Although Microsoft has said its Windows 10 preview program has some 2.8 million participants, just over half of those are using the early version on a regular basis, according to Web metrics estimates.
For March, U.S.-based analytics company Net Applications pegged Windows 10's user share, a rough projection of the fraction of online users running the OS, at 0.09%, or nine PC users out of every 10,000.
That represented 0.1% of all Windows PC users, the slight difference due to the fact that Windows does not account for all personal computer operating systems, but instead about 91%.
Windows 10's user share of all Windows PCs last month was slightly less than Windows 8's 0.12% in March 2012, seven months before the latter's official launch. Microsoft ran a preview program for Windows 8 in the year prior to the OS's October 2012 on-sale date.
This time, however, Microsoft will ship the upgrade at least one, perhaps several, months sooner on the calendar than it did Windows 8: The Redmond, Wash. company has promised to release Windows 10 this summer, a wide window that could mean as early as June or as late as September.
Windows 10's user share translated into approximately 1.5 million users, assuming about 1.52 billion Windows PCs in operation across the globe.
Microsoft last gave an Insider participant count a month ago, when Stephen Elop, formerly the CEO of Nokia and now the head of Microsoft's Device and Services division, said there were 2.8 million registered testers.
The gap between 2.8 million Insider participants and 1.5 million Windows 10 users is not unusual in beta testing circles: More people download and try a preview than run it regularly after that. Some may have returned to their older OS after dipping a toe in the Windows 10 waters, while others may have installed it on a secondary system, or in a separate disk partition or in a virtual machine, and then use it intermittently.
Windows users can register with the Insider program on Microsoft's website to access the early version of Windows 10.