How Google Measures Cumulative Layout Shift (CLS) Explained

Ever visited a website where the content shifts suddenly just as you're about to click a button? Annoying, right? That’s what Cumulative Layout Shift (CLS) is all about. It's one of the key metrics Google uses to evaluate your website's performance. So, how exactly does Google measure CLS, and why should you care? Let's break it down in a way that’s easy to digest.

In this article, we’ll explain how Google measures Cumulative Layout Shift (CLS), how it affects your website, and what you can do to improve it. By the end of this guide, you'll not only understand CLS but also have actionable insights to optimize it for your website's success.

What is Cumulative Layout Shift (CLS)?

Before diving into how Google measures CLS, let’s first tackle what it is. CLS refers to unexpected layout shifts that happen during the lifespan of a webpage. These shifts typically occur when a webpage element, like an image or ad, loads late, causing the content around it to move unexpectedly.

Now, Google has decided this is a problem—and they’re right! This metric affects Core Web Vitals, a set of guidelines Google uses to ensure a smooth user experience. If your webpage has poor CLS, it can harm your search rankings and annoy your visitors. But how exactly does Google measure CLS? Let's explore.

How Google Measures Cumulative Layout Shift (CLS) Explained: The Core Methodology

1. What Exactly Causes Layout Shifts?

Google defines a layout shift as any visible element moving from one frame to another unexpectedly. These shifts can happen for several reasons, such as:

• Images without defined dimensions
• Ads, banners, or iframes injected into the page dynamically
• Fonts loading late, causing text to shift
• Elements like buttons or forms that appear at different stages of page load

Each time a shift occurs, Google measures it using two main factors: the impact fraction and the distance fraction.

2. Impact Fraction and Distance Fraction

• Impact Fraction: This measures how much space an unstable element takes up in both its old and new positions. Essentially, it’s the percentage of the viewport affected by the shift.
• Distance Fraction: This measures the movement distance of the shifting element, based on the viewport height.

Google multiplies these two values to get the Layout Shift Score. The higher the score, the more severe the shift. For example, if a large image suddenly moves down the page, affecting a wide section of the screen, it will result in a high layout shift score.

3. How Google Calculates Your Final CLS Score

Google doesn’t just focus on one shift; they evaluate the entire lifespan of the webpage. The final CLS score is determined by adding up all the individual layout shift scores that occur while users interact with your site.

Optimizing Your Site for Better Performance

Now that we’ve explained how Google measures Cumulative Layout Shift (CLS), let’s look at ways to improve it.

1. Predefine Dimensions for Media

One common cause of layout shifts is images or videos loading late, pushing content around. The fix is simple: always specify width and height attributes for your media. This reserves space for the element even before it loads, preventing any unexpected shifts.

2. Avoid Ads Without Fixed Sizes

Ads are notorious for causing CLS issues. They load unpredictably, changing the layout of your page in ways that frustrate users. To avoid this, use reserved ad space by setting a minimum height for your ad containers.

3. Ensure Consistent Fonts

Fonts can be another sneaky culprit. Web fonts sometimes load slower than expected, causing text to shift. Use font-display: swap in your CSS to avoid a "flash of unstyled text" (FOUT) and keep the layout stable while the correct font loads.

4. Manage Dynamic Content Carefully

Be mindful when adding dynamic elements like pop-ups, forms, or chat boxes. Ensure they load without disrupting the layout by inserting them below existing content or using animation effects that don’t cause page shifts.

5. Lazy Load Below-the-Fold Images

Lazy loading images is great for performance, but it can also cause layout shifts if the image loads late. To avoid this, only lazy load below-the-fold content, while above-the-fold images should load as soon as possible.

FAQs About How Google Measures Cumulative Layout Shift (CLS) Explained

Q: How does CLS affect my website's SEO?

A: CLS is a part of Google's Core Web Vitals, which directly impacts your SEO rankings. Websites with poor CLS scores will see a dip in their rankings, as Google prioritizes user experience.

Q: Can a high CLS score increase bounce rates?

A: Absolutely. When users experience sudden layout shifts, especially when they’re trying to click something, they get frustrated and are more likely to leave your site, increasing bounce rates.

Q: What CLS score should I aim for?

A: Google recommends a CLS score of 0.1 or lower. Anything higher than that could lead to a subpar user experience.

Q: How can I check my website's CLS score?

A: You can measure your CLS score using tools like Google’s PageSpeed Insights, Lighthouse, or Web Vitals. These tools will give you an exact breakdown of your CLS issues and suggestions on how to fix them.

Q: Do mobile websites have different CLS considerations?

A: Mobile websites often suffer more from layout shifts due to varying screen sizes. Optimizing CLS for mobile involves making sure images and ads are responsive and properly sized for smaller screens.

Final Thoughts: Google isn’t just about fast loading times; it’s about creating seamless, frustration-free browsing experiences. Now that you know how Google measures Cumulative Layout Shift (CLS) and why it’s so important, it’s time to take action and refine your website for optimal performance.