A new report calculates that over 100 million people now use an Apple Watch. That’s one out of every 10 iPhone users, an excellent attach rate.

“More than 100 million people wear an Apple Watch,” Above Avalon’s Neil Cybart writes. “Based on my estimates, Apple surpassed the important adoption milestone this past December.”

Cybart further estimated that 30 million of those 100 million users bought their first Apple Watch in 2020, thanks to the device’s focus on fitness and the late 2020 launch of the Fitness+ service.

That said, it took Apple 5.5 years to reach this milestone in part because the Apple Watch got off to a slow start. Apple had sold an estimated 10 million units in its first year, and just 8 million in year two. But the pace picked up from there, with total sales of 50 million units by 2018 and then 100 million this past year.

Apple Watch usage has also grown as a percentage of the iPhone user base. In 2015, just 3 percent of iPhone users owned an Apple Watch too. But that number grew to 6 percent in 2018 and then 10 percent by the end of 2020. Today, Apple Watch is the company’s fourth-largest product by installed base behind iPhone, iPad, and Mac. And Cybart predicts that Apple Watch usage could surpass that of the Mac by 2022.

Apple reported last month that its total installed base of iPhone users had surpassed one billion, so if one out of every 10 iPhone users also owns an Apple Watch, the 100 million figure certainly makes sense.