The weight of airplane fuel, primarily kerosene-based Jet A or Jet A-1, is calculated differently than common liquids like water or automotive gasoline. Jet fuel’s volume is highly variable, making its mass the only reliable metric for flight planning. Aviation uses density—weight per unit volume—to manage this variability and ensure aircraft are safely loaded. This approach is necessary because an aircraft’s performance, balance, and structural limits are directly tied to the total mass carried.
The Standard Density of Jet Fuel
Aviation professionals use a standard density figure for initial planning of common jet fuel types, Jet A and Jet A-1. Density is the mass of the fuel contained within a specific volume. For planning, the industry approximates the weight as 6.7 pounds per US gallon at the standard temperature of 59°F (15°C). In metric terms, this is roughly 0.8 kilograms per liter, or 800 kilograms per cubic meter.
The actual density of Jet A-1 can legally range between 775 and 840 kilograms per cubic meter, based on the American Society for Testing and Materials (ASTM) specifications. This variation translates to a range of 6.50 to 7.01 pounds per US gallon at the reference temperature. These standard figures are averages, offering a quick reference point, but they are insufficient for the precision required during actual refueling operations.
Variables That Change Fuel Weight
Jet fuel density is not fixed and is constantly affected by temperature. Fuel expands when heated, meaning a given volume of warm fuel weighs less than the same volume of cold fuel. For instance, a gallon of jet fuel at 100°F may weigh 6.58 pounds, while the same gallon at 0°F could weigh 6.91 pounds.
This temperature-driven change requires precise weight calculation for every batch delivered. To achieve this accuracy, fuel suppliers use Specific Gravity (SG), which is a ratio comparing the fuel’s density to the density of water at 60°F. The SG figure acts as a multiplier, allowing ground crews to convert the volume of fuel pumped into the tanks into its exact weight, regardless of the ambient conditions during refueling.
Translating Weight into Flight Logistics
Aviation focuses on fuel mass rather than volume because aircraft performance is governed by weight and balance. The energy engines extract from the fuel is proportional to its mass, not its volume. Consequently, engine performance charts and fuel burn rates are calculated in pounds or kilograms per hour.
Flight safety calculations, including the aircraft’s center of gravity and structural stress, depend entirely on the total mass of the fuel load. The total fuel weight is a major component of the Maximum Takeoff Weight (MTOW), a structural limit the aircraft cannot safely exceed. Fueling based only on volume could cause a cold, denser load to exceed the MTOW.
Commercial aircraft use sophisticated Fuel Quantity Indicating Systems (FQIS) to manage this. These systems often incorporate densitometers to measure the fuel’s density in real-time within the tanks. The FQIS converts the volume measured by tank probes into an accurate mass reading, displayed to the pilots in pounds or kilograms, ensuring the aircraft remains within its certified operating limits.