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Flight emissions calculator comparison: Google, ICAO, Oncarbon

Why do different flight emissions calculators show different results for the same flight itinerary? Let’s compare methodologies of available flight emissions calculators, including ours.
Written by Team Oncarbon
Climate footprint data for travel products
August 29, 2022

Airline passengers today have no trouble finding out about the carbon footprint of their flight. But what are the differences between some popular flight emissions calculators? We decided to compare them.

Oncarbon flight emissions calculator

We at Oncarbon offer an automated flight emissions calculator for airlines and travel agencies. Our solution is built with the intention for it to be integrated into our customer’s online shops, providing regular consumers with emissions information of different flight options.

This is what it looks like on the website of the IMT travel agency:

Example of Oncarbon app in IMT web store.
Example of Oncarbon app in IMT web store.

In addition to just the emission numbers, passengers can view an emissions comparison to other possible connections and receive a full breakdown the parameters that go into and come out of the model – including a breakdown to a flight’s CO2 and non-CO2 emissions:

For these calculations, we follow our methodology, which we have described in detail here. In short, the calculation looks like this:

1. Calculation of fuel burn of the entire flight. To do this, we use four main parameters: plane model, the engine types installed in the aircraft, the great circle distance of the flight, and airport busyness of the departure and destination airports.

2. Conversion of fuel consumption into emissions. Burning one kilogram of jet fuel produces 3.157 kg of carbon dioxide, and we consider this ratio is constant. We also consider non-CO2 emissions and include them in our calculations: nitrogen oxides (NOx), water vapor, and contrails (increased cloudiness ).

3. Adding emissions generated during 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.

4. Attributing the warming effect to a seat on a flight. At this point, we have calculated the warming effect for an entire flight. We then calculate the CO2e emissions for one seat on that flight.

For more details on our method, including references to scientific articles, please see our flight emissions methodology.

Google Flight Emissions Calculator

Now let us take a look at Google’s tool for displaying flight emissions. When passengers search for flights through Google Flights, they can see the emissions of a particular flight and how they compare to the average. Google explains that “to create carbon emissions estimates, Google uses data provided by third parties, such as airlines. For example, the data includes the type of aircraft and the seating arrangement on the aircraft. In some rare cases, this data may differ slightly from reality due to various factors, including a late change of aircraft during flight operations.”

Google adds that the following factors are taken into account: aircraft model and configuration, aircraft speed and altitude, and distance between origin and destination. Google’s full methodology for calculating flight emissions can be found on Github1, where Google also states that its model doesn’t account for non-CO2 emissions – which is the subject of some recent controversy.

The main difference is that at Oncarbon we account for non-CO2 emissions and use the radiative forcing index (RFI). The RFI describes the rate at which aircraft emissions warm the atmosphere at high altitudes compared to associated CO2 emissions alone. A recent study concluded that a realistic RF index for today’s flights is 3.02, and this is the value we use in our calculations. This means that the total global warming caused by aviation is three times that of CO2 alone. Before, Google used the RFI value of 1.6, but in July changed its calculation model and removed the RFI from its model entirely. Consequently, Google severly underestimates the total climate footprint of a flight.

Also, Google assumes of how many people are going to board the plane (load factor set at 84.5%), and calculates emissions per passenger. At Oncarbon, we attribute the footprint to one seat rather then a passenger.

ICAO flight emissions calculator

Another calculator for aviation emissions is provided by ICAO (International Civil Aviation Organization). As described in its methodology3, ICAO only calculates the amount of direct CO2 released into the atmosphere during a flight. In addition, the ICAO calculator does not take into account emissions generated during fuel production. All these factors lead to significant differences between the estimated flight emissions.

Also, ICAO does not provide separate calculations for different plane models.

The following table illustrates the main differences in methodology:

As a result, emissions for the same flight route and aircraft can vary significantly. If we take a flight from London Heathrow to New York JFK with a Boeing 777, we arrive at the following results:

We would like to emphasize that there is no universal agreement on how aviation emissions should be calculated and that we still know too little about some complex processes in the Earth’s atmosphere. However, at Oncarbon, we provide our customers with accurate emissions calculations based on science and the best available data, and as an independent third party, we are able to verify miscalculations. We are also constantly working to improve our methodology to keep it up to date with the latest science.

If you’re interested in learning about our other features and see the demo, please book a call here. We'll be happy to answer all your questions.

Cover picture: 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.