How do we calculate flight emissions at Oncarbon?

Flight carbon emissions calculators can show different results for the same flight itinerary due to various methodologies applied. This is how our flight emissions calculator works at Oncarbon.
Written by Team Oncarbon
Climate footprint data for travel products
August 12, 2022

There are many online flight emissions calculators, but you may wonder why they show different results for the same flight itinerary. The reasons may be due to different input data or calculation methodology. Some emissions calculators also take into account more factors than others or use different coefficients.

In this article, we explain how Oncarbon calculates the carbon footprint of a flight – and what factors we take into consideration compared to other popular flight emissions calculators.

How do you calculate the carbon footprint of a flight?

Step 1: calculate the fuel burn of the entire flight

To calculate the carbon footprint of a flight, you need to know how much fuel the plane uses. For this estimation, we use information on:

  • Aircraft type

  • Engine models

  • Airport busyness factors

  • The great circle distances between origin and destination, and any stops along the way

What is the airport busyness factor?

In our calculation model, we take into account how busy the airport is at the origin and destination, as this influences the taxi out, taxi in, and approach times of the aircrafts, which affect the overall carbon footprint of the flight.

What is the great-circle distance?

Flight distances are estimated as the great circle distance between two locations, which is the shortest distance between two points on a sphere (an approximation of the Earth) and is derived from the departure and destination location coordinates using a mathematical formula.

In reality, the exact amount of fuel burned during the flight depends on also on some other factors which are hard to know before the flight has even taken off.

“The amount of fuel burned can depend on many factors: the length of the flight, the speed, the altitude, the type of fuel used, and even the wind.” Julia Bondarchik, Chief Data Officer and Scientist at Oncarbon

Step 2: Convert fuel burn into emissions

When fuel is burned in an aircraft engine, the chemical reaction produces several products, one of which is carbon dioxide (CO2). Burning one kilogram of jet fuel produces 3.157 kg of carbon dioxide, and we treat the ratio as constant. Therefore, converting fuel combustion into direct CO2 emissions is a simple matter.

CO2 is a major greenhouse gas, but atmospheric warming effects also happen from the release of nitrogen oxides (NOx), water vapor and contrails (increased cloud cover). All of these emissions cause more warming at high altitudes compared to if the same amounts were emitted at sea level.

Radiative Forcing Index

The warming effect of these non-CO2 factors is accounted for by the Radiative Forcing Index (RFI) 1, which describes the rate at which in-flight emissions warm the atmosphere compared to the CO2 emissions alone.

Recent studies conclude that a realistic RFI for today’s flights is 3.0, although previous estimates use indices between 1.9 and 3.0. At Oncarbon, we use the value from the most recent research on this topic – the RFI of 3.0.2

The warming potential of these non-CO2 effects is communicated as carbon dioxide equivalents (CO2e). CO2e is a yardstick that compares the warming potential of non-CO2 effects to the warming potential of CO2: what is the amount of CO2 that would have the same warming potential as these non-CO2 effects have?

Step 3: Add the emissions that result from fuel production

Not all carbon footprint calculators include the emissions that result from fuel production. Fossil fuels do not just cause emissions when they are burned – large amounts of CO2 and other gasses are also released during extraction, drilling, refining, and transportation. At Oncarbon, we use a constant of 0.617 kg of CO2 for these emissions per kilogram of jet fuel produced.

Step 4: Attribute the warming effect to one seat of a flight itinerary

At this point, we have calculated the warming effect for an entire flight. We then calculate the CO2e emissions per one seat on that flight. To do this, we use an estimate of the amount of cargo on the flight based on information about the movement of global air cargo between geographic regions, and attribute part of the emissions to this cargo. For the rest of the emissions, we attribute them to the seats installed on the aircraft – more to a first class and business class seat than to an economy class seat.

Why do we base our model on the number of seats rather than the number of passengers? First of all, it’s not simple to predict how many passengers will actually board a flight that will happen in the future. Moreover, calculating emissions per passenger could lead to a logical dead end: if there is only one passenger on an otherwise empty plane, should all of the emissions be put into this passenger’s personal carbon account?

Example of flight emissions calculations

Let’s calculate flight emissions for a round-trip flight from London Heathrow to New York JFK on American Airlines. The airline typically uses Boeing 777 300ER’s for this route, the distance is 5,665 km and the number of seats on the plane – 304. The plane carries medium amount of cargo. The estimated fuel burned of that flight: 54,898 kg per entire flight one way.

After converting fuel burn into emissions, adding the emissions that result from fuel production and attributing them to one seat on this plane, we arrive at 1396 kg of CO2e per seat for one way, and 2792 kg of CO2e per seat per round trip.

Carbon footprint of a flight London-New York
The Oncarbon calculation of the carbon footprint of a return flight from London Heathrow to New York.
Carbon footprint of a flight London-New York
Emissions according to flight phases.

Other carbon footprint calculators

There are many flight carbon footprint calculators that can give you an estimated carbon footprint of your flight. However, most of them do not take into account all the components we consider, or use a slightly different methodology, so you may get different results.

ICAO carbon emissions calculator

For example, the ICAO (International Civil Aviation Organization) Carbon Footprint Calculator will give different results for the LHR-JFK itinerary that we used as an example here. According to ICAO, one passenger will emit only 630.5 kg of CO2 on this flight. Why the difference?

ICAO flight emissions calculator

ICAO doesn’t state which aircraft type they use for the calculation, and calculates only the amount of direct CO2 emitted into the atmosphere during a flight.

It doesn’t account for Radiative Forcing effects (RFI), which we mentioned earlier. Also, ICAO’s calculator doesn’t include emissions that result from fuel production. All of the above factors lead to significant difference between estimated flight emission.

If you’d like to learn more about differences in calculations and our methodology, book a demo – we’d be happy to show you how our model works in practice.

Original cover photo: USGS/Unsplash.

Team Oncarbon
We’re on the mission to show you that we need – and we can – travel more consciously. We bring closer to you the topics of carbon footprint, sustainable travel and aviation, and transparency in emissions reporting.