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ballmerandroidWindows Phone revenue rose by $1.2 billion in the company’s 2013 fiscal year, according to new numbers released by Microsoft this afternoon as part of its Form 10-K filing with the Securities and Exchange Commission. But the definition of “Windows Phone revenue” isn’t what you might think.

The number reflects “an increase in patent licensing revenue and sales of Windows Phone licenses,” the filing says.

In other words, an unspecified chunk of that revenue is coming not from Windows Phone, but from patent licensing deals that Microsoft has struck with companies that make phones based on Google’s Android operating system.

Those deals include a licensing agreement with Samsung, which has been dominating the smartphone market with skyrocketing sales of Android-based devices.

So how much is Microsoft making on Android patent deals? The company isn’t saying, but here’s some back-of-the-napkin math, with some help from my spreadsheet of sales stats.

Microsoft hasn’t disclosed the number of Windows Phone units shipped during the year, but based on IDC’s past reports and Nokia’s public reports of Lumia sales, my estimate is that there were somewhere around 26 million total Windows Phone units shipped during Microsoft’s fiscal 2013, which ended in June.  This is a rough estimate based on the typical ratio of Lumia units to the overall Windows Phone market.

(Nokia reported 20.3 million Lumia shipments over the last four quarters, and on average Lumia represents around 77 percent of the Windows Phone market.)

Analysts have estimated in the past Microsoft is getting roughly $15 in licensing fees for each Windows Phone unit. If true, those 26 million units would translate into less than $400 million in actual revenue from Windows Phone during the fiscal year.

Going back to the $1.2 billion figure reported by Microsoft, keep in mind that it’s an increase over an unspecified number, so the total revenue figure from Windows Phone and patent licenses is actually higher.

Yes, these are all estimates, and we won’t know for sure unless Microsoft releases the actual numbers, but it’s quite possible that Android has become Microsoft’s latest billion-dollar business. And if not yet, then soon.

PreviouslyMicrosoft Surface revenue so far: $853 million

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