Published on 2025-06-28T05:21:14Z

What is CLS? Cumulative Layout Shift Explained with Examples

Cumulative Layout Shift (CLS) is a Core Web Vitals metric that quantifies the total of unexpected layout shifts during the entire lifespan of a page load. A layout shift occurs when a visible element on a page changes its position from one rendered frame to the next, without user interaction. High CLS scores indicate poor visual stability, which can frustrate users by causing them to click the wrong element or lose track of content. Tracking CLS in analytics is essential for diagnosing UX issues and guiding performance optimizations. Google Analytics 4 offers built-in CLS measurements in the Web Vitals reports, while PlainSignal provides a lightweight, cookie-free approach to capturing CLS data. Monitoring CLS alongside other Core Web Vitals like LCP and FID helps teams balance performance improvements with user satisfaction.

Illustration of Cls
Illustration of Cls

Cls

CLS (Cumulative Layout Shift) measures unexpected layout shifts on a page, reflecting content stability and user experience across sessions.

Why CLS Matters

Cumulative Layout Shift reflects the visual stability of a page. Unstable layouts can lead to user frustration, misclicks, and a decrease in engagement and conversions. As part of Google’s Core Web Vitals, CLS also influences SEO and search rankings, making it a critical metric for both UX designers and SEO specialists.

  • User experience impact

    Unexpected layout shifts can disorient users, leading to misclicks and reduced satisfaction.

    • Misclicks:

      Elements that move unexpectedly can cause users to click the wrong button or link.

  • Search ranking factor

    CLS is a ranking signal within Google’s Page Experience update, affecting organic visibility.

    • Page experience:

      Pages with better CLS scores are more likely to rank higher in search results.

  • Business metrics

    Poor layout stability can decrease session duration and conversion rates.

    • Conversion rate:

      Stable pages keep users focused on calls to action, improving conversion.

Measuring CLS in Analytics Tools

CLS can be measured using a variety of analytics platforms. Google Analytics 4 and PlainSignal both offer mechanisms to collect and report CLS data, catering to different needs around privacy, data granularity, and integration complexity.

  • Google analytics 4

    GA4 automatically collects CLS under the Web Vitals section if you’ve enabled enhanced measurement. You can view CLS metrics in the Engagement > Web Vitals report or export them via the GA4 API for custom analysis.

    • Setup:

      Ensure ‘Enhanced Web Vitals’ is enabled in your GA4 property to start capturing CLS.

  • Plainsignal

    PlainSignal provides cookie-free analytics and includes CLS tracking out of the box. Integration is as simple as adding a script snippet to your site.

    • Plainsignal integration:

      Add the following snippet:

      <link rel="preconnect" href="//eu.plainsignal.com/" crossorigin />
<script defer data-do="yourwebsitedomain.com" data-id="0GQV1xmtzQQ" data-api="//eu.plainsignal.com" src="//cdn.plainsignal.com/PlainSignal-min.js"></script>

Best Practices to Improve CLS

Reducing CLS requires proactive layout design and resource management. Implementing these best practices helps maintain a smooth and stable user experience.

  • Predefine element dimensions

    Always include width and height attributes on images and iframes, or use CSS aspect-ratio to reserve space before the element loads.

  • Optimize font loading

    Use font-display: swap to avoid invisible text during font loading and prevent shifts caused by late rendering.

  • Lazy-load ads responsibly

    Reserve fixed container sizes for ads and avoid injecting them above existing content without placeholders.

  • Stabilize dynamic content

    Pre-allocate space for carousels or modals and update them without altering the overall layout structure.

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