Over the past five years working on technical SEO, I’ve realized that optimizing for core web vitals goes beyond just trying to reduce site loading speed.
Metrics like LCP, CLS, INP, and even the older FID each highlight different parts of the user experience. One issue I see repeatedly is this: a page loads fast, but the layout keeps shifting just as users try to interact with it.
That’s exactly where cumulative layout shift SEO becomes critical, especially if you’re trying to understand what CLS is in core web vitals from a practical standpoint.
In this article, I’ll break down:
- What CLS is
- How CLS actually works
- What causes it
- And how to improve it
If you’ve ever wondered how to fix CLS issues beyond just following generic advice, this will give you a clearer direction grounded in real-world audits rather than theory.
What Is Cumulative Layout Shift (CLS)?
CLS is a Core Web Vitals metric that measures how much visible content on a page unexpectedly shifts during loading, affecting how stable the page feels to users.
In simple terms, it tracks those moments when buttons move, text jumps, or images suddenly push content down just as someone is about to click.
What Causes Layout Shifts on a Webpage
Layout shifts occur when elements load without reserved space or when elements appear dynamically after the page has started rendering.
The most common issues I see in audits are:
- Images without fixed dimensions
- Ads loading late
- Scripts injecting content above existing elements
From a user’s perspective, the page feels like it’s “moving around” unpredictably. In real projects, it’s rarely just one issue.
A combination of:
- Font swapping late
- Embeds resizing
- Banners appearing after load can stack up quickly
Even small shifts, when repeated across a session, contribute to a poor CLS score and a noticeably unstable experience.
Why CLS Is Part of Core Web Vitals
CLS is part of Core Web Vitals because Google wants to measure not just how fast a page loads, but how usable it feels while loading.
A page that loads quickly but shifts constantly can be frustrating, especially when users misclick or lose their place mid-scroll. Stability is a key part of the overall page experience.
From an SEO standpoint, I treat CLS as a trust signal. If your layout keeps shifting, users hesitate to interact, and that impacts engagement. Over time, that friction adds up. Even if your content is strong, a poor layout experience can quietly reduce its effectiveness.
How Cumulative Layout Shift Works
CLS works by measuring how much visible elements move within the viewport while a page is loading, focusing specifically on unexpected shifts that disrupt the user’s experience. It doesn’t count intentional animations or interactions, only movements that happen without user input.
What Counts as a Layout Shift
A layout shift occurs when an element changes its position between rendered frames without the user triggering it. This could be a button moving down because an image suddenly loads above it, or a paragraph shifting because a font swaps late.
In audits, I often point this out using real examplesnlike a “Buy Now” button that jumps just as someone tries to click it.
Not every movement is counted. If the shift happens within 500 milliseconds of a user interaction, it’s typically excluded because it’s considered expected. What CLS really captures are those unexpected movements that break the flow and catch users off guard.
How CLS Is Calculated (Impact Fraction × Distance Fraction)
CLS is calculated from two components: the area of the screen affected and the distance the elements move. The impact fraction measures the portion of the viewport that shifts, while the distance fraction measures how far those elements travel relative to the screen size. These two values are multiplied to produce a layout shift score.
In practical terms, I don’t expect clients to memorize the formula, but understanding the logic helps. A small shift affecting a large portion of the screen can be just as damaging as a large shift affecting a smaller area. When multiple shifts occur during a session, their scores are accumulated to form the final CLS value.
Why Unexpected Shifts Affect User Experience
Unexpected shifts affect the user experience by interrupting actions at the worst possible moment. A user might be about to click a link, only for it to move, leading to a misclick. I’ve seen this happen on landing pages where banners load late, pushing everything down—it creates friction immediately.
Over time, these small frustrations build up. Users lose confidence in the interface, especially on mobile, where screen space is limited. From my experience in performance audits, even minor instability can reduce engagement. It’s not just about annoyance—it directly affects how trustworthy and usable the site feels.
What Is a Good CLS Score?
A good CLS score is one in which the page remains visually stable during loading, so users don’t experience unexpected shifts when interacting with content. From what I’ve seen in real audits, once layout movement becomes noticeable, users start hesitating before clicking, which is exactly what this metric is designed to prevent.
