One in four websites is now powered by WordPress.

Today is a big day for the free and open-source content management system (CMS). To be perfectly clear, the milestone figure doesn’t represent a fraction of all websites that have a CMS: WordPress now powers 25 percent of the Web.

The latest data comes from W3Techs, which measures both usage and market share: “WordPress is used by 58.7% of all the websites whose content management system we know. This is 25.0% of all websites.” While these numbers naturally fluctuate over the course of the month, the general trend for WordPress has been slow but steady growth.

“We should be comfortably past 25% by the end of the year,” Automattic founder Matt Mullenweg declared. “The big opportunity is still the 57% of websites that don’t use any identifiable CMS yet, and that’s where I think there is still a ton of growth for us (and I’m also rooting for all the other open source CMSes).”

Indeed, WordPress hasn’t grown much in terms of market share over the past few years. Its share has actually slipped over the past year:

Nonetheless, controlling more than half the market share pie is impressive. Such a prime position does, however, lead to being the target of various attacks, and WordPress has had numerous security issues throughout its lifetime. (Please update to the latest version of WordPress, or whatever CMS you run.)

W3Techs uses a very specific methodology to provide the above numbers.

Websites, not individual web pages, are examined — if a technology is on any of a website’s pages, it is considered to be used by the website. Furthermore, W3Techs doesn’t technically scan the whole World Wide Web. To limit the impact of domain spammers, only the top 10 million websites are investigated, based on the popularity rankings provided by Alexa, using a three-month average ranking.

That said, W3Techs excludes subdomains (all the subdomains of wordpress.com are counted only as one website) and redirected domains (Sun.com redirects to Oracle.com so it is not counted). As such, because it defines “website” differently than Alexa, the “top 10 million” websites are actually fewer than 10 million, but this “has no statistical significance,” according to W3Techs.

WordPress powers 25 percent of… the 10 top million websites… that are actually fewer than 10 million… but effectively represent the entire Web.