The 2025 factors per passenger kilometer
The UK Department for Energy Security and Net Zero (DESNZ) publishes greenhouse gas conversion factors every year for company reporting. The 2025 set, published on 10 June 2025, gives air travel in kilograms of CO2 equivalent (CO2e) per passenger kilometer. Each factor comes in two versions: direct emissions only, and with the indirect effects of non-CO2 emissions such as water vapour, contrails and NOx, often called radiative forcing (RF).
| Flight | Class | g CO2e/km | With RF |
|---|---|---|---|
| Domestic (UK) | Average | 136 | 229 |
| Short-haul (UK) | Economy | 74 | 126 |
| Short-haul (UK) | Business | 112 | 189 |
| Long-haul (UK) | Economy | 69 | 117 |
| Long-haul (UK) | Premium economy | 111 | 187 |
| Long-haul (UK) | Business | 201 | 339 |
| Long-haul (UK) | First | 277 | 468 |
| International (non-UK) | Average | 84 | 143 |
The haul groups are defined from the UK: domestic means within the UK, short-haul within Europe, long-haul outside Europe, and international means flights between two non-UK destinations. The international factors are an average of short and long-haul flights. For trips that do not touch the UK, those are the ones to use. Per mile, multiply by 1.609344: international economy is about 104 grams per passenger mile without RF.
Two details matter when you use them. First, every factor already includes an 8 percent distance uplift for planes not flying the most direct route, so you multiply by the straight great-circle distance, which the distance between cities tool gives for any two places. Second, the UK domestic factor is the highest per kilometer in the table, so per kilometer a short domestic hop counts more than a long-haul economy seat.
Worked trips
Multiply the one-way distance by the factor. The examples use the international factors for economy and business, at round distances rather than specific routes. Double them for a round trip.
| One way | Economy | Economy + RF | Business | Business + RF |
|---|---|---|---|---|
| 800 km | 52 kg | 87 kg | 150 kg | 253 kg |
| 2,000 km | 129 kg | 218 kg | 374 kg | 633 kg |
| 5,000 km | 322 kg | 546 kg | 935 kg | 1,583 kg |
| 9,000 km | 580 kg | 982 kg | 1,683 kg | 2,849 kg |
Class changes the result more than most other choices. A business seat carries 2.9 times the economy factor in the international set, and first class 4.0 times. DESNZ explains why: the factors share a flight's emissions by the area of the plane each passenger takes up, and a cabin of business seats carries fewer people.
| International class | g CO2e/km | With RF |
|---|---|---|
| Economy | 64 | 109 |
| Premium economy | 103 | 175 |
| Business | 187 | 317 |
| First | 258 | 437 |
With or without radiative forcing
DESNZ applies the entire indirect effect to the CO2 part of the factor, which raises it by 70 percent. It recommends that organisations include the indirect effects to capture the full climate impact, and in the same paragraph says there is significant scientific uncertainty about how large they are. It also asks reporters to stay consistent and not switch between the two versions from one year to the next. If you compare two numbers for a flight, check first whether both include RF; a factor of 1.7 between two estimates is often only this choice.
How the ICAO calculator works
The International Civil Aviation Organization calculator takes another route. Its methodology (version 13.1, August 2024) starts from the airports, not from a factor per kilometer:
- Look up the scheduled flights between the two airports and the aircraft types that fly them.
- Map each aircraft to one of 336 equivalent types with a known fuel burn by distance.
- Compute the great-circle distance and add a correction: 50 km below 550 km, 100 km up to 5,500 km, 125 km above.
- Take the fuel for that distance, weighted by how often each type flies the route.
- Keep the passengers' share using a passenger-to-cargo factor, which counts each passenger as 100 kg with baggage plus 50 kg for seats, galleys and crew.
- Divide by the occupied seats, using the route group's load factor, and weight by cabin class with the Yseat factor.
- Multiply by 3.16, the tonnes of CO2 from burning one tonne of aviation fuel.
A worked example with numbers from the methodology's own appendices. Its fuel table gives 7,772 kg for the equivalent aircraft coded 320 over 1,000 nautical miles (1,852 km). For the route group Intra North America it lists a load factor of 79.1 percent and a passenger-to-cargo factor of 93.35 percent. Assume a hypothetical all-economy cabin of 150 seats, so about 119 are occupied:
7,772 × 0.9335 ÷ 118.7 × 3.16 = 193 kg CO2 per passenger
That is about 104 grams per passenger kilometer, CO2 only. Real routes use real cabin layouts, a mix of aircraft and their own load factors, so treat this as a demonstration of the formula, not a figure for any flight.
Why estimates differ
- Non-CO2 effects. ICAO counts CO2 only. DESNZ offers both, and the RF version is 70 percent higher on the CO2 part.
- Gas basis. DESNZ factors are CO2e, which adds small amounts of methane and nitrous oxide. ICAO's result is CO2.
- Distance. DESNZ adds 8 percent; ICAO adds a fixed 50 to 125 km.
- Averages. DESNZ gives one factor per haul and class. ICAO works per route, aircraft and airline seating.
- Connections. ICAO handles one flight number at a time; for a connection, it says to calculate each leg and add them up.
For a sense of scale next to driving, the guide car CO2 emissions per mile works through tailpipe and electricity emissions for cars. A car's total is shared by the people in it, just as a flight's is shared by its passengers.
Frequently asked questions
How much CO2 does a flight emit per passenger?
With the UK government 2025 factors for international flights in economy, about 64 grams of CO2e per passenger kilometer without the non-CO2 effects and about 109 grams with them. A 5,000 km one-way trip comes to roughly 320 kg or 550 kg. Business and first class are several times higher per passenger because the seats take more space.
Why is business class so much higher?
Both methods share out a flight's emissions by the floor space a seat takes. DESNZ explains that a cabin of business seats carries fewer passengers, so each one takes a larger share. ICAO does the same with its Yseat factor: a business seat with three times the area of an economy seat gets three times the share.
What is radiative forcing, and should I include it?
Planes also emit water vapour, nitrogen oxides and contrails at altitude, which add to warming. DESNZ publishes one set of factors with these indirect effects, adding 70 percent to the CO2, and one without. It recommends including them for organisations reporting air travel while noting significant scientific uncertainty about their size. ICAO's calculator counts CO2 only.
Do I need to add extra distance for detours?
Not with the DESNZ factors: they already include an 8 percent uplift for planes not flying the most direct route, so you multiply them by the great-circle distance. ICAO adds 50, 100 or 125 km to the great-circle distance depending on how long the flight is.
How do I count a trip with a connection?
Add up the legs. ICAO's calculator only handles one flight number and tells users to calculate each leg separately and add them. Use the great-circle distance of each leg, not of the whole journey.
Sources
- UK Department for Energy Security and Net Zero, Greenhouse gas reporting: conversion factors 2025, published 10 June 2025 (page with the condensed set, methodology and major changes)
- DESNZ, Conversion factors 2025: condensed set, sheet Business travel- air (kg CO2e per passenger km by haul and class, with and without indirect effects; 70% increase for non-CO2 effects; 8% distance uplift; haul definitions; why business class is higher)
- ICAO Carbon Emissions Calculator Methodology, version 13.1, August 2024 (great-circle distance and correction table, 336 equivalent aircraft, load factor and passenger-to-cargo factor, 100 kg plus 50 kg per passenger, Yseat cabin factor, the 3.16 factor, Appendix A load factors, Appendix C fuel table)
- ICAO, Carbon Emissions Calculator page (four cabin classes, passenger load factors and cargo in the methodology)