How this calculator works
Burning one kilogram of jet fuel releases about 3.16 kg of carbon dioxide, and a modern wide-body in cruise burns a few tonnes of fuel an hour. Per passenger, the figure depends on how many seats are filled and how the airline splits the plane, so the cleanest public figures are per passenger-kilometre at a stated load factor. The UK conversion factors give about 0.148 kg CO2e per passenger-kilometre for long-haul economy, with a radiative forcing multiplier that accounts for the extra warming from contrails and nitrogen oxides. At a typical 800 km/h cruise this is about 118 kg CO2e per hour in the air. Dividing by the occupancy divided by 0.8 adjusts for flights that are emptier or fuller than the reference cabin, and a return trip doubles the total. The result is an estimate of economy-class travel: business and first class occupy more floor area and carry roughly two to four times the per-passenger emissions.
Formula
distance (km) = hours x 800 x trips, where trips is 1 for one way and 2 for a return. emissions (kg CO2e) = hours x 118 x (0.8 / occupancy) x trips. Time within a budget = budget / (118 x 0.8 / occupancy). The share of the budget is emissions / 2300.
Worked example
A seven-hour one-way flight at 80 percent occupancy: 7 x 118 x 1 = 826 kg CO2e, over 5,600 km at 800 km/h. That is 36 percent of a 2,300 kg annual budget. If the cabin is only 60 percent full, the per-passenger figure rises by 0.8 / 0.6, to 1,101 kg CO2e.
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Frequently asked questions
Why multiply by 0.8 and divide by occupancy?
The published emission factor is measured at a reference load factor of about 80 percent, meaning each passenger is credited with 80 percent of a seat. On a plane that is 60 percent full, the same fuel is spread over fewer passengers, so each one carries more: the factor is multiplied by 0.8 / 0.6.
What is the non-CO2 multiplier?
Aircraft also warm the climate through contrails, water vapour and nitrogen oxides. The DEFRA factors include a multiplier of about 1.9 for these effects on long-haul flights, which roughly doubles the CO2-only figure. Some researchers argue the multiplier is uncertain and could be higher or lower.
Does flying economy really matter?
It is the single largest item in most personal footprints. One return long-haul flight can exceed the entire annual carbon budget of an average person in many countries, which is why a small number of trips dominates the total.
Can I compare flying with driving?
Yes, but use distance rather than hours. A petrol car with one occupant emits roughly 0.17 to 0.25 kg CO2e per kilometre, so a 7-hour flight at 800 km/h is closer to a 5,600 km drive for one person. Filling the car changes the comparison.