The usage share of operating systems is the percentage of the operating systems used in computing devices (approximately market share, also discussed in the article).

There are “three big personal computing platforms”,two of which claim over 1.4 billion users, Android and Windows. The third “platform” – or strictly two (or three) platforms – Apple’s iOS and macOS combined, have over 1 billion users Historically Windows running on desktop computers was dominant (Macintosh computers were previously the more popular desktop computers, and those followed the mainframe computer-era), for a 25 year period from the early 1990s to 2016 From late 2016 the mobile era took over, with the desktop computers’ market share (measured by web usage; including macOS) was down to 45.22% in January 2017, an “end of an era” was declared by StatCounter for Windows (and the desktop in general) as no longer most popular, since smartphones alone – excluding tablets – because of Android, have majority use globally.

Different categories of computers use a wide variety of operating systems (OS). Windows gained majority usage share in the 1990s, on desktops (the computer platform then taking over), eventually described as “dominant” (and still has a majority as a “desktop OS”) but is not dominant across all platforms (while latest version runs on all platforms). On smartphones, Android is dominant by any metric; its installed base is 1.8 billion, which tops Windows on PCs. No operating system has a clear majority across all platforms or all regions; as judged by web use – a proxy for all use – Android has surpassed Windows, when counted across all platforms and regions. Android is the highest ranked OS in most countries of the world (even a few in Europe, such as Poland with majority use); leading to it (with some help from other mobile operating systems such as Apple’s) eventually, in late 2016, making the world smartphone-majority. Android alone explains that to a large degree, smartphones alone have majority use, where Android is dominant. Android has over half the usage share across platforms (even with it not used much on desktops) in the two biggest continents (and most populous with 76% combined), Africa and Asia. For brief periods, countries on other continents, and also South America as a whole, have lost desktops-majority share. As of June 2017, the United States and the UK have also lost desktops-majority share.

Since 2013, devices running Android have been selling more than Windows, iOS and macOS devices combined. That made Android the most popular operating system that runs on smartphones, while iOS is more used on tablets. Most desktop and laptop computers use Microsoft Windows, while virtually all supercomputers (and for over a decade back) use Linux. In the servers category, there is more diversity, with Linux and Windows Server most popular, and many fewer mainframes. Data about operating system share is difficult to obtain, since in most categories there are few reliable primary sources or agreed methodologies for its collection.