°F), or 0.7196 kg per litre. This is the value used in most US pilot training and the FAA Pilots Handbook of Aeronautical Knowledge. Some sources round to 6.0 lb/US gal for simplicity. At higher ambient temperatures the fuel is less dense — at 35°C the density is approximately 5.91 lb/US gal. At lower temperatures it is denser — at 0°C approximately 6.08 lb/US gal. The ASTM and IATA standard reference temperature for density measurement is 15°C. When fuelling on a very hot ramp, the fuel loaded at a given volume will weigh less than the nominal 6.01 lb/US gal figure suggests.

100LL (100 Low Lead) is the most common general aviation piston fuel worldwide. It contains a small amount of tetraethyllead as an anti-knock additive, is dyed blue, and has a motor octane number of 99.5. UL91 (Unleaded 91) is a lead-free alternative approved for use in aircraft whose engines do not require the higher octane protection. UL91 has a slightly lower density (0.71 kg/L vs 0.7196 kg/L for 100LL) and a lower energy content per unit volume. G100UL (General Aviation 100 Unleaded) is a newer high-octane unleaded alternative designed as a direct drop-in for all engines currently approved for 100LL. The practical fueling difference between 100LL and UL91 is small — a 40-gallon fuel load in 100LL weighs approximately 3.24 lb more than the same volume in UL91.

Fuel density decreases as temperature increases because thermal expansion causes the fuel to occupy more volume for the same mass. The correction factor for aviation fuels is approximately 0.08% per degree Celsius change from the standard 15°C reference. So at 35°C (ISA+20°C), fuel density is approximately 0.08% × 20°C = 1.6% lower than the reference value. For 100LL: 6.01 × (1 − 0.0008 × 20) = 6.01 × 0.984 = 5.91 lb/US gal. This effect is particularly relevant at tropical airports where ramp temperatures of 40°C+ are common. A pilot loading 50 US gallons on a 40°C day gets approximately 5 lb less fuel weight than the nominal calculation suggests — not operationally critical in most cases, but important for precise weight and balance on aircraft with limited payload margins.

A US gallon equals 3.78541 litres. An imperial (UK) gallon equals 4.54609 litres — approximately 20% larger. This distinction is critical in aviation because aircraft POHs and fuel dipstick charts from US manufacturers use US gallons, while UK fuel trucks and some European documentation may use imperial gallons or litres. A common fuelling error occurs when a pilot requests fuel in gallons without specifying US or imperial, resulting in either too little or too much fuel uplift. Always specify US gallons or imperial gallons explicitly at any airport where ambiguity may exist — particularly in the UK, Ireland, Canada, and at international airports serving multiple markets. The safest method is to specify fuel in kilograms or pounds — mass units are unambiguous regardless of temperature or unit system.

Specific gravity (SG) is the ratio of the density of a substance to the density of water at 4°C (1.000 kg/L). For 100LL avgas at 15°C, SG ≈ 0.7196 (density 0.7196 kg/L compared to water at 1.000 kg/L). For Jet A, SG ≈ 0.8041. Specific gravity is measured during fuel quality testing to verify the fuel has not been contaminated or mixed with another product. A fuel sample with an unexpected specific gravity reading is a red flag for contamination. Hydrometer or densitometer readings are taken during quality checks at the fuel farm and sometimes at the aircraft before boarding. Specific gravity combined with temperature gives the most accurate density for weight conversion — this is what bulk fuel suppliers use when calculating the mass of a delivered load.

Fuel endurance is calculated by dividing the usable fuel weight by the fuel burn rate in the same units. If the aircraft has 240 lb of usable fuel and burns 60 lb per hour, endurance is 240 ÷ 60 = 4 hours. Convert fuel flow from volume to weight using fuel density: if the aircraft burns 10 US gal/hr of 100LL at 6.01 lb/gal, fuel burn is 60.1 lb/hr. Deduct reserve fuel requirements (FAA VFR: 30 minutes at cruise power for day; 45 minutes at night; IFR: destination alternate plus 45 minutes) before calculating usable endurance. Always plan fuel for the flight, not just endurance — block time, wind corrections, and extended routing must be added to the planned fuel load before departure.

The clearest method is to specify fuel quantity in mass units (kilograms or pounds) to eliminate any ambiguity from volume unit differences (US vs imperial gallons) and temperature effects. "Please upload 150 kg of 100LL" is unambiguous. If requesting by volume, always specify US gallons or litres explicitly — never just "gallons". At airports where fuel pricing is per litre, you can request a specific spend or ask for "full tanks" if weight and balance permits. After fuelling, verify the uplifted quantity by checking the fuel gauges and comparing with the dipstick or totaliser. Discrepancies between requested and actual fuel should be resolved before departure. For large aircraft on commercial operations, the fuel order is prepared by dispatch and must be signed off by the flight crew.

The Cessna 172S has a total fuel capacity of 56 US gallons (28 per wing), of which 53 gallons are usable. At standard density (6.01 lb/US gal for 100LL), full usable fuel weighs 53 × 6.01 = 318.5 lb (144.5 kg). This is one of the most important constraints in the 172S: with two adults (340 lb) and full fuel (318.5 lb) the combined payload is 658.5 lb. The 172S useful load is approximately 860–900 lb depending on the specific aircraft empty weight. This leaves approximately 200 lb for baggage and additional passengers — commonly less than one additional adult. The 172 is a classic example of the "full fuel or full payload — not both" constraint. Before every flight in a 172, pilots must calculate which combination of fuel and payload fits within the useful load while keeping the CG in limits.

Total fuel capacity is the total volume the fuel tanks can hold. Usable fuel is the quantity that can actually be used during flight — the remainder is trapped in the fuel system sumps, lines, and lowest points of the tanks in positions from which it cannot reach the fuel outlet. On the Cessna 172S, total capacity is 56 US gallons but usable is 53 gallons — 3 gallons are unusable. Unusable fuel is still part of the aircraft weight and must be included in weight and balance calculations (it is already included in the aircraft empty weight as a fixed component). POH performance data for range and endurance uses usable fuel only. When calculating fuel weight for weight and balance, use usable fuel weight — the unusable fuel is part of the empty weight already accounted for in the empty weight figure.