In humans, live births tend to follow nine months after conception. The gestation period of an iOS device is closer to that of a whale or rhinoceros: new model Macs, iPads and iPhones tend to arrive 12 to 15 months after a bump in capital expenditures (plants, equipment, etc.).
Actual shipments: 247 million, according to the Form 10-K that Apple filed Monday. Or 251 milllion if you count the Apple TV.
On Tuesday, Dediu used this year's anticipated Cap Ex spending -- $12.4 billion, not counting retail -- to predict next year's iOS shipments.
Result: 310 million iOS devices, give or take a few million Apple Watches.