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.
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.
What to verify next
- Approved aircraft documentation and current limitations.
- Local fuel, insurance, maintenance, hangar and financing quotations.
- The exact aircraft configuration, loading and maintenance status.