Different emissions

Why do different offset calculators, calculate different emissions for the same route?

There are many different flight-emission calculators available on the internet (see e.g. Atmosfair, My Climate, Green Miles, Live Neutral and many more). Calculating the exact emissions of every single flight is almost impossible: quite apart from the different aircraft in operation, different routes are taken according to air-traffic and weather conditions and flights often have to fly several circles before being allowed to land (for more detail see: how precise are the calculations?).
Due to this multitude of factors that affect the emissions of an individual flight, average values are used for calculation (detailed calculations can be downloaded here). These average values apply to the type of aircraft (and accordingly, fuel consumption), but also the detour factor (factor with which the shortest distance between departure and arrival airport is multiplied to account for air traffic and weather conditions), the number of seats and the percentage of seats taken, which all have an affect on the emissions per passenger. The factor that accounts for non CO2 emissions is also applied differently.
Different flight-emission calculators use different average values when calculating the emissions of particular flights, which lead to different results. The detour factor also explains why different calculators also calculate differing distances for the same route.