Almost every four-seat aircraft is really a three-seat aircraft with full tanks: the marketing seats and the full fuel capacity rarely work at the same time. This calculator subtracts occupants, baggage and fuel weight from the useful load to show what margin genuinely remains, before the aircraft tells you on the takeoff roll.
How this calculator works
Payload remaining = useful load minus occupants, baggage and usable fuel weight.
Worked example
With 520 kg of useful load, 300 kg of occupants and 35 kg of baggage, loading 120 litres of avgas (about 86 kg) leaves a margin of roughly 99 kg. That is one more light passenger or another 130 litres of fuel, but not both, which is exactly the trade the brochure never mentions.
Who and what fits, against fuel loaded
Every litre of fuel is weight that cannot be a passenger. This grid takes a 520 kg useful load, the calculator's opening figure and a fair value for a four-seat piston single, and shows what is left after fuel, baggage and occupants. A negative figure is an aircraft that cannot legally depart in that configuration.
| Occupants | 60 L of avgas | 100 L of avgas | 140 L of avgas | 180 L of avgas |
|---|---|---|---|---|
| 1 pilot, 85 kg | 357 kg spare | 328 kg spare | 299 kg spare | 270 kg spare |
| 2 people, 170 kg | 272 kg spare | 243 kg spare | 214 kg spare | 185 kg spare |
| 3 people, 255 kg | 187 kg spare | 158 kg spare | 129 kg spare | 100 kg spare |
| 4 people, 340 kg | 102 kg spare | 73 kg spare | 44 kg spare | 15 kg spare |
What the table holds fixed
- Useful load 520 kg, baggage 35 kg, avgas at 0.72 kg per litre.
- Useful load is specific to the airframe and its equipment fit, so take yours from the weight and balance sheet rather than from a type average. PlaneFit stores payload net of typical mission fuel, which is a different figure.
- Being inside the useful load is not the same as being inside the centre of gravity envelope. Both have to be checked.
Frequently asked questions
What is the difference between useful load and payload?
Useful load is everything the aircraft can lift above its empty weight: people, baggage and fuel together. Payload is what remains for people and baggage after the fuel you need is on board. A generous useful load with big tanks can still translate into a small full-fuel payload.
Can I fly with full fuel and full seats?
In most light aircraft, no, and the type's honest answer is one of the most useful facts to know before buying. Sellers quote maximum seats and maximum range in the same sentence; weight and balance mathematics only ever grants one of them at a time.
How do I convert fuel litres into kilograms?
Multiply by the fuel density: avgas is about 0.72 kg per litre and Jet A about 0.80 kg per litre. So 120 litres of avgas weighs roughly 86 kg while the same volume of Jet A weighs about 96 kg, a difference worth respecting when switching between types.