Over 270 million users have either upgraded to Windows 10 or bought a new Windows 10 PC since the operating system shipped in July of last year, a number that Microsoft says outpaces Windows 7 adoption by 145 percent. Microsoft CEO Satya Nadella says that the operating system is the "fastest growing version of Windows for both consumers and enterprises," and Windows and Devices group VP Terry Myerson says it has been used for more than 75 billion hours.
Windows 10's adoption rate has been brisk since it was released thanks to positive buzz and the return of the Start menu: there were 14 million upgrades in the first day, 75 million in the first month, 110 million installs by early October, and 200 million by early January. Developers looking to build Universal Windows Platform (UWP) apps for the Windows Store should be heartened by these numbers—the install base is already quite large, and it's continuing to grow.
Of course, Windows 10 adoption has also been helped along greatly by the fact that the operating system is available as a free download to all Windows 7 and Windows 8 users (in a handful of cases, it has been installed on systems whether users wanted it or not). The free upgrade is scheduled to end in July barring some extension from Microsoft, so we may see this relatively brisk adoption rate drop off after that.