Term
CLS (Cumulative Layout Shift)
CLS measures unexpected layout shifts during page load. A Core Web Vital — "good" is ≤ 0.1 (unitless score).
CLS — explained in more detail
CLS (Cumulative Layout Shift) sums all unexpected layout shifts of visible elements during the lifetime of a page. The score is unitless and computed as impact fraction (portion of viewport that shifts) × distance fraction (how far). Since 2021, sessions are windowed to a maximum of 5 seconds — the worst window counts.
Thresholds: ≤ 0.1 = good, ≤ 0.25 = needs improvement, > 0.25 = poor. As with the other Core Web Vitals, the 75th percentile from CrUX field data is what counts.
Example / In practice
Classic CLS culprits: images without width/height (or aspect-ratio), late-loading webfonts without font-display: optional, ads or iframes with dynamic height, cookie banners that push content down. Levers: reserve dimensions, set font-display, predefine container heights for ads.
Distinction from similar terms
CLS only measures unexpected shifts. Layout changes within 500 ms after a user interaction (clicking “show more,” activating a filter) do not count — those are expected.
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Core Web Vitals
Core Web Vitals are three Google metrics (LCP, INP, CLS) measuring loading speed, interactivity and visual stability of a page — part of the Page Experience signals.
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