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CORE WEB VITALS & PERFORMANCE

Cumulative Layout Shift: what it means and how it works

How much content jumps around unexpectedly; good is 0.1 or less.

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All glossary terms
  • Intermediate level
  • 2 primary sources
  • Reviewed 29 October 2026
At a glanceSEO glossary
  • TopicCore Web Vitals & performance
  • LevelIntermediateAssumes the basics of how search works
  • Also calledCLS
  • Sources2 primary sourcesweb.dev
  • Reviewed29 October 2026Re-checked every 12 months
On this page
  1. Quick definition
  2. How it works
  3. Example
  4. How to check it
  5. Common mistakes
  6. Related concepts
  7. Check it on your site
  8. Keep learning
  9. Sources and review

Quick definition

Cumulative Layout Shift (CLS) measures how much visible content moves unexpectedly while a page is open. It is the Core Web Vital for visual stability; a score of 0.1 or less at the 75th percentile is good.

How it works

Each unexpected shift gets a score from how much of the viewport moved and how far. Shifts are grouped into bursts (session windows of up to five seconds), and CLS is the largest burst. Shifts within half a second of a user’s input do not count, because the user expected a change.

  • Images, videos and iframes without width and height, which push content down when they load.
  • Ads, embeds and cookie banners inserted above existing content.
  • Web fonts that swap in with different sizes.
  • Content injected by JavaScript after the first render.

Example

A news site’s article pages score 0.32 on mobile. The cause is an ad slot at the top with no reserved height: when the ad loads, the headline and first paragraph drop by 250 pixels, sometimes as a reader taps a link. Reserving the slot’s height with CSS brings CLS under 0.05.

How to check it

  1. Find which elements shift with the browser’s performance panel or PageSpeed Insights’ layout shift diagnostics.
  2. Give every image and embed explicit dimensions or an aspect ratio.
  3. Reserve space for ads and late-loading components.
  4. Check field data, since shifts that happen during scrolling may not appear in a lab load test.

Common mistakes and misconceptions

  • Myth CLS only counts during page load.

    In fact Unexpected shifts across the visit count, including ones caused by content loading as the user scrolls.

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Sources and review

Written and maintained by Ndovu SEO Labs. First published 29 October 2026. Facts last checked against the sources above on 29 October 2026. How we write and review the glossary.