The impressive increase came after Microsoft began what will likely be its last big push to put the free Windows 10 on customers' PCs, a campaign that started mid-May and featured a much-derided trick to get users to approve the upgrade from Windows 7 and Windows 8.1.
According to U.S.-based analytics vendor Net Applications, Windows 10 powered 19.4% of all Windows PCs in May, a 2.1-point increase from the month before. Net Applications measures user share -- an estimate of the percentage of the global PC population that runs a particular operating system -- by tallying unique visitors to clients' websites.
The 2.1-point rise in user share was the second largest one-month gain for Windows 10, bested only by August 2015's whopping 5.3 percentage points. Microsoft released Windows 10 on July 29 of last year.
With the big boost in user share, Windows 10 will easily surpass the 20% mark by the end of June, as Computerworld forecast four weeks ago. If the growth trend over the last 12 months holds up, Windows 10 will reach the 22% milestone -- 22% of all Windows PCs -- by the end of July, just days after Microsoft issues its next major upgrade for the operating system, which it has tagged "Anniversary Update."
Assuming Windows 10 makes reaches 22% of all Windows personal computers in its first year, it will have beaten the adoption record of 2009's Windows 7, which collected a 20% share in its first 12 months.
Windows 10 now powers approximately 292 million systems, according to Computerworld's calculations, which used Net Applications' numbers and Microsoft's oft-cited claim that 1.5 billion machines run Windows. That represented an increase of about 33 million in May, or just over 1 million each day. By comparison, nearly 4 million Windows PCs were upgraded to Windows 10 each day during the first complete week of its availability, Aug. 2-8, 2015.
In early May, Microsoft claimed that 300 million "active devices" were then running Windows 10. Unlike Net Applications, which tallied only personal computers, the Redmond, Wash., company also counted tablets, smartphones, the Xbox One video game console and other devices that run Windows 10 or a variant.
Other sources confirmed Windows 10 growth during May.
Irish vendor StatCounter pegged Windows 10's global usage share -- a different metric than Net Applications' that reflects online activity -- at 20% for May, an increase of 2.1 percentage points from the month prior and the largest gain since September 2015. Meanwhile, the Digital Analytics Program (DAP) tapped Windows 10 at 25.3% of all Windows PCs, an increase of 1.5 percentage points over April.
DAP tracks visits to more than 4,000 websites run by the U.S. government, so its data is highly U.S.-centric. Because the U.S. has been a stronghold of Windows 10, the smaller increase measured by DAP may be due to the already-brisk uptake of the new OS, and thus a smaller pool of customers who haven't accepted the free offer.
May's robust gains may have stemmed from Windows users reacting to Microsoft's May 5 notice that it will end the free upgrade offer on July 29. By restating the deadline, Microsoft may have prompted large numbers of laggards to get in under the wire.
A more likely cause, however, would be the mid-month push by Microsoft, the latest in a long series of campaigns, which switched the automatically-offered Windows 10 upgrade to "Recommended" in Windows Update. That, in turn, scheduled the upgrade process unless the user interfered.
Those users faced a dialog box that reinterpreted a click on the red "X" in the upper-right corner as approving the scheduled upgrade, even though closing a window or notification by clicking the X has been defined for decades as a rejection of the offered action. Microsoft broke its own design rules to assume that closing the scheduling notice meant the customer authorized the upgrade. A significant number of the reports of Windows 10 installing without user approval were probably due to Microsoft's counter-intuitive interpretation.
So while Microsoft may have added more Windows 10 PCs to its ongoing tally -- the company last year set a goal of 1 billion Windows 10 devices by mid-2018 -- the ham-handed approach infuriated those who saw it as deceptive.