TL;DR: Cumulative Layout Shift (CLS) is a Core Web Vitals metric that measures the visual stability of a webpage. A good CLS score should be 0.1 or less; anything higher can hurt user experience and SEO rankings of your website. This guide includes everything about CLS, including measuring tools and improvement strategies.
Do you know what is more frustrating than a slow website? It’s a Cumulative layout shift (CLS). When you’re about to click a button on a site, and it just moves away at the last second. This unexpected shift is called a Cumulative Layout Shift.
Now, think as a website owner, when a user is about to interact with any visual element on your website, and it just suddenly takes a shift as the page loads. This shift not only ruins the entire user experience but can also frustrate them to leave your site, especially when this unexpected click is on ad banners.
CLS in core web vitals is one of the most overlooked yet impactful metrics. In this blog, we’ll break down what a Cumulative Layout Shift is and why it matters in Core Web Vitals.
What does Cumulative Layout Shift (CLS) mean?
Cumulative Layout Shift (CLS) is a Core Web Vitals metric that measures the stability of a webpage as it is loaded. It counts layout shifts that happen while the page is loading, which can annoy users and cause them to click on things they didn’t mean to.
For instance, you’re reading an article when an image suddenly appears above the text, pushing the text down. Or you want to buy something, but the page jumps and you click on an ad instead. These are real-life instances of Cumulative Layout Shift that show why it’s so important for a smooth browsing experience.
Google introduced CLS as part of the webpage performance indicators to see how steady your website seems as it loads. A high CLS score suggests that the parts of your site are not stable, while a low score means that the rendering is smooth.
What is a Good Score For CLS?
Google divides CLS scores into three levels to assist website owners in checking their site performance:
Good (≤ 0.1): Scoring 0.1 or less in CLS means your website is very stable. Most people won’t notice any shifts.
Needs Improvement (0.1- 0.25): This is the moderate score, which means your site’s layout is taking shifts that are clear and need to be fixed.
Poor (≥ 0.25): Indicates that your site is very unstable, which makes it hard for users to browse and could harm your rankings and user experience.
CLS is an important factor for website optimization. Your goal should be to achieve a CLS score of 0.1 or under to make sure your SEO and user experience are good enough for conversions.
How to Measure Cumulative Layout Shift?
The Cumulative Layout Shift (CLS) score is measured through a scoring system that measures how much visible site components shift unexpectedly during page load. This system quantifies the major shifts based on these two measures:
1. Impact Fraction:
The amount of space the unstable element takes up on the screen. It is basically the proportion of the layout shift area and the viewport, before and after the shifting.
Formula: Impact Fraction = Total area of viewport / Total area of layout shifts
2. Distance Fraction:
It is the distance of the element moved in the viewport between two rendered frames. Basically, it is the major horizontal or vertical distance at which any element has taken a shift.
Formula: Fractional Distance = Fraction × Total Distance travelled by the element
For instance:
The CLS score = Impact Fraction × Distance Fraction, summed up for all layout shifts on the page.
If a picture fills up half of the screen and moves 25%, the score would be 0.125 (0.5 × 0.25). Google keeps track of these scores all the time as people use your site to figure out the total CLS.
Tools to Measure Cumulative Layout Shift
To measure CLS and check out your site speed to see if it is providing a good user experience. You can use various tools to assist you in calculating these Core Web Vitals metrics.
Google PageSpeed Insights
This tool gives you CLS ratings and tips on how to make your pages load faster. Google PageSpeed Insight provides you with warnings regarding layout shifts in the “Opportunities” and “Diagnostics” sections. It is one of the best tools to measure Core Web Vitals (FCP, LCP, CLS, INP).
Lighthouse (Chrome DevTools)
It shows how the layout changes on certain pages. And its animation tool shows the exact part that is taking a shift while the page loads. As this is a Google tool, it is more reliable and efficient to calculate your Core Web Vitals score, including First Contentful Paint (FCP), Largest Contentful Paint, Cumulative Layout Shift, and more.
Google Search Console
Shows sitewide CLS problems that affect more than one URL. Google Search Console uses real-world data from Chrome users to show how visitors actually feel when visiting your site. This approach makes Search Console Core Web Vitals reports more accurate.
WebPageTest & GTmetrix
Third-party tools that visualize layout shifts for deeper insights. These tools use browser data to calculate CLS scores, helping analyze the issues and fixes.
You can make sure your site stays optimized for both visitors and search engines by checking the site’s Cumulative Shift regularly.
How to Fix Cumulative Layout Shift (CLS)
To fix Cumulative Layout Shift optimization issues, you need to deal with the things that usually cause instability. Here are some useful tips:
1. Always Add Size Attributes to Videos and Pictures
One of the most typical reasons for layout changes is when media loads without set dimensions. By setting explicit width and height attributes, the browser reserves the required space, preventing sudden jumps.
2. Fix the Space for Ads and Embeds
Dynamic ads, iframes, and third-party embeds can interrupt the smooth page loading. Reserve the fixed dimensions and aspect ratios in containers, even if the ad or embed hasn’t yet loaded.
3. Use CSS Aspect Ratio Boxes
Modern CSS allows you to define aspect ratios for elements like images and videos. This will allow elements to load in their decided scale without disturbing the nearby content placement.
4. Use Preload Fonts to Avoid Flash of Unstyled Text (FOUT)
Slow-loading fonts cause shifting and re-rendering of the content. Thus, preloading critical fonts reduces this risk.
5. Avoid Placing Content Above Existing Elements
Never add new banners, forms, or content above what’s already loaded, unless user-initiated. Instead, reserve space or position for new elements below the fold.
6. Optimize for Web Vitals with Modern Frameworks
If you’re using platforms like WordPress, Shopify, or Squarespace, use themes and plugins to optimize page experience metrics. Many modern frameworks now prioritize stability alongside speed.
7. Monitor Changes with Real User Monitoring (RUM)
Technical testing is useful, but real-world user data offers the most accurate insights. Use tools like Google Analytics or performance monitoring solutions to track CLS and other CWV metrics, including LCP, INP, and more, under real conditions.
CLS in Core Web Vitals & SEO Impact
CLS is not just about your site’s design or layout, but it is directly related to SEO and conversions. CLS is a Google Core Web Vitals metric and a vital ranking factor. The high layout shift scores can lower your chances of ranking high in search results.
Moreover, user experience is everything for a successful business. Users will usually leave the unstable sites. Studies show that a bad user experience causes higher bounce rates, lower conversions, and fewer repeat visitors.
Investing in CLS performance optimization not only means rankings but also helps you achieve good SEO and optimal business growth in the long run.
Conclusion
Cumulative Layout Shift (CLS) is a critical metric in Core Web Vitals that checks the visual stability of a website. You should focus on achieving a good CLS score that is less than 0.1. Having a good CLS score means that the user can browse and scroll on your website without frustration, improving conversions and enhancing user experience as well.
Things to Remember for Ranking Higher:
- Aim for a CLS score of 0.1 or less.
- Measure CLS with tools like PageSpeed Insights, Lighthouse, and Search Console.
- Fix issues by reserving space for images, ads, and embeds, preloading fonts, and avoiding sudden content injections.
By taking these steps, you can reduce layout shifts, improve your page speed optimization, boost SEO, and deliver a smoother, more reliable user experience.
FAQs
Q1. What causes a high CLS score?
Q2. What is a good CLS score?
Q3. How can I check my website’s CLS score?
Q4: What can I do to fix problems with cumulative layout shifts?
Q5. What does CLS have to do with SEO?
Q6. Does CLS affect sales and conversions?
September 26, 2025
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Ishan Makkar
Ishan Makkar is the Founder of Website Speedy, a performance optimization platform dedicated to making websites faster, more user-friendly, and SEO-ready. With deep expertise in technical SEO and Core Web Vitals, Ishan helps businesses reduce bounce rates and deliver lightning-fast digital experiences.
