Some calculators allow a 'via' airport to be input. I thought it would be a good check to see if 'via' gives the same result as the sum of the two direct legs, eg: ZRH-PRG-PVG vs ZRH-PRG + PRG-PVG. To think of a reason how these two numbers could be different really requires some gymnastics, like, 'via' happens on one fuel efficient airline whereas two direct legs happen on different, less fuel efficient airlines. We also have to be aware of rounding errors and precision, since some calculators give emissions to the nearest 10 kg. As we see in the table below, mostly but not always the 'via' result is consistent with the sum of the two direct legs.
| Calculator | ZRH-PRG-PVG | SOU-MAN-EDI | EDI-BUD-IST | JFK-ORD-SFO | EWR-KEF-SVO |
|---|---|---|---|---|---|
| atmosfair | 0 ± 1 | 0 ± 1 | 0 ± 1 | 0 ± 1 | 0 ± 1 |
| carbon footprint | 0 ± 10 | 0 ± 10 | 0 ± 10 | -10 ± 10 | 0 ± 10 |
| ClimateCare | 0 ± 10 | 0 ± 5 | 10 ± 10 | 10 ± 10 | -10 ± 10 |
| FlyGreen | -10 ± 10 | 0 ± 10 | -10 ± 01 | 0 ± 10 | 0 ± 10 |
| myclimate | -49 ± 50 | -6 ± 1 | -6 ± 1 | -7 ± 1 | 43 ± 50 |
| mcl-luft/swiss | 0 ± 1 | 0 ± 1 | 0 ± 1 | -1 ± 1 | 0 ± 1 |
| south pole | 2 ± 1 | -89 ± 1 | 0 ± 1 | 14 ± 1 | 12 ± 1 |
Calculators accessed 2019-10-06.