Microsoft revealed today that it has sold more than 100 million licenses to Windows 8 in its first 6 months in the market, equaling the tally for its well-received predecessor, Windows 7, for the same time frame. But this means that Windows 8 is also tracking well below the average monthly sales of Windows 7 over its three years in the market.

"We recently surpassed the 100 million licenses sold mark for Windows 8," Microsoft Chief Marketing Officer Tami Reller said. "This number includes Windows licenses that ship on a new tablet or PC, as well as upgrades to Windows 8. This is up from the '60 million licenses' number we provided in January. We’ve also seen the number of certified devices for Windows 8 and Window RT grow to 2,400 devices, and we’re seeing more and more touch devices in the mix."

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Three years ago, Microsoft reported that Windows 7 had gotten off to “a strong start”: The firm sold more than 100 million licenses to that version of Windows in its first six months in the market. This week’s revelation mirrors that milestone perhaps a bit too closely: Microsoft has now sold more than 100 million licenses to Windows 8 in exactly the same time frame.

I’ve written repeatedly, however, that Microsoft averaged 20 million Windows 7 license sales per month over that OS’s three-year life span. So it’s reasonable to rate Windows 8 license sales against that particular number. And if you go back to the firm’s previous (and, to date, only) Windows 8 sales figure, from January 8, you’ll find that Microsoft claimed to have sold 60 million licenses at that time, three months into the OS’s lifecycle. Three times 20 million is 60 million—right on schedule.

But 100 million in 6 months, alas, is just 16.7 million licenses per month. Worse, if you're looking just at the three months since the last milestone, it’s only 13.3 million licenses per month. This means that Windows 8 experienced a temporary bump in its first quarter of sales because of the normal launch bump, but also because of a temporary upgrade offer during which Windows 8 Pro Upgrade could be had for next to nothing. Since then, Windows 8 license sales have fallen off sharply.

Granted, the first half of the year isn’t a traditional high point for PC sales, and of course many who might otherwise buy a Windows PC or device now are waiting for new models based on new hardware architectures as well as the rumors of mini-tablets. Those latter machines should provide a nice license sales bump, just as netbooks did for Windows 7.

But there's one more thing.

In addition to repeating the “20 million licenses per month” mantra as a measurement of Windows 8’s success, I’ve also pointed out that because Windows 8 targets a much bigger potential market than did Windows 7—which is to say, PCs plus hybrid PCs plus tablets plus other devices, instead of just PCs—that this figure should be considered a minimum for Windows 8 to be successful. Arguably, it should be much higher, especially considering the growth rate in the tablet market in particular. So, 16.7 million per month, let alone 13.3 million, just isn’t cutting it.

You’re going to see some interesting cheerleading pieces today from the usual suspects, as well as boilerplate doom-and-gloom stories from the anti-Microsoft crowd. The truth, as always, lies somewhere in the middle, and you should simply ignore the partisan attack dogs. Windows 8 is obviously a long-term play, and the story is still unfolding. This level of sales going forward—or, worse, the downward trend suggested by these numbers—is not acceptable. For now, we’re in a wait-and-see mode. And the coming quarters, as well as pending announcements about Microsoft’s plans to improve Windows 8 with “Blue,” will do a lot to help a clearer picture emerge.

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