What is page weight?
Definition Page weight is the total amount of data a browser downloads to display a web page: the HTML plus every stylesheet, script, font, image, video and third-party resource it requests, usually measured as compressed transfer size in kilobytes or megabytes. Images and JavaScript make up most of it on typical sites. Lighthouse recommends staying below 1,600 KiB, and MobileCheck flags pages over 3 MB.
How to fix it: Page Weight: How to Reduce Page Size for Mobile
How is page weight measured?
Tools add up the bytes transferred over the network for the page and all its subresources. Two numbers are easy to confuse:
- Transfer size: what crosses the network, after gzip or Brotli compression. This is what costs time and mobile data.
- Resource size: the uncompressed size the browser has to parse and run. 300 KB of compressed JavaScript can be over 1 MB to parse and execute.
Page weight also depends on the device: a site using srcset sends smaller images to phones, and lazy-loaded images below the fold aren't downloaded until you scroll, so they don't count in an initial measurement.
Why page weight matters on mobile
- Speed. More bytes take longer on slow or congested mobile connections and delay Largest Contentful Paint.
- CPU. JavaScript must be parsed, compiled and run; on a mid-range phone this costs far more than on a laptop, and hurts INP.
- Cost. Many visitors pay for data or have capped plans.
Example
A quick way to see where the weight is: open DevTools, Network tab, reload, and sort by size. Typical wins:
<!-- Modern format, right size for the screen, lazy below the fold -->
<img src="photo-800.webp"
srcset="photo-480.webp 480w, photo-800.webp 800w, photo-1600.webp 1600w"
sizes="(min-width: 800px) 50vw, 100vw"
width="1600" height="1067" loading="lazy" alt="…">Others: replace autoplaying background video with a poster image, subset web fonts, remove unused JavaScript and third-party tags, and serve text with Brotli compression.
Remember that third-party code counts too. Analytics, chat widgets, video embeds, ad and consent scripts are part of the page weight your visitors download, even though they aren't in your repository, and they often load more resources of their own.
How MobileCheck checks it
MobileCheck adds up the transfer size of the page and its resources during the load on each device, and flags pages over 3 MB. The number reflects what that device actually downloaded, so a page with good responsive images can be lighter on a phone than on a tablet.