Offset flight emissions figures begin with an estimate of the CO2 a trip releases per passenger, and the method behind that estimate shapes the number a traveller sees.
This page is part of the series on what is carbon offsetting, and it explains the calculation steps used by published methods rather than recommending any calculator or credit.
How a distance-based calculator works
The methodology published for the International Civil Aviation Organization (ICAO) calculator is a useful reference because it is public. According to the ICAO methodology document, the user enters the origin and destination airports along with the cabin class (economy, premium economy, business or first), and the tool works out fuel use from there.
In the same document, each aircraft on a route is mapped into one of the 336 equivalent aircraft types to calculate the fuel consumption for the trip, based on the Great Circle Distance (GCD) between the airports. The great circle distance is the shortest distance between points on the surface of a sphere, so it is a straight-line estimate rather than the path actually flown.
The calculator does not add up journeys with connections: the ICAO document states that the tool does not compute total emissions for journeys with different flight numbers. A trip with a change of plane therefore needs to be treated as separate flights.
Where the input data comes from
A calculator is only as current as its inputs. The ICAO document says schedule data are obtained from the global airline schedules data in the official airline guide, and that the flight schedule data are based on the latest available information and are updated on a two-month’s basis. Aircraft are matched to the routes actually scheduled, rather than to a single average plane.
Load factors are assigned by region. The same methodology assigns them to a city pair using 53 international route groups plus 11 domestic areas plus 11 intra areas, so a passenger on a thinly filled route is allocated a larger share of the fuel than one on a full route of the same length.
Why distance is corrected upward
Real flights do not follow the great circle exactly. The ICAO document explains that a correction is applied to include the emissions of distance flown in excess of the GCD, stacking, traffic and weather-driven corrections, and it shows the correction in a table.
| Great circle distance | Distance added |
|---|---|
| Less than 550 km | 50 km |
| Between 550 km and 5500 km | 100 km |
| Above 5500 km | 125 km |
The added distances in the table come from the correction factor listed in the ICAO methodology.
Passenger share, cargo and cabin class
A flight carries more than passengers, so the fuel has to be shared out. The ICAO document says the passenger load factor and the passenger to cargo factor, obtained from traffic and operational data collected by ICAO, are applied to obtain the proportion of total fuel used which can be attributed to the passengers carried.
Cabin class changes the share again. The ICAO method uses a seat factor called Yseat, which represents the ratio between the surface for a specific seat divided by the minimum surface for a seat, usually an economy seat. A seat that takes more floor area is allocated more of the fuel burn, which is why the same flight gives a larger figure for a premium seat.
From fuel to CO2
Once fuel burn per passenger is known, it is converted to CO2. The ICAO document says the result is multiplied by 3.16 to obtain the amount of CO2, in kg, attributed to each passenger travelling between the airports. It defines 3.16 as the constant representing the number of tonnes of CO2 produced by burning a tonne of aviation fuel.
The output of this step is a CO2 figure only. Methods that report a larger number usually do so because they add further effects on top of it, which the next section describes.
Non-CO2 effects and the radiative forcing multiplier
The European Commission notes that, in addition to CO2, aircraft engines release other gases such as nitrous oxides, sulphur dioxide, water vapour and particulate matter, and it states that these gases can lead to persistent contrail cirrus clouds, which also contribute to global warming by retaining the Earth’s heat.
Some reporting frameworks account for this with a multiplier. The UK government methodology paper for company reporting says that when using the direct conversion factor for aviation turbine fuel, applying a 1.7 multiplier to the CO2 is applicable to account for the radiative forcing effects of emissions at altitude. A calculator that includes such a multiplier and one that does not will give different totals for the same flight, so the method should be read before the numbers are compared.
From a number to credits
A calculated figure is only the starting point for an offset. The ICVCM explains that each carbon credit represents 1 tonne of CO2e, so the quantity of credits is tied to the tonnes in the estimate. What a credit actually stands for is set out in the guide to how carbon credits work.
Aviation also has an official offsetting scheme. The Commission describes the international measure as the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA), and the ICVCM guide adds that aeroplane operators are required to purchase offset credits to compensate for CO2 emissions exceeding an agreed baseline. That is a duty on airlines, separate from a passenger’s voluntary choice.
Official schemes alongside voluntary offsets
Voluntary offsetting sits next to rules that already apply to airlines. The European Commission explains that the EU emissions trading directive applies CORSIA to extra-European flights only, and that for flights within the European Economic Area, only the EU ETS will continue to apply. These are obligations on operators, so a passenger’s offset is a separate, optional step and does not replace them.
The Commission also notes that the eligibility of credits to be used for CORSIA offsets is subject to the participation of issuing countries in the Paris Agreement and in CORSIA, as well as a prohibition of double counting. That condition applies to the official scheme, and it shows why a traveller buying credits separately may still want to ask how double counting is prevented.
A traveller or business that publicises an offset should be careful about wording. The difference between offsetting and cutting emissions is explained in carbon neutral vs net zero, and anyone making public climate claims should check the advertising and consumer rules where they operate.
Frequently asked questions
Why do two calculators give different totals for the same flight?
They can use different distance corrections, seat allocations and non-CO2 multipliers. The UK methodology paper, for example, describes applying a 1.7 multiplier to the CO2 for radiative forcing, while the ICAO method reports CO2 only.
Does a connecting journey count as one flight?
Not in the ICAO tool, which does not compute total emissions for journeys with different flight numbers, so each flight is estimated separately.
What are contrails and why do they matter?
The European Commission says they are persistent contrail cirrus clouds formed by gases from aircraft engines that contribute to global warming.
How many credits match a calculated figure?
The ICVCM says each credit represents 1 tonne of CO2e, so credits are counted in tonnes.
