During the opening keynote at Build 2017, Microsoft CEO Satya Nadella announced that there are now over 500 million active Windows 10 PCs worldwide. The previous milestone was 400 million, from last October.

Microsoft told me ahead of Build that it “felt really good” about the adoption rate of Windows 10, and as I’ll demonstrate below there is good cause for this.

But a little math does show two disappointing trends: Windows 10 adoption has only slowed over the past year. And it is dramatically slower than the adoption of Windows 7, month-to-month, over its first three years.

That last comment requires a bit of a caveat: When Windows 7 was still the current Windows version, Microsoft massaged the way it reported Windows license sales in order to make them more consistent month-to-month. So Windows 7 adoption rose by 20 million units per month for three straight years.

Windows 10 has been in the market for less than two years (22 months), and there are 500 million active devices. At two years, there were (allegedly) 480 Windows 7 million sales. Does that mean Windows 10 is selling at a faster clip than did Windows 7? No, because Microsoft held back on reporting on licenses sold so that it could backfill later when sales slowed. But Microsoft is right to be pleased by Windows 10 uptake, given that the PC market has shrunk since 2009-2010, and that users are not upgrading as quickly.

But yes, it’s also fair to say that Windows 10 adoption has only slowed, and dramatically, over time. (I do realize that “license sales” and “active devices” are two different things. But I also think they’re roughly comparable.)

During its first year in the market—yes, when it was free—Windows 10 was adopted at an average rate of over 31 million new devices per month. It makes sense that adoption would be high in the beginning and, in this case, during a temporary free promotion.

From there, adoption slowed to 29 million per month and then 17 million. With this new figure, we can see that the number of new Windows 10 devices coming online per month, on average, is now at a bit over 14 million units.

You may recall that Microsoft had at one time expected to hit 1 billion active Windows 10 devices within 2-3 years of the initial release. At the current rate, it will take Microsoft 36 more months, or three years, to hit that target. (That’s a total of almost five years.) But nothing is ever that stable: Enterprise adoption could speed things up. Or the continued decline of the PC could weigh on the monthly average. There’s just no way to know.

Anyway, Microsoft is right to be pleased. For a mature product in a declining market, Windows 10 is doing pretty damned well.