If you listen to Windows Weekly or What the Tech, you know that I’ve regularly described a future in which Microsoft and its Windows products split a more heterogeneous general computing market with Android and iOS. The result will be much less market share but, I think, a bigger overall market for Microsoft as well. But I’m not the only one thinking this way: Forrester is on board with this vision now as well.

I read about Forrester’s predictions for the future of what it calls the “personal computing devices” market—which includes smart phones, tablets, and PCs—in the Wall Street Journal. But I was particularly taken by a chart that I think tells only half the story.

It looks like so.

personal-computing-devices-

In this chart, we can clearly see that Microsoft dominated the personal computing devices market of 2008 since it consisted mostly of PCs. But as we switch to a mix of smart phones, tablets, and PCs going forward, Microsoft’s share—in Forrester’s predictions—drops down to about one-third of the market this year and in 2016.

I don’t have an opinion about the actual share numbers—again, if you’ve heard me speak about this, I’ve stated that something akin to an even split between Windows, iOS, and Android seems to make sense—nor do I have a stake in it. It is what it is.

But there’s a bigger story here that isn’t addressed by the Forrester report, or at least by the WSJ article about that report. And that’s about the size of this new market of personal computing devices. How does it compare to the PC-only market that Microsoft now dominates? And will Microsoft’s 30-35 percent of that new market be bigger than the PC-only market of the past?

I think it will.

And this means that even though Microsoft will lose share via a broadening of what constitutes personal computing devices and by new more agile competitors like iOS and Android, Microsoft should in fact still grow in the future too.

So let’s plug in some numbers and see if this makes any sense. I’m going to have to mix and match a bit.

In 2008, as a frame of reference, PC makers sold 297 million units, according to IDC. Windows represented about 95 percent of that, and probably about 90 percent of the overall personal computing market, as Forrester calls it. So let’s call it 267 million units.

Again, this is rough. But looking ahead, Gartner expects 1.1 billion annual smart phone sales by 2015. Gartner also expects 294 million annual media tablet sales by 2015. And last year, IDC predicted that total PC sales in 2015 would be 535 million. That last figure is too high, as is the firm’s 2012 prediction of 390 million units. But in that WSJ article, there is a second chart that breaks down PC sales for 2016, and while it’s not terribly clear, it looks like the number is about 380 million. So let’s use that.

1.1 billion smart phones + 294 million tablets + 380 million PCs = about 1.77 billion personal computing devices.

One-third of that number is 590 million units. That’s almost exactly twice as big as the number of devices that shipped with Windows on them in 2008. That’s “growth” even if the cost of each Windows license goes down a bit. (And since Forrester thinks that tablet sales will actually just about equal PC sales in 2016, the numbers could be bit bigger, all around.)

It’s also not a bad future at all. Choice is good.