Free CLS Tester & Analyzer

Cumulative Layout Shift measures how much page content unexpectedly shifts during loading. Lower is better.

Under 0.1 is Good. 0.1–0.25 needs improvement. Over 0.25 is Poor.

Yes, completely free.

Absolutely. The CLS Tester & Analyzer processes everything client-side in your browser. No data is uploaded to or stored on any server. Your content remains private on your device at all times.

Yes, the CLS Tester & Analyzer is fully responsive and works on smartphones and tablets. You can use it on any device with a modern web browser -- no app download required.

No account or registration is needed. Simply open the CLS Tester & Analyzer in your browser and start using it immediately. There are no sign-up walls or usage restrictions.

Yes, the CLS Tester & Analyzer is built following current Google Search guidelines and best practices. We regularly update the tool to reflect changes in search engine requirements.

Simply enter your input in the provided field, adjust any settings to your preference, and the tool will process it instantly. You can then copy the result to your clipboard or download it.

The CLS Tester & Analyzer works in all modern browsers including Chrome, Firefox, Safari, Edge, and Opera. For the best experience, use the latest version of your preferred browser.

CLS is not a measure of time but a score that sums every unexpected layout shift across a page's lifespan. Each shift is graded by two factors multiplied together: the impact fraction, which is how much of the visible viewport moved, and the distance fraction, which is how far the elements traveled relative to the viewport. A shift that pushes a large block of content a long way scores far higher than a tiny nudge. The browser then adds up all these unexpected shifts to produce one final number. Crucially, movements that happen within roughly 500 milliseconds of a user click or tap are excluded, since those are usually intentional. This tool mirrors that exact logic, ignoring input-flagged shifts and totalling only the unexpected ones. Open it on any page to watch the running total build live in your browser.

Lab tools measure a single page load on one device under controlled conditions, while field tools like the Chrome User Experience Report average many real visits across different screens, networks, and behaviors. That difference explains why your lab number and your field number rarely match. A page can look stable in one synthetic test yet still shift for visitors on slow connections or smaller viewports where late-loading images and ads behave differently. To pass Core Web Vitals, Google looks at the 75th percentile of real-user CLS across both mobile and desktop, which should sit at or below 0.1. The live measurement in this tool reflects your own device, screen size, and network at that moment, so it behaves like a single field sample rather than a lab average. Run it a few times to see how conditions change the result.

High CLS almost always traces back to a few repeat offenders. Images and iframes without explicit width and height attributes, or a CSS aspect-ratio, force the page to reflow once they load. Ad slots that appear without reserved space shove content down suddenly. Content injected above existing elements, like banners or notices, pushes everything below it. And web fonts that swap in late can reflow whole paragraphs of text. The reliable cure is to reserve space ahead of time for anything that loads asynchronously, so the layout never has to jump to accommodate it. For movement you do want, animate using transform and opacity rather than properties that trigger a full layout recalculation. This tool lists these patterns alongside their fixes, so paste or measure your score and use the guidance to target the exact shifts hurting your page.

When the page loads, the analyzer attaches a PerformanceObserver to layout-shift events and requests buffered entries. That buffering detail matters: it captures shifts from the very moment loading began, not just from when the measuring script started running, so nothing early is missed. As shifts occur, it reads the live Layout Instability API, the same browser interface Google's own tools rely on, and keeps a running total rounded to three decimals beside a rating badge. Because the value comes straight from your real device, screen size, and network at that instant, it reflects genuine conditions rather than an idealized simulation. There is no crawling or server round-trip involved; everything happens locally. Open the tool on the page you want to evaluate and watch the score and its Good, Needs Improvement, or Poor rating update as the layout settles.

The in-browser measurement only reads the page you currently have open, because it listens to live layout-shift events on that document. It does not crawl or fetch a remote URL itself. To analyze a different site, you have two options here. First, use the manual score analyzer: enter any value between 0.0 and 1.0 from a report you already have, and the tool instantly returns whether it is Good, Needs Improvement, or Poor against Google's thresholds. Second, paste a URL into the dedicated field and the tool hands you off to Google's PageSpeed Insights, opening that report in a new tab where you get both lab and real-user field data. So while it does not directly scan external pages, it gives you a fast rating and a one-click route to deeper data. Try the score analyzer to interpret any number you have.