In a report issued Friday, Consumer Intelligence Research Partners estimated that Apple’s installed base of 94 million iPhones includes 25 million iPhone 6 models and 13 million iPhone 6 Plus.
The installed base includes 26 million iPhone 5s models, but these are at most 18 months old. It also includes 8 million iPhone 4S phones, dating back to 2011.
“While they are ripe for upgrade,” says CIRP’s Josh Lowitz, “many of these consumers have owned their iPhone 4S for only a fraction of its three and a half years on the market. The size of this upgrade cohort, relative to the number of new iPhone 6 and 6 Plus units sold, suggests that the coming quarters may prove challenging for Apple in the US.”
Follow Philip Elmer-DeWitt on Twitter at @philiped. Read his Apple (AAPL) coverage at fortune.com/ped or subscribe via his RSS feed.
