Page weight

Page weight counts the bytes that actually travel over the network. A text file therefore counts at its compressedCompressionEncoding a file so that it needs fewer bytes in transit, after which the browser decodes it. The server names the method in the Content-Encoding header, usually gzip, Brotli or Zstandard.In the glossary size, an image at the size of its file. The total covers everything the browser fetches for one view: the HTML document, stylesheets, JavaScript, images, fonts, and whatever third parties add, such as analytics, maps or video players. Browser developer tools and audits such as Lighthouse report this figure as transferred data. A page can keep growing after the first view, for example when images further down load as the reader scrolls.

In 2025 the median mobile home page transferred 2.56 MB, according to the HTTP Archive Web Almanac. Images accounted for 911 KB of that, JavaScript for 632 KB and fonts for 122 KB, while the HTML itself came to only 22 KB. In July 2015 the same median stood at 845 KB, so the typical page has roughly tripled in ten years.

Every byte has to arrive before the page is complete, and the mobile network is usually the slowest and least predictable part of the journey. A heavy page delays the moment a visitor sees content and can act on it, and on a plan with a data cap it costs them money. Speed also shows up in revenue: Deloitte and 55 examined 37 European and American brand sites in a study commissioned by Google. When a mobile site became 0.1 seconds faster, retail conversion rates rose by 8.4 percent and retail customers spent 9.2 percent more; in travel, conversion rates rose by 10.1 percent. Weight is one of several causes of slowness, and the one a site owner controls most directly.

Weight is rarely spread evenly. Images and JavaScript make up most of it on the typical page. The largest savings therefore come from modern image formats in the size the screen needs, and from shipping only the code a page runs. Text files shrink considerably when the server compresses them, and a single well-prepared font file can replace several. Third-party scripts deserve their own review, because they add weight that the site owner neither wrote nor can easily reduce.

A sensible weight budget starts from the content of the page: an article with two photos needs a fraction of what a product configurator needs, whatever the median says. What a returning visitor downloads also depends on cachingCachingKeeping a copy of a response so it does not have to be fetched or computed again. The Cache-Control header tells browsers and CDNs how long a copy stays valid.In the glossary, since files kept from an earlier visit are not transferred again.