Cumulative Layout Shift
Also called CLS
A Core Web Vital that measures how much visible content unexpectedly moves while a page is in use. Good is 0.1 or less.
Cumulative Layout Shift, or CLS, measures visual stability: how much the content on a page moves around unexpectedly. Each unexpected shift gets a score based on how much of the screen moved and how far. Shifts that happen close together are grouped, and CLS reports the largest such group during the visit. Good is 0.1 or less, needs improvement is up to 0.25 and poor is above 0.25, assessed at the 75th percentile of page loads. CLS has no unit; it is a score, not a time.
Layout shifts are frustrating on any page and costly on a landing page. A visitor starts reading the headline and it jumps down; they go to tap a button and a banner pushes it away, so they tap the wrong thing. That breaks trust and can cause accidental clicks.
The usual causes are images and videos without width and height set, ads, embeds and iframes that load without reserved space, cookie banners or promo bars inserted above existing content, and web fonts that change the size of text when they load. Shifts that happen within half a second of a user's own action, such as opening a menu they tapped, are not counted, because the user expects them.
How to improve it
- Set width and height attributes, or a CSS aspect ratio, on all images and videos.
- Reserve space for embeds, ads, review widgets and iframes before they load.
- Show cookie banners and promo bars as overlays or in reserved space, not by pushing content down.
- Use font loading settings and fallback fonts that keep text size stable.
- Avoid inserting new content above what the visitor is already reading.
How PageScore checks it
PageScore reports CLS from a Google PageSpeed Insights mobile test of your page.