Free tool
What does one flight put into the air?
Distance and cabin class drive most of the difference. The multiplier for non-CO2 effects is stated openly so you can remove it.
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The assumptions, stated plainly
The estimate uses 0.09 kg of CO2 per passenger-kilometre for economy, a cabin multiplier (premium 1.6, business 2.9, first 4.0) and a radiative-forcing index of 1.9 to account for non-CO2 effects at altitude. These are commonly used planning figures, not a measurement of a specific flight.
Real emissions depend on the aircraft, load factor, route, altitude and fuel. Short flights have higher per-kilometre emissions because take-off and climb are amortised over fewer kilometres.
What to do with the number
Use it to compare options, not to buy absolution. A round-trip long-haul business-class seat can exceed several tonnes; the useful comparison is against the same trip in economy, or against a train on a route where one exists.
Frequently asked questions
Does the calculator include non-CO2 effects?
Yes, through a radiative-forcing multiplier of 1.9. If you want CO2 only, divide the result by 1.9.
Why does cabin class change the estimate by so much?
A premium seat occupies more floor area, so a larger share of the aircraft's emissions is allocated to it. The multipliers are allocation assumptions, not measurements.
Is a round trip simply double?
For the same route and aircraft, broadly yes. The tool has a round-trip output that doubles the one-way figure.
Can I offset the result?
Offsets vary widely in quality and price. Estimate the emissions first; the number is the part you can check.