What is CLS?
TL;DR
CLS (Cumulative Layout Shift) is a Core Web Vital that measures how much the page layout unexpectedly shifts during loading, affecting user experience.
Last updated: 2026-03-09
Definition#
Cumulative Layout Shift (CLS) measures the total of all unexpected layout shifts that occur during the entire lifespan of a page. A layout shift happens when a visible element changes position without user action — for example, when an ad loads and pushes text down, or when an image renders and bumps buttons to a new location.
CLS is scored on a scale where 0 means no shifts and higher numbers mean more visual instability. Google considers a CLS of 0.1 or less to be "good." Between 0.1 and 0.25 "needs improvement." Over 0.25 is "poor."
Common causes include images and videos without explicit dimensions, dynamically injected content (ads, banners, cookie notices), web fonts that cause text to resize (FOUT/FOIT), and content inserted above existing content.
Why It Matters for AI Readiness#
While CLS primarily affects human users, it is also a signal of overall page quality that contributes to your site's reputation with search engines. The Speed & Performance factor includes layout stability metrics.
Fix CLS by setting explicit width and height on all media, reserving space for dynamic content, and using CSS
aspect-ratio. See How to Improve Page Speed.Related Concepts#
CLS is one of the three Core Web Vitals, alongside LCP and INP. It is measured in the Speed & Performance factor.
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