In aerospace engineering, payload fraction is a common term used to characterize the efficiency of a particular design. The payload fraction is the quotient of the payload mass and the total vehicle mass at the start of its journey. It is a function of specific impulse, propellant mass fraction and the structural coefficient. In aircraft, it is standard practice to load less than full fuel for shorter trips to reduce weight and fuel consumption. For this reason the useful load fraction calculates a similar number, but based on the combined weight of the payload and fuel together in relation to the total weight.

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Propeller-driven airliners had useful load fractions on the order of 25–35%. Modern jet airliners have considerably higher useful load fractions, on the order of 45–55%.

For orbital rockets the payload fraction is between 1% and 5%, while the useful load fraction is perhaps 90%.

For payload fractions and fuel fractions in aviation, see Fuel Fraction.

• Tsiolkovsky rocket equation