How do you work out the calories?

tefleon

I went for a bike ride this morning and uploaded the completed route with both speeds and cadence onto two different websites. One was mapmyfitness and the other Strava.

Both these sites analysed the route and gave the same speeds, gradients etc. however the used calories are very different. One site was 431 and the other 628

How can it be so different using the same source data and other than using the lower just because it's lower, what do others do to establish calories burned during exercise?

Cheers