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Minimize layout shift

A layout shift occurs when a visible element on your page changes position or size, affecting the position of content around it. Sometimes the shift is intended, such as when a container expands as a result of a user action. However, the dynamic nature of ads can lead to unexpected layout shifts, which have a negative effect on the user experience and can cause serious usability problems.

This guide explains how to measure and minimize layout shift when working with Google Publisher Tags (GPT).

How ads cause layout shift

Ads are usually requested asynchronously and dynamically add content to your page during or after page load. While ads are being fetched, the rest of the page continues to load and non-ad content may become visible to the user. If you don’t reserve sufficient space for the ads being loaded, they can end up displacing visible non-ad content when they are eventually added to the page.

When working with Google Publisher Tags, there are a few points in the ad lifecycle where layout shift can occur:

  1. When display() is called. A slot may expand or collapse, depending on how it was configured.
  2. When ad content is rendered. A slot may be resized if served a fluid ad or if insufficient space is otherwise available. A slot may also expand or collapse at this point, depending on how it was configured.
  3. After ad content is rendered. Certain creative types are designed to expand after they appear on the page.

Keep in mind that the higher an ad slot is within the viewport, the more content it has the potential to displace. Take special care with slots near the top of the initial viewport (above the fold). These slots can cause a disproportionate amount of layout shift because they are more likely to be visible when their ad content is loaded.

Measuring layout shift

Cumulative Layout Shift (CLS) is a Core Web Vitals metric that you can use to quantify the impact of layout shifts on your site, both in the lab and in the field.

In the lab

Lab tools measure CLS synthetically. This means that they don't rely on real user interaction, making them well suited for use in continuous integration and local development workflows. However, these tools usually only measure performance during page load and are limited in the conditions they can simulate, so reported values may be lower than what a real user would experience.

Publisher Ads Audits for Lighthouse is a tool that can be used to measure CLS specifically attributable to GPT ads. For details, see the reduce ad-related layout shift audit documentation.

Chrome DevTools can also be configured to highlight layout shifts as you navigate your site. This can be used to manually identify layout shifts that occur near ad slots on your page.

In the field

Field tools measure CLS experienced by real users as they interact with your site. This process is commonly known as real user monitoring (RUM). RUM allows you to observe how CLS is affected by real-world factors—such as device capabilities, network conditions, user interaction, and page personalization—which are often difficult or impossible to simulate with synthetic tests.

Minimizing layout shift

The only guaranteed way to avoid layout shift is to reserve a sufficient amount of space for a given ad slot using CSS. Setting a fixed height and width directly on the ad slot div is the most effective way to do this. However, while this works well for static, fixed-size ad slots, more complicated scenarios may require a more nuanced approach. Some common scenarios are explained in the following sections.

Multi-size ad slots

For ad slots that accept multiple sizes, we recommend one of the following approaches:

  • Reserve space for the largest size configured to serve.
  • Reserve space for the smallest size configured to serve.
  • Reserve space for the size most likely to serve, determined by examining historical fill data from Google Ad Manager reporting.

Choosing the right approach

Reserving space for the largest size configured to serve is the most effective way to eliminate layout shifts. However, this method can result in extra blank space around the ad, in the event a creative smaller than the reserved size is served. Shrinking the ad slot to match the size of the served creative would incur an additional layout shift, so it's recommended to avoid doing this. Instead, vertical and horizontal centering can be used to reduce the visual impact of excess blank space.

On the other hand, reserving space for the smallest size configured to serve avoids excess blank space around your ad. This can be a good option if your ad slot is commonly filled with smaller creatives, or in cases where all of the sizes the slot supports are similar. However, this method results in a layout shift in the event a creative larger than the reserved size is served (although such shifts are generally smaller when compared to not reserving space at all).

To strike a balance between blank space and layout shifts, you can reserve an "average" amount of space for your ad slots. For example, reserving 100px vertically incurs a little blank space when a 320x50 creative is served, but reduces the CLS score compared to reserving no space at all. You'll need to experiment with different sizes to find the best balance for your own site.

When determining how much space to reserve for a given slot, examining historical fill data from Google Ad Manager reporting can help you make a more informed decision. This can be best illustrated by exploring some examples.

Example #1
Creative size (delivered) Ad Server Impressions (%)
300x250 70%
320x50 30%

In this case, reserving 250px vertically can greatly reduce CLS since the majority of served creatives are 300x250.

Example #2
Creative size (delivered) Ad Server Impressions (%)
970x90 60%
728x90 10%
320x50 20%
728x250 5%
300x250 5%

In this case, a majority of ads are at most 90px tall, so reserving 90px vertically should avoid a layout shift most of the time.

How to reserve space

We recommend tackling this issue by specifying the size of your ad slot via min-height and min-width CSS properties:

<div id="ad-slot" style="min-width: 300px; min-height: 250px;"></div>

Using min-height and min-width attributes lets you reserve a minimum amount of space for your ad slot, while still allowing the browser to increase the size of the container as necessary. This ensures that no content gets cut-off in the event that a larger creative is served.

You can combine this technique with CSS media queries to specify different minimums for different screen sizes:

@media screen and (max-width: 960px) {
  #ad-slot {
    min-height: 100px;
  }
}

Reserving space with JavaScript should be avoided, since doing so can result in a layout shift at the time the script loads. Reserving space with CSS avoids this risk.

Fluid ad slots

Fluid ad slots do not specify a fixed set of sizes that they support. Instead, these slots automatically resize to fit the creative content served to them, allowing them to support creatives of indeterminate size. As a result, it's not possible to reserve space for these slots prior to requesting ad content, and fluid sized ads always cause layout shifts.

To mitigate the effects of layout shifts when working with fluid ad slots, we recommend the following:

  • Only use the fluid size for slots below the fold.
  • Fetch fluid slots as early as possible to minimize the chance of a user scrolling them into view before the slot is resized.

Collapsing and expanding ad slots

The collapseEmptyDivs() method allows you to collapse ad slot divs so that they don't take up space on the page when there is no ad content to display. However, this feature should be used with caution because it can introduce unintended layout shifts. As noted in the preceding section, collapsing and expanding ad slots can cause layout shifts at two different points in the ad lifecycle.

If you need to make use of this feature, you can reduce the impact of layout shifts by using historical fill data from Ad Manager reporting to implement the following best practices:

  • Slots which are likely to be filled should always start out expanded.
  • Slots which are unlikely to fill should always start out collapsed.

For an example implementation, see the Collapse empty ad slots sample.