Cumulative Layout Shift (CLS) is a Core Web Vitals metric developed by Google that measures the visual stability of a web page during loading. It quantifies how often users experience unexpected layout shifts — instances where page elements (text, images, buttons) move around while the page is still loading, often causing users to accidentally click the wrong element. CLS is scored on a scale from 0 (no shifting) to any positive number.