Google’s CLS Performance Thresholds
Google’s CLS performance thresholds define how stable your page is based on real user behavior. These thresholds aren’t arbitrary; they reflect how much visual disruption users are likely to tolerate before the experience feels unreliable. In practice, I use these benchmarks as a quick way to prioritize fixes, especially on pages that drive conversions.
Good (≤ 0.1)
A CLS score of 0.1 or below means the page is visually stable, and most users won’t notice any unexpected movement. When I review sites with scores in this range, interactions usually feel smooth and predictable, which builds confidence during navigation.
Needs Improvement (0.1 – 0.25)
A CLS score between 0.1 and 0.25 indicates some visible shifting that may not break the experience entirely but can still cause minor frustration. I often see this on pages where images or ads load slightly late, creating small but noticeable jumps.
Poor (> 0.25)
A CLS score above 0.25 indicates a clear usability issue: frequent or severe layout shifts that disrupt user actions. At this level, users are more likely to misclick or abandon the page, especially on mobile, where even small movements feel amplified.
Why Real User Data Matters for CLS Evaluation
Real user data matters for CLS evaluation because it reflects how your page behaves across different devices, screen sizes, and network conditions, not just in a controlled test environment. Lab tools are useful for identifying issues, but they don’t capture how layout shifts actually affect users in everyday scenarios.
In my experience, I’ve seen pages perform well in testing tools but still show poor CLS in real user data due to dynamic content or third-party scripts. That’s why I treat field data as the final benchmark. If users are experiencing instability, that’s what needs to be fixed, regardless of lab scores.
Why Cumulative Layout Shift Matters for SEO
Cumulative Layout Shift matters for SEO because it measures how stable your page feels during loading, which directly affects user experience and how Google evaluates page quality. From my experience, even small layout issues can undermine an otherwise well-optimized page, especially when users start to lose confidence while navigating.
How CLS Affects Page Experience Signals
CLS affects page experience signals by influencing how Google assesses usability alongside metrics such as loading speed and responsiveness. A page that shifts unexpectedly creates friction, even if it loads quickly, and that weakens the overall experience Google is trying to reward.
In real audits, I’ve seen pages with strong content and decent performance still struggle because layout instability made interactions unreliable. When elements move unpredictably, users hesitate, and that hesitation signals a poorer experience over time.
The Relationship Between CLS and Core Web Vitals Ranking Signals
CLS is part of Core Web Vitals, which are used as ranking signals within Google’s broader page experience framework. That means layout stability plays a role in how pages are evaluated, especially when competing content offers similar relevance and quality.
From a practical SEO standpoint, I treat CLS as a supporting ranking factor. It won’t push weak content to the top, but when everything else is equal, a more stable page can gain an edge by delivering a smoother experience.
How Layout Instability Impacts Conversions
Layout instability impacts conversions by interrupting user actions at critical moments, often without the user even realising why they feel frustrated. A shifting button or moving form field can lead to misclicks, hesitation, or abandonment.
I’ve reviewed landing pages where late-loading banners pushed content just as users were about to convert. The result wasn’t always obvious in analytics, but session recordings made it clear. When users can’t trust the interface, they disengage, and that directly affects business outcomes.
What Causes Poor CLS Scores?
Poor CLS scores usually happen when elements on a page load without reserved space or appear after the initial layout has already been rendered. In most of the audits I’ve done, the issue isn’t a single mistake but a combination of design and loading decisions that cause content to shift unexpectedly.
Images Without Dimensions
Images without defined width and height cause layout shifts because the browser doesn’t know how much space to allocate before the image loads. As a result, when the image finally appears, it pushes the surrounding content down or to the side.
I still come across this quite often, especially on older sites or CMS setups. Once proper dimensions are added, the layout stabilizes immediately. It’s one of the simplest fixes, yet it has a direct impact on CLS performance.
Ads, Embeds, and Dynamic Content
Ads, embedded videos, and third-party widgets can introduce layout shifts when they load unpredictably. If space isn’t reserved in advance, these elements expand into the page and displace existing content.
In real scenarios, this is a major contributor to poor CLS. I’ve seen pages where ad slots collapse initially, then suddenly expand mid-load. That movement not only affects CLS but also disrupts user interaction, especially on mobile devices.
