How we calculate your flight
Every figure in the calculator comes from the same five steps: distance, emission factor, what the factor includes, price, and how other ways to travel compare. Here is each one, with the numbers we use.
1. Distance
We measure the great-circle distance between the departure and arrival airports, the shortest path over the Earth's surface. A return trip counts the distance twice. Real routes are usually slightly longer, which the emission factors allow for.
2. Emission factor
We multiply the distance by an official UK DESNZ conversion factor for one economy passenger. Shorter flights emit more per kilometre, because take-off and climb burn the most fuel.
3. What the factor includes
The factor covers the full climate impact of the flight, expressed in CO₂ equivalents (CO₂e). The shares below are those of a short-haul flight; the breakdown in the calculator shows your own flight's.
- Burning the fuel (TTW) 51% Tank-to-wake. CO₂ and other greenhouse gases from combustion in the engine.
- Producing the fuel (WTT) 14% Well-to-tank. Extracting, refining and transporting the fuel to the aircraft.
- High-altitude effects (RFI) 35% Contrails and nitrogen oxides warm more at cruise altitude. DESNZ recommends a 70% add-on to combustion.
4. From emissions to price
The flat rate covers both parts of the switch: SAF replacing the fossil fuel for the combustion emissions, and Gold Standard credits for the rest. Every batch of SAF is certified under ISCC (International Sustainability & Carbon Certification) or RSB (Roundtable on Sustainable Biomaterials) with at least 75% lower lifecycle emissions than fossil jet fuel.
5. Comparing ways to travel
Emissions per passenger-kilometre over the same distance, from the same DESNZ dataset. Cars assume two people on board.
Limitations
This is an estimate, not a measurement. The aircraft, load factor, weather and route on the day all change real emissions. High-altitude effects are the largest uncertainty: depending on the flight, the real effect can be anywhere between 0 and 200% of combustion.