The conclusion reached by the Airports Commission was that a third runway at Heathrow would add roughly 240,000 ATMs per year to capacity.
What they did was divide the current number of movements by 2, and assume that would be how much capacity would be added. There simply isn't the airspace capacity to increase movements into and out of LHR by 51%.
Whilst we're on the subject, Gatwick has 18% more movements and 14% more passengers per runway than Heathrow, in it's current state - and Gatwick would have no airspace restrictions with a second runway.