CLS Tester Tool
Measure your website's Cumulative Layout Shift (CLS) score using real Lighthouse data and Chrome User Experience Report (CrUX) field data. Identify layout shift sources and get actionable optimization recommendations.
Ready to Test Your CLS Score
Enter a website URL above to get real CLS measurements from Google PageSpeed Insights, including Lighthouse lab data and Chrome real-user field data.
Good
0.1 or less
Excellent visual stability. Users can confidently interact with page elements without unexpected content jumps.
Needs Improvement
0.1 to 0.25
Moderate layout shifts that may occasionally frustrate users. Optimization recommended for better experience and SEO.
Poor
Greater than 0.25
Significant layout shifts creating a frustrating experience. Immediate optimization needed for usability and rankings.
Cumulative Layout Shift (CLS) is a Core Web Vitals metric that measures the visual stability of your webpage. It quantifies how much visible content shifts unexpectedly during the page loading process, providing a score that reflects the user experience quality. Google uses CLS as a ranking factor since 2021.
How CLS is Calculated
CLS is the sum of all individual layout shift scores for every unexpected layout shift during the page's lifespan. A layout shift occurs when a visible element changes position from one frame to the next without user interaction.
CLS = Σ (Impact Fraction × Distance Fraction)Why It Matters for SEO
- User Experience: Prevents accidental clicks on wrong elements
- SEO Ranking: Core Web Vital used by Google's algorithm
- Conversions: Better stability leads to higher engagement
- Mobile: Especially critical on mobile devices
Images Without Dimensions
Images without width/height attributes cause content to shift when loaded.
Ads Without Reserved Space
Ad containers without pre-reserved space cause major shifts when ads load.
Dynamic Content Injection
New content above existing elements pushes everything down.
Web Font Loading
Font swaps can cause text reflow leading to layout shifts.
Non-Composited Animations
Animations changing element dimensions trigger layout shifts.
Embeds and Iframes
Third-party embeds without dimensions cause shifts when loaded.
Quick Wins
- Add width/height to all images
- Reserve space for ads and embeds
- Preload critical web fonts
- Use CSS aspect-ratio for responsive media
Advanced Techniques
- Implement proper font loading strategies
- Use transform for animations instead of layout props
- Optimize third-party script loading
- Implement skeleton screens for dynamic content
Best Practices
- Test on real devices and slow networks
- Monitor CLS with RUM tools continuously
- Prioritize above-the-fold stability
- Set performance budgets for shifts
How does this CLS tester work?
This tool uses the Google PageSpeed Insights API, which runs a full Lighthouse audit on your page. It measures real CLS by rendering your page in a controlled Chrome environment and tracking all layout shift events. When available, it also shows CrUX field data from real Chrome users over the past 28 days.
What's the difference between Lab and Field CLS data?
Lab data (Lighthouse) is measured in a controlled environment during a simulated page load — ideal for debugging. Field data (CrUX) comes from real Chrome users visiting your site, and reflects actual user experiences. Google uses field data for ranking decisions when available.
Why might my CLS score differ from other tools?
CLS can vary based on viewport size, network conditions, third-party scripts, and user interaction patterns. Lab tests measure CLS during initial page load only, while field data captures shifts throughout the entire page session. Results may also differ between mobile and desktop testing.
How long does it take CLS improvements to affect rankings?
CrUX data is aggregated over 28 days. After fixing CLS issues, it typically takes 4-6 weeks for the improvements to appear in Search Console and 8-16 weeks for ranking changes to manifest as Google processes the updated data.