Microsoft announced today that Windows 10 is now running on over 400 million active devices. This is up from 300 million as of May, and 207 million as of end of the March. The company says that it deems devices that have been active in the past 28 days as "active." Microsoft added that this 400 million active devices figure include tablets and phones as well as Xbox One consoles, HoloLens, and Surface Hubs running Windows 10. Paul Thurrott adds:Microsoft last provided a Windows 10 usage milestone on June 29, when it said that there were 350 million active Windows 10 devices. At that time, I noted that the Windows 10 adoption had accelerated from the previous milestone, hitting an average of almost 29 million new devices per month. But 50 million additional devices over three months is a much slower pace of about 17 million per month. This is the slowest rate since Windows 10 was first announced. Again, no surprise there: Windows 10 was free for its first year, and over that time period it averaged roughly 31.25 million new devices per month (if you assume a figure of 375 million after one year, as I do). Does this mean that Windows 10 will see fewer than 20 million new devices each month, on average, going forward? No, of course not. There's no way to accurately gauge how things will go, given that most future devices will be new PCs purchased by businesses or consumers, or business PCs upgraded to Windows 10.