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Performance · Free Tool

Free CLS Checker

CLS (Cumulative Layout Shift) measures visual stability — how much page elements unexpectedly move while loading. A high CLS score means visitors accidentally click on things that shift, hurting UX and rankings. This tool measures your CLS score and identifies the worst-offending elements.

scheduleSpeed tests typically take 15–30 seconds.

What This Tool Checks

  • check_circleCLS score with Good/Needs Improvement/Poor verdict
  • check_circleVisual stability score breakdown
  • check_circleShifting element identification (images without dimensions, fonts, ads)
  • check_circleLayout shift impact score per element
  • check_circleRecommendations specific to your CLS root cause
  • check_circleComparison between mobile and desktop CLS scores
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Why This Matters for SEO

CLS is the most frustrating Core Web Vital for users — it's the experience of trying to click a button as the page shifts and accidentally clicking something else. Google's "Good" threshold is under 0.1. High CLS is caused by very specific, fixable issues: mostly images without explicit dimensions and dynamically injected content. It's often the easiest Core Web Vital to fix.

Want this audit running on every page of your site, every day? See PerfBee Performance Boost — Continuous Core Web Vitals monitoring with deploy comparisons and AI fix suggestions.

Frequently Asked Questions

What is Cumulative Layout Shift (CLS)?expand_more
CLS measures visual stability — how much the visible content shifts unexpectedly during page load. It's calculated as the sum of all individual layout shift scores (impact fraction × distance fraction). A score of 0 means no shifting. Google's thresholds are: Good = under 0.1, Needs Improvement = 0.1–0.25, Poor = over 0.25. Common causes include images without size attributes, ads, embeds, and late-loading fonts.
What causes high Cumulative Layout Shift?expand_more
The main CLS causes are: (1) Images without explicit width/height attributes — the browser doesn't reserve space, so the image pushes content down when it loads. (2) Ads, embeds, or iframes loading dynamically — no space reserved. (3) Font swap (FOUT) — text reflows when a web font loads and replaces the fallback font. (4) Dynamically injected content like cookie banners, cookie notification bars. (5) Animations using properties that trigger layout (top/left instead of transform).
How do I fix CLS?expand_more
The most effective CLS fixes are: (1) Add explicit width and height attributes to all images and video elements. (2) Use CSS aspect-ratio or padding-top trick to reserve space for dynamic embeds/ads. (3) Use font-display: optional or swap with size-adjusted fallback fonts to minimize font-swap layout shift. (4) Avoid inserting content above existing content. (5) Use CSS transform for animations instead of top/left/margin which trigger layout.
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Monitor CLS across all pages after every deploy

CLS regressions often happen when new features ship — an ad, a banner, a lazy-loaded image. PerfBee catches these before they affect your rankings. Get daily crawl reports, broken link alerts, Core Web Vitals tracking, and AI fix suggestions — across your entire site.

  • checkCrawl unlimited pages — daily, automatic
  • checkBroken link detection across your whole site
  • checkCore Web Vitals & Lighthouse audits per page
  • checkAI-powered code fix suggestions
  • checkWhite-label PDF reports for clients
  • checkEmail alerts for new issues & downtime
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