“Phil Schiller stated (under oath) that when assessing sales for a new model of the iPhone, Apple used an easy shorthand: ‘Each new generation sold approximately equal to all previous generations combined,’” Horace Dediu reports for Asymco.
“In order to test this rule of thumb we need to know generational sales. Although we know overall unit sales for a given quarter, since Apple has been selling several versions simultaneously, we need to make some guesses about the ratio of generations in the mix. My guess is about 10% for 3G vs. 90% 3GS and 5% 3GS vs. 10% 4 vs. 85% 4S,” Dediu writes. “The assumptions show that… The 4S is only half way through beating the 4+3GS+3G+original. Assuming the current quarter’s sales roughly equal the last quarter’s, the total for 4S will reach two thirds of the sum of the previous generations (i.e. 100 million) by the end of September. Bearing in mind that the 4S is likely to remain in production at least one more year means it has a potential to come close to the target of 162 million.”
“This leaves us with the question of how many iPhone ’5′s will sell. As it stands today, the cumulative total would have to be greater than 263 million units,” Dediu writes. “Unfortunately, this is a moving target. As the 4S (and maybe 4) remain in production, the target continues to increase with time… If the iPhone ’5′ remains in production for at least as long as the 3GS then it might cross 200 million… Underlying all this is the question of a change in strategy toward more penetration (vs. current skimming.) That might allow for sustaining the 100% historic growth rate. If so then Apple will easily sell 250 million iPhone ’5′s.”