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Measure And Optimize Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) is one of the three Core Web Vitals metrics that impact search result rankings in Google.

CLS is a measure of how user experience is impacted by unexpected layout shift. Ever clicked a button only to have it move at the last second? That's layout shift in action, created when content moves around on the page after the initial load.

This article takes a closer look at what the CLS metric is, how you can measure it, and how it can be optimized.

What is Cumulative Layout Shift?​

The Cumulative Layout Shift metric exists to measure visual stability because of its importance to user experience. For example, you may have experienced a website where the page loaded initially and then a header or advertisement was injected into the top of the page causing the rest of the page to shift downwards.

CLS is used to measure this kind of visual instability as it may interfere with user actions such as clicking buttons or just reading text. It measures user frustration caused by a jumpy webpage.

In its worst examples, visual instability of this kind can lead to incorrectly making a payment or submitting a form.

Cumulative Layout Shift Example​

This filmstrip demonstrates content shift after initial render. The banner's load pushes the heading and article downward.

For this page the CLS value is 0.13.

Comulative Layout Shift filmstrip showing different rendering stages

If you look closely there are actually two layout shifts on this page. The first one occurs after 2.5s when the web font loads, causing the title and description to re-render. After this, the description takes up a little less space, and the content below it shifts upward.

Changes to how Cumulative Layout Shift is defined​

CLS measurement varies slightly across Chrome versions. Google documents CLS definition changes here.

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How is CLS calculated?​

Think of Cumulative Layout Shift as a "unexpected movement score" for your webpage. It's calculated by summing up individual layout shifts in a certain time window, which can cover up to 5 seconds.

Originally CLS measured total layout shift throughout the lifetime of the page. The new windowed definition was introduced in May 2021.

Each individual layout shift is scored based on two components:

  • the impact fraction measures how much of the screen area is impacted
  • the distance fraction measures how far an element has moved

The final score is calculated by multiplying the two numbers.

Example calculation​

Take the example layout shift below:

  • the main text takes up 50% the screen
  • the ad takes up 20% of the screen

So the total impact fraction for the shift is 70%. The main text moves down by 20% of the screen – that's the distance fraction.

The final layout shift score is 70% * 20% = 0.14.

Example layout shift

What is a good Cumulative Layout Shift score?​

A Cumulative Layout Shift score below 0.1 is considered good for user experience. Scores ranging from 0.1 to 0.25 are considered to need improvement, while scores above 0.25 are considered poor.

Cumulative Layout Shift is one of the Core Web Vitals metrics that Google uses as a ranking signal.

How does Cumulative Layout Shift affect Lighthouse scores?​

As of Version 10 (Feb 2023) Cumulative Layout Shift determines 25% percent of the overall Lighthouse Performance score.

The performance subscore for CLS is visible in DebugBear for example:

Performance score breakdown in DebugBear

What causes unexpected layout shift?​

Layout shifts during the initial page load usually happen when the initial page HTML has been returned by the server but additional resources like JavaScript and images are still loading. When these resources do load the page content changes and the existing page contents moves around, causing layout shift.

There are several common causes of Cumulative Layout Shift to be aware of:

Missing Single Page Application container height​

Single Page Applications often load content dynamically after initial render. This can cause layout shift when inserted elements have variable heights, leading to visual instability.

You can prevent this particular kind of layout shift by setting a min-height to a particular height on the application container for example 900px on the root element of your application to prevent CLS of this kind for viewports up to that size.

Missing image width and height attributes​

You can prevent some layout shifts if you know the size of the element that's being loaded. In that case, you can provide an empty placeholder with the appropriate height, and then fill in the contents later.

Setting the height and width value on images is considered a good practice to prevent CLS.

Web fonts​

Web fonts may trigger layout shifts when loaded text size differs from initial render.

You can reduce these layout shifts by using setting the font-display CSS property to either optional or fallback. Both will hide the text for up to 100ms, completely preventing layout shifts if the web font loads quickly. If the font loads slowly the browser will use a system font initally.

font-display: fallback switches to the web font if it loads within 3s. font-display: optional never shows the web font if it takes more than 100ms to load.

Simon Hearne has written an article all about preventing layout shifts caused by web fonts.

Web fonts causing layout shifts

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Why is Cumulative Layout Shift different in field and lab data?​

Lab data is in an isolated test environment while field data is collected from real users. Lighthouse lab data only tests layout shifts during the initial page load, while field data also counts layout shifts that users experience when scrolling down the page. This can lead to discrepancies and sometimes make Cumulative Layout Shift difficult to debug.

How to measure Cumulative Layout Shift​

There are several tools which can help measure and diagnose Cumulative Layout Shift issues:

  • Chrome DevTools has a layout shift highlight feature
  • Lighthouse performance tests measure CLS
  • DebugBear contains Cumulative Layout Shift measurements in the Overview tab
  • The Layout Instability API programmatically detects layout shift with JavaScript

Continue reading to learn how to run a CLS test on your website.

Layout Shifts in Chrome DevTools​

Chrome DevTools can highlight layout shifts as part of its Animation tooling. First, click the three dots in the top right corner, then select More tools, and finally click Animations.

DevTools More tools menu item

You can then enable highlighting for Layout Shift Regions.

DevTools animations pane

Now, when content moves on the page, Chrome will show a blue highlight rectangle for the affected DOM nodes.

Layout Shifts highlighted on the inspected page

Cumulative Layout Shift in Lighthouse​

You can find the CLS metric as one of the 6 key metrics at the top of each Lighthouse report.

The filmstrip below can help identify what's causing the layout shift.

Performance metrics at the top of the Lighthouse report

There's also a more detailed "Avoid large layout shifts" audit, which breaks down different LayoutShift events as reported by Chrome.

CLS audit in the Lighthouse Performance section

Use the DebugBear CLS Checker​

Running a website speed test with DebugBear gives you access to the CLS checker.

You can find the Cumulative Layout Shift metric in the page Overview tab of the DebugBear application, just below the filmstrip.

Run a test today with DebugBear to identify your CLS metric score.

Cumulative Layout Shift in DebugBear

The Web Vitals tab includes a CLS debugger that shows all layout shifts on the page and provides in-depth diagnostics to help you check and improve your Cumulative Layout Shift score.

Detailed list of layout shifts

Layout Instability API​

The Layout Instability API detects layout shifts and calculates CLS in real-time within your web application.

Here's how to use a PerformanceObserver to retrieve and monitor layout-shift entries:

var cumulativeLayoutShift = 0;

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    // Don't count if the layout shift is a result of user interaction
    if (!entry.hadRecentInput) {
      cumulativeLayoutShift += entry.value;
    }
    console.log({ entry, cumulativeLayoutShift });
  });
});

// `buffered: true` to report layout shifts that have already happened on the page
observer.observe({ type: "layout-shift", buffered: true });

A layout-shift entry looks like this – each event is attributed to a set of DOM nodes.

Entry in the Layout Instability API

Monitoring Cumulative Layout Shift over time​

You can use DebugBear to measure Cumulative Layout Shift over time and optimize your CLS scores.

The Web Vitals tab shows both real-user data and the result of lab-based performance test. The ral user data comes from the Chrome User Experience Report (CrUX), which Google uses as a ranking signal.

CLS web vitals

You can also get a high-level view of page speed and Core Web Vitals on your website.

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