The State of Web Vitals

Iframe dimension sizing and Cumulative Layout Shift

Unsized iframes (81.6%) versus explicitly sized iframes (51.4%): how reserving embed dimensions affects Cumulative Layout Shift across iframe-using sites.

Field data PhoneDesktopAll Scope All sites October 2026 edition · Phone field outcomes

81.6% of sites use unsized iframe dimension sizing, and 85% of those sites have a good CLS, against 89% of all sites.

The State of Web Vitals · October 2026 · 210,902 crawled sites · 126,761 with phone field data · 86,714 sites with iframe dimension sizingBy Arjen Karel · corewebvitals.io/state-of-cwv
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CLS pass rate

Sites with unsized iframe dimension sizing have a good CLS most often: 85%, against 89% of all sites. Sites with explicitly sized iframe dimensions have it least often, at 85%.

SitesGood, needs improvement, poor CLSMedian
All sites126,76189%0.00
Unsized iframe (no width/height) 81.6%45,81885%0.00
Explicit width & height attributes 51.4%27,37985%0.00
GoodNeeds improvementPoorSites: with Chrome field data
The State of Web Vitals · October 2026 · 210,902 crawled sites · 126,761 with phone field data · 86,714 sites with iframe dimension sizingBy Arjen Karel · corewebvitals.io/state-of-cwv
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Iframe dimension sizing usage

Unsized is the most used: 81.6% of sites.

Sites using each option (sites can use more than one) 86,714 sites
Unsized iframe (no width/height) 81.6% Explicit width & height attributes 51.4%
Unsized iframe (no width/height) 81.6% Explicit width & height attributes 51.4%
The State of Web Vitals · October 2026 · 210,902 crawled sites · 126,761 with phone field data · 86,714 sites with iframe dimension sizingBy Arjen Karel · corewebvitals.io/state-of-cwv
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Understanding iframe dimension sizing

Why visible iframes cannot infer an aspect ratio without attributes

Unlike images, an <iframe> has no intrinsic dimensions in its network header; without explicit width and height attributes or a sized container, the browser defaults the frame to 300x150 pixels until scripts resize it. Across iframe-using sites, 51.4% size their embeds explicitly (85% good CLS) and 81.6% include unsized iframes (85% good CLS, largely hidden utility sandboxes).

Reserve fixed width and height attributes or a CSS aspect-ratio container around every visible iframe so late postMessage resizes do not shift surrounding layout. Across sites with iframes, 51.4% set explicit iframe dimensions (85% good CLS) while 81.6% contain at least one unsized iframe (85% good CLS).

Go fix it Perfect YouTube Core Web Vitals → Google Maps with 100 percent PageSpeed → CLS from images and media →
The State of Web Vitals · October 2026 · 210,902 crawled sites · 126,761 with phone field data · 86,714 sites with iframe dimension sizingBy Arjen Karel · corewebvitals.io/state-of-cwv
Platform & Framework Takeaways

How iframe dimension sizing plays out per platform

What the field data shows when you isolate a single CMS or framework.

WordPress 30,414 sites
62.8% size embeds explicitly (87% good CLS)

62.8% of iframe-using WordPress sites set explicit iframe dimensions (87% good CLS).

Across iframe-using WordPress sites, 62.8% include explicit width and height attributes on iframes (87% good CLS) and 82.1% include unsized iframes (87% good CLS).

Read the WordPress iframe dimensions breakdown →
Shopify 6,746 stores
97.7% unsized hidden frames (93% good CLS)

97.7% of iframe-using Shopify stores include unsized iframes (93% good CLS).

Across iframe-using Shopify storefronts, 97.7% contain an iframe without HTML width or height attributes (93% good CLS) and 15.8% use explicit dimensions (90% good CLS).

Read the Shopify iframe dimensions breakdown →
Related signals Scripts per page → DOM depth → DOM size → Page weight & requests → By Arjen Karel · 210,902 crawled sites (126,761 with phone Chrome field data, 86,714 sites with iframe dimension sizing, October 2026) · How we measured