How carbon emissions are estimated

Google Flights shows carbon emission estimates next to each flight. Flights are labelled as higher, typical, lower or unknown emissions.

Where does Google get this information?

To generate carbon emission estimates, Google uses data provided by third parties such as airlines. The data includes aircraft type and aircraft seating layout, for example. In some rare cases, this data might be slightly different from reality due to various factors, including a late change of aircraft during operations.

Typical emissions

Typical emissions are the median carbon emissions for your searched route. The median is calculated as the middle value amongst all the possible carbon emissions per route, and considers all available dates and flights.

Carbon emission estimates for each flight are compared to the route's median. This is how Google identifies flights with higher, typical or lower emissions.

For some searches, you may find no 'lower emission' flights. This happens when the flights on your searched dates aren't less polluting than the route's median. To find lower emission flights, try different dates.

Unknown emissions

For some flights, we don't have emissions data available, nor are we able to make a close estimate. This might happen for a very specific aircraft type, for example. In these cases, we will not show any carbon emission estimates and the flight will be labelled as 'unknown emissions'.

Factors impacting carbon emissions

Actual carbon emissions may vary and depend on factors such as:

Aircraft model and configuration
Speed and altitude of the aircraft
Distance between origin and destination

The most commonly used aircraft types are supported. If we don't have estimates for a new aircraft, we'll use the closest estimate that we have available. As new aircraft enter the market and the scientific community advances their calculation methods, we'll continue to update our algorithms to the latest scientific insights and standards.

Actual carbon emissions between route options may vary and depend on a number of factors that we consider. To understand the carbon emission estimates that we display, it's important to know a few things.

Non-stop flights aren't always less polluting, especially for long routes. It is possible for a multi-stop flight on fuel-efficient aircraft to emit less than the non-stop option.
Aircraft with a similar capacity and range can have very different emissions. Contributing factors include the aircraft type or the seating layout used by the airline.
Our emission estimates don't yet consider factors such as direction of flight or the use of sustainable aviation fuel.

Google will continue revising carbon emission estimates over time to improve accuracy.

Train emission estimates

To calculate carbon emission estimates for trains, Google uses a very simple method that considers the kilometres travelled and number of passengers in your search. Carbon emissions for trains are likely to be overestimated. But they are still a reliable estimate, especially when comparing trains with flights.

Carbon dioxide (CO₂) equivalent emissions

A flight's carbon emissions are measured in CO₂e (CO₂ equivalent) – that's the carbon dioxide and equivalent greenhouse gases emitted by the aircraft.

Aircraft operations cause CO₂ emissions and non-CO₂ effects, such as contrail formation. The total output implies a higher contribution to climate change than carbon emissions alone. Estimates for non-CO₂ effects are based on lower bounds from scientific research, such as those found in Kärcher's 2018 paper published in Nature.

Emission estimates shown on Google Flights include all applicable CO₂e emissions and non-CO₂ effects.