The UK government publishes emission factors for flights two ways. One counts the fuel burned. The other multiplies the carbon dioxide by 1.7 to cover contrails and other effects at altitude, which makes the same flight about 70% higher. Google's Travel Impact Model, the model behind many of the estimates in Google Flights, leaves contrails out. Every one of these follows a published method.
That's one reason airline emissions reports don't match. Before you put a figure in a company report, you need to know which choices produced it. This guide covers the four that matter most, what to ask your data provider, and how to keep your numbers consistent from year to year.
Four choices behind every flight's number
Every flight emissions figure depends on the same four decisions:
- whether it counts warming effects beyond carbon dioxide
- whether it includes the emissions from producing the fuel
- how it splits the flight's emissions between cabin classes
- whether it uses your actual flight or an average for the route
1. Carbon dioxide only, or other warming effects too
Planes warm the climate in ways beyond their carbon dioxide, through contrails, water vapor and nitrogen oxides at altitude. How big that effect is remains uncertain, so calculators handle it differently.
- The UK's 2026 conversion factors recommend a multiplier of 1.7 as a central estimate, applied to the carbon dioxide only. The same paper calls the value "subject to significant uncertainty" and publishes factors both with and without it.
- Google's Travel Impact Model "provisionally" removed contrail effects from its estimates and doesn't yet include other non-CO2 effects. It now shows contrail warming risk as a separate rating.
- The ICAO calculator estimates fuel burned and multiplies it by 3.16 to get carbon dioxide. It doesn't add anything for other effects.
The GHG Protocol's guidance on business travel allows a multiplier, as long as you disclose the factor you used. Either choice is defensible. Mixing them isn't.
2. Burning the fuel, or producing it too
Some figures cover only what comes out of the engine. Others add the emissions from extracting, refining and transporting the fuel, which is known as well-to-tank. Google's model reports three numbers: fuel production, fuel burned, and the two together. Ask which one you're getting, because a provider that quotes the combined figure will look higher than one that quotes the engine alone, for the same flight.
3. How much more a business or first class seat counts
A business class seat takes up more of the plane than an economy seat, so it gets a bigger share of the emissions. How much bigger depends on the method.
Google's model splits emissions by floor space. On a wide-body plane, a business seat counts as four economy seats and a first class seat as five. On a narrow-body plane, both count as one and a half. The UK factors weight a long-haul business seat at 2.9 economy seats and first class at 4. ICAO also uses seat space, averaged across the airlines that fly the route.
On long flights, the cabin can move your number more than the multiplier does: a business seat counts as 2.9 to 4 economy seats, against a multiplier of 1.7. It's also the one choice your travel policy controls. Our piece on business class travel policy covers how the federal government decides who flies in which cabin.
4. Your actual flight, or an average
The most accurate figure uses the aircraft that flew and how full it was. Many calculators use averages instead.
ICAO estimates fuel burn from the scheduled aircraft and the great-circle distance between the airports, because most airline fuel data isn't public, then applies load factors from its own traffic data. Google's model uses US Department of Transportation data on how full US flights were, and assumes 84.5% where it has no data.
An average is fine for a first inventory. Once flying is a large part of your footprint, ask for figures based on the flights your people took.
Why the same flight can report nearly five times the emissions
These choices multiply. Inside the UK method, a long-haul business seat counts as 2.9 economy seats, and the version with the multiplier puts 1.7 on the carbon dioxide. Report one trip in economy without the multiplier, then in business with it, and the second figure is close to five times the first, because 2.9 times 1.7 is about 4.9.
Keep the two apart in your report: the cabin is the traveler's choice and the multiplier is yours. Otherwise a reader can't tell whether emissions fell because people traveled differently or because the method changed.
How three common methods compare
| Method | Other warming effects | Fuel production | Cabin split |
|---|---|---|---|
| UK government factors (2026) | Separate factors with a 1.7 multiplier on the carbon dioxide | Separate well-to-tank factors | By cabin; long-haul business counts as 2.9 economy seats |
| Google Travel Impact Model | Not included; contrail risk shown separately | Reported separately and combined | By floor space, by aircraft type |
| ICAO calculator | Not included | Not included | By seat space, averaged across airlines |
What to ask your data provider
- Which method and version do you use, and which year are the factors from?
- Do you apply a multiplier for other warming effects? If so, which one?
- Do your figures include fuel production?
- Do you use each traveler's cabin class, or an average?
- Is each figure based on the actual flight, or a route average?
- Can you export it per trip, by traveler and cost center?
The same questions belong in the questionnaire you send to airlines and your travel agency. Our travel supplier climate questionnaire has the full list.
Pick one method and keep it
A change of method can move your number even when nobody travels any differently. So choose one, write down the four choices above, and use them for every airline and every year. If you do switch, restate the earlier years so the trend still means something.
If you can't decide on the multiplier, report both figures, one without it and one with it. The extra line costs nothing, and nobody can say you picked the lower number.
Then put one sentence in your report that states those choices. For example:
Flight emissions are calculated with [method and version], [with / without] a [1.7] multiplier for non-CO2 effects, [including / excluding] fuel production, using each traveler's cabin class and [actual flights / route averages].
This will matter more soon. The California Air Resources Board has proposed that companies with more than $1 billion in annual revenue that do business in the state report business travel emissions from 2027, under the GHG Protocol's Scope 3 Standard. The rules aren't final. Either way, a method you can explain will make them easier to meet. Our guide to calculating business travel emissions covers the rest of the inventory.
FAQ: airline emissions data
Should we include radiative forcing in our flight emissions?＋
Either choice is defensible. The GHG Protocol allows a multiplier if you disclose the factor, and the UK government recommends 1.7 as a central estimate. What matters is using the same choice for every airline and every year.
Why is business class so much higher than economy?＋
Methods give a seat a share of the flight's emissions based on how much of the plane it takes up. In Google's model, a business seat on a wide-body plane counts as four economy seats.
Is the ICAO calculator good enough for corporate reporting?＋
It's a reasonable starting point. It estimates carbon dioxide only, from scheduled aircraft and average load factors, so it won't reflect the specific flights your travelers took or other warming effects.
Do we have to report flight emissions?＋
It depends on where you operate. The California Air Resources Board has proposed that companies with more than $1 billion in annual revenue that do business in the state report business travel emissions from 2027. The rules aren't final yet.
