It’s an XP world out there still, but that fact is slowly degrading as the venerable operating system cedes market share each month to Windows 7. While that has been going on since Windows 7 first came out, what remains yet unknown is the amount of time left that it will require for Windows 7 to overtake Windows XP as the most popular operating system in the world.

Fortunately for you, we have decided to do the math. We’ll compare the growth rate of Windows 7 with the rate of decline that Windows XP endures, add the two, and divide their total market share gap by that monthly rate. Let’s start with Windows XP (all data via NetMarketShare):

Windows XP

  • Market share June 2011: 54.04%
  • Market share December 2011: 46.52%
  • Total average monthly decline: 1.253%

Windows 7

  • Market share June 2011: 28.68%
  • Market share December 2011: 36.99%
  • Total average monthly growth: 1.385%

Synthesis

  • Current total difference in market share: 46.52%-36.99% = 9.53%
  • Total monthly decline in separation: 1.253+1.385 = 2.638%
  • Which allows us to take the total separation and divide by our monthly decline in that separation: 9.53/2.638 = 3.61 months until parity
  • In short, Windows 7 is set to overtake Windows XP in a little under 4 months.

Here’s the takeaway: the popularity of Windows 7 and the age of Windows XP are set to allow for a changing of the guard in the second quarter of this year. That Windows 7 is competing with Windows XP, and not Windows Vista, is a testament to how poorly Vista was received. Even more, if you notice, Windows 7 is gaining market share faster than XP is losing it, which indicates that Windows 7 is eating both XP and Vista at the same time.

And in case you were curious, 3.61 months after January first is April 19th. Mark you calendars.