What does an assumed fuel density change in a mass conversion?

Where the density enters

The unit screen's fuel converter works in two stages. A volume is first converted to US gallons with the exact definition 1 US gal = 3.785411784 L. The gallons are then multiplied by a density in pounds per US gallon to give pounds, and pounds can be converted on to kilograms with 1 lb = 0.45359237 kg. Going the other way divides by the density. A litre-to-gallon or pound-to-kilogram conversion needs no density at all.

The density is chosen by fuel type: 6.0 lb/US gal for avgas and 6.7 lb/US gal for Jet-A. The app labels these as standard planning densities and notes that actual density varies with temperature.

Worked example

Ten US gallons at an assumed 6.0 lb per US gallon gives 60 lb. The arithmetic does not establish that a particular ten-gallon quantity actually has that measured mass.

Planning mass (lb)

60

Assumed density (lb/US gal)

6

Convert a fuel quantity

1. Pick the fuel type, which fixes the planning density.
2. Enter the quantity and its unit: US gallons or litres for volume, pounds or kilograms for weight.
3. Choose the target unit and read the result together with the density line in the steps.
4. Carry the density note into any weight-and-balance row that uses the converted weight.

Three conversions with one rule

Ten US gallons of Jet-A at 6.7 lb/US gal is 67 lb. A hundred litres of avgas is 100 ÷ 3.785411784 ≈ 26.417205 US gal; multiplied by 6.0 that is about 158.503231 lb, or about 71.895856 kg after the pound-to-kilogram step.

10 US gal of Jet-A (lb)

67

100 L expressed in US gallons

26.417205

100 L of avgas (lb)

158.503231

100 L of avgas (kg)

71.895856

The same 100 L converted with the Jet-A density would weigh about 177 lb, so choosing the wrong fuel type changes the result by more than 18 lb.

Common mistakes

• Treating an Imperial gallon as a US gallon; the converter's gallon is the US gallon only.
• Leaving the fuel type on avgas while converting a turbine-fuel quantity.
• Assuming the planning density is the measured density of fuel on a hot or cold day.

Scope

A planning density gives a consistent estimate, not a weighed result. The NIST conversion table below defines the gallon, litre and pound used here; the fuel supplier or aircraft documentation gives a measured density.

Primary sources and related work

• FAA Pilot’s Handbook of Aeronautical Knowledge
• NIST SI conversion factors