Web Fonts Causing Layout Shifts
Web fonts can trigger layout shifts when the browser swaps from a fallback font to the final font after loading. If the two fonts have different spacing or sizing, text reflows, causing visible movement.
This tends to be subtle but noticeable. On content-heavy pages, even small shifts in line height or letter spacing can accumulate. I usually recommend controlling font loading behavior to minimize these changes.
Late-Loading Content Above the Fold
Late-loading content above the fold causes shifts because it appears after the main layout has already been displayed. This is common with banners, popups, or dynamically injected elements that load after user interaction or delayed scripts.
From what I’ve observed, this is one of the most frustrating experiences for users. When something appears at the top of the page and pushes everything down, it interrupts whatever the user was about to do. That disruption directly contributes to poor CLS and weaker engagement.
How to Fix CLS Issues (Cumulative Layout Shift How to Improve)
You fix CLS issues by ensuring the layout is predictable from the moment the page starts loading, so elements don’t shift unexpectedly as content loads. In most of the optimization work I’ve done, improving CLS comes down to controlling how space is allocated and when elements are rendered.
Always Set Width and Height for Images and Videos
Setting width and height ensures the browser reserves the correct space before media loads, preventing content from being pushed around. Without these attributes, the layout adjusts only after the asset appears, which creates visible shifts.
In practice, this is usually the first thing I check. Once dimensions are defined properly, especially for above-the-fold content, the page immediately feels more stable even before deeper optimizations are applied.
Reserve Space for Ads and Embedded Content
Reserving space for ads and embeds prevents layout shifts by ensuring placeholders are in place before these elements load. If the space is not defined, the page expands dynamically when the content arrives.
I’ve seen this frequently on sites with multiple ad slots. When placeholders are implemented correctly, ads load into a fixed space rather than disrupting the layout, improving both CLS and overall usability.
Avoid Injecting Content Above Existing Content
Avoiding content injection above existing elements helps maintain layout consistency during loading. When new elements appear at the top of the page, they push everything downward, creating abrupt shifts.
From my audits, this often happens with promotional banners or delayed notifications. Moving these elements below existing content or loading them in a reserved area keeps the layout stable and avoids disrupting user actions.
Optimize Font Loading to Prevent Shifts
Optimizing font loading prevents layout shifts by reducing the difference between fallback fonts and final fonts. When fonts are swapped after loading, text can reflow, causing subtle but noticeable movement.
I usually address this by controlling font display behavior and ensuring fallback fonts are visually similar. It’s a small adjustment, but on content-heavy pages, it makes a clear difference in stability.
Use Performance Tools (e.g., WP Rocket) to Reduce Layout Shifts
Using performance tools helps reduce CLS by automating optimizations such as lazy loading, resource prioritization, and script management. Tools like WP Rocket can handle common layout issues without requiring manual code changes.
In real projects, I treat these tools as a support layer rather than a complete solution. They help streamline fixes, but understanding the root cause of layout shifts is still essential to achieving consistent results.
How to Measure CLS
You measure CLS by analyzing both real user data and controlled testing data, since each reveals different aspects of layout stability. In my experience, relying on a single view often leads to incomplete conclusions, especially when issues only manifest in real-world conditions.
In the Field
You measure CLS in the field by looking at how real users experience layout shifts across different devices, browsers, and network conditions. This is the data that reflects actual behaviour, not simulated environments, so it carries more weight when evaluating performance.
Measuring CLS with PageSpeed Insights (CrUX Data)
PageSpeed Insights uses Chrome UX Report data to show how real users experience CLS on your site. I use this as a quick benchmark to understand whether a page is considered stable from Google’s perspective.
Monitoring CLS with Real User Monitoring Tools
Real User Monitoring tools track layout shifts across live sessions, giving deeper visibility into when and where issues occur. When reviewing these, I look for patterns, such as shifts happening only on mobile or after specific scripts load.
In the Lab
You measure CLS in the lab by simulating page loads in controlled environments, which helps isolate technical causes of layout shifts. While this doesn’t fully reflect real usage, it’s useful for debugging specific problems.
Using Lighthouse to Detect Layout Shifts
Lighthouse highlights layout shifts during page load and provides a breakdown of elements causing instability. I often use it during development or audits to quickly identify problem areas.
Using Chrome DevTools for Layout Shift Debugging
Chrome DevTools allows you to record performance and visually track layout shifts frame by frame. This is especially useful when diagnosing complex issues tied to scripts or dynamic content.
Measure CLS in JavaScript
You can measure CLS in JavaScript using browser APIs that capture layout shift events in real time. This approach is more technical but gives precise insight into how shifts occur during user sessions.
Differences Between CLS Metric and API
The CLS metric aggregates layout shifts across a session, while the API provides raw event-level data. In practice, this means the values may not match exactly, so I treat the API as a diagnostic tool and the final CLS score as the benchmark for performance evaluation.
CLS vs Other Core Web Vitals
CLS differs from other Core Web Vitals because it focuses on visual stability, while the others measure speed and responsiveness. When I audit performance, I don’t look at CLS in isolation. It only makes sense when viewed alongside how fast the page loads and how quickly it reacts.
CLS vs Largest Contentful Paint (LCP)
CLS measures layout stability, while LCP measures how quickly the main content becomes visible. LCP is about first impressions — whether users see meaningful content fast enough to stay on the page.
That said, a fast LCP doesn’t guarantee a good experience. I’ve seen pages load quickly but then shift repeatedly as images or banners appear. From a user’s perspective, that still feels broken. LCP gets users in, but CLS determines whether the layout stays reliable.
CLS vs Interaction to Next Paint (INP)
CLS tracks how stable the layout is, while INP measures how quickly the page responds to user interactions. One focuses on visual consistency, the other on responsiveness during use.
In practice, these two often overlap. If a user clicks a button while the layout shifts, it affects both CLS and INP. I usually review them together because instability during interaction creates a compounded issue that users notice immediately.
How All Core Web Vitals Work Together
All Core Web Vitals work together to define the full user experience from start to finish. LCP ensures the page loads quickly, CLS ensures it stays visually stable, and INP ensures it responds smoothly when users engage.
From an SEO perspective, I treat them as a system rather than separate metrics. Improving how to reduce LCP without fixing layout shifts or interaction delays only solves part of the problem. When all three are aligned, the site feels fast, stable, and usable — which is what both users and search engines ultimately seek.
Fix Layout Shifts Before They Affect Rankings
CLS in SEO is about making your site feel stable and trustworthy from the moment it loads.
In most of the projects I’ve worked on, knowing how to fix CLS issues comes down to getting the fundamentals right. This includes reserving space, managing dynamic content, and tightening how assets load across the page.
If you’re looking at minimizing CLS but aren’t sure where to start, this is where having the right team matters.
At First Page Digital, we approach CLS as part of a bigger SEO strategy, not a standalone fix. If you want a clearer roadmap tailored to your site, you can book a FREE strategy call (worth $2000), and we’ll walk you through exactly what’s holding your performance back and how to fix it properly.
Frequently Asked Questions About CLS
What is CLS in Core Web Vitals?
CLS in Core Web Vitals is a metric that measures how much elements on a page move unexpectedly while it is loading, affecting how stable the layout feels to users. In simple terms, it tracks those moments when content shifts without warning, which can disrupt interactions and reduce trust in the page.
Why is cumulative layout shift important for SEO?
Cumulative layout shift is important for SEO because it reflects the quality of user experience, specifically how stable and predictable a page feels during loading. When layouts shift unexpectedly, users are more likely to hesitate, misclick, or leave, and while CLS acts as a supporting ranking factor, it can influence performance when competing pages offer similar content quality.
How to fix CLS issue?
You fix CLS issues by ensuring that space is properly reserved for all elements before they load, which prevents unexpected layout shifts and improves stability. This typically involves setting dimensions for images, controlling how dynamic content appears, and managing font loading so that elements render predictably.
How to improve cumulative layout shift?
You improve cumulative layout shift by making the page layout stable from the start, ensuring that all elements load within predefined space and do not shift during rendering. In practice, this means identifying unstable elements, fixing their loading behaviour, and maintaining consistency across how content is displayed.
What causes cumulative layout shift?
Cumulative layout shift is caused by elements loading without predefined space or appearing after the page has already rendered, such as images without dimensions, late-loading ads, or font swaps that change text layout. In most cases, multiple small shifts combine over time, which is why addressing all contributing factors is necessary for improving CLS.
