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Climate change

How we rank CO2 emissions

Flyrejser.dk commissioned the world famous KTH Royal Institute of Technology in Stockholm and Stockholm University to help develop a mathematical model to estimate C02 emissions produced by flights and a multitude of additional factors. With KTH’s help and expertise we hope to refine and develop the model and encourage others to contribute their data and research so that we can make it as accurate and as easy-to-understand as possible. Quite simply, we want to empower you to make informed decisions about the environmental consequences of flying. One very pleasing result from the study is that just because a flight is more expensive, it is does not automatically mean that it is better for the environment. It’s quite the opposite in many cases and means that even if you buy cheap flight tickets you can limit your environmental impact. But what you need to know is which cheap flight ticket you should buy.

The parameters

  • Fuel consumption on start, taxiing, take-off, flying at cruising altitude and approach.
  • Distance covered; an approximation of the actual distance, not just in a straight line.
  • If the aircraft is equipped with winglets (wingtips devices that control aircraft drag)
  • Increased effect of cloud formation when flying at high altitude
  • Seating configuration of relevant airline’s aircraft type on the route
  • Seat occupancy rate for the relevant airline
  • Carbon dioxide conversion factor to calculate the amount of CO2 per kg of aviation fuel

Impact of data quality

Data gathered from airlines, travel agencies and others is not always of the highest quality and we have used approximations based on relevant routes and airlines in this case. The airline and travel industry is improving at providing primary, real-time data and this will increase the quality of the calculations.

After analysing a huge amount of data on actual flight routes, aircraft types and configurations used on relevant routes, there exist approximated values on actual routes, airlines and aircraft types based on data being of deficient quality at the time of the user performing the search.

General conclusions

Fewer stopovers, more modern aircraft and more passengers per flight have less impact on the environment, as do airlines with modern fleets that configure their aircraft with more seats. But even a flight with an airline with older aircraft that is configured to carry more passengers, or is a direct flight, produces the same effect. You may think that this gets very complex and you would be right. That is why you should only use this tool for your planned trip to figure out the optimal conditions between your various travel options, based on airline, aircraft and actual route. This is why you should not arrive at the hasty conclusion that you should only ever choose one, particular airline. Instead you should take the whole picture into account when the model is applied in the search results.

Have a great trip!

You can download the full study (Master’s Thesis) from KTH and Stockholm University for more information. The thesis resulted in a paper published in Sustainable Environment Research in 2019. You can read the paper here.

Climate compensation

Our partner for Climate compensation is Go Climate Neutral.


Starry night sky over pine trees