More than an estimated 100 million iPhones are being used in the United States, according to new data shared by Consumer Intelligence Research Partners [PDF]. At the end of the September 2015 quarter, more than 101 million iPhones were in use, and two-thirds of those iPhones were newer iPhones that have been released in 2014 and 2015.

An estimated 58 million of the 101 million iPhones in use were the iPhone 6 or 6 Plus, while four million were iPhone 6s and 6s Plus models. The iPhone 6, 6s, 6 Plus, and 6s Plus are the four iPhones that have larger 4.7 and 5.5 inch screens, and the 4.7-inch iPhone 6 appears to be the most popular model. CIRP's data was gathered in September, shortly after the iPhone 6s launch. iPhone 6s and 6s Plus numbers are likely much higher now.

"The analysis shows the continued strength of the iPhone 6 and 6 Plus, now one year old," said Mike Levin, Partner and Co-Founder of CIRP. "We estimate a combined 60 million of these flagship phones sold in the US from the September 2014 launch through September 30, 2015. For comparison, the iPhone 5S sold about 28 million in the same period in 2013-2014. With only a weekend of availability in the quarter, we estimate 4 million of the new iPhone 6s and 6s Plus sold in the US."

CIRP's data suggests that while the larger-screened iPhones make up a large chunk of the iPhones in use in the United States, iPhone adoption is slowing as the U.S. market matures. For the past eight quarters, the iPhone installed base grew an estimated average of eight percent, but the installed base grew four percent in the September 2015 quarter and six percent in the September 2014 quarter. Growth numbers were as high as 17 percent in the December 2013 quarter following the launch of the iPhone 5s and 5c.

As the U.S. market becomes saturated with smartphone owners, Apple has increasingly focused its attention on luring customers from Android and other competing platforms. Apple has introduced a new "Move to iOS" Android app, an Android-switching microsite, and a trade-in program for Android users looking to switch to an iPhone.

Apple's efforts have been successful, and in October, Apple CEO Tim Cook said Apple saw the highest rate of Android switchers it had ever measured in the fourth quarter of 2015. 30 percent of customers who were upgrading from an existing smartphone to an iPhone were former Android users.

CIRP's numbers are extrapolated from a September 2015 survey of 500 U.S. Apple customers who purchased an Apple product in the preceding quarter. CIRP analyzes data on iPhone buyers, their new model selection, and their prior phones and compares that to iPhone sales data adjusted to the U.S. market.