For April, Windows 8's average usage share was 4.2% of all Windows PCs, according to Net Applications' data. That number included what Net Applications labeled as "touch" for Windows 8 and Windows RT, those tablets and touch-enabled notebooks that browse the Web from the "Modern" user interface (UI) rather than the mouse-and-keyboard UI of the traditional desktop.
Four-point-two percent of 1.4 billion equals approximately 58.6 million, the number of devices using Windows 8 last month.
Those same calculations also illustrate the huge chore Microsoft and its customers, predominantly enterprises, have in ditching the 12-year-old Windows XP
before April 2014's support retirement deadline. Using the same math, XP powers about 584.4 million machines, or well over half a billion PCs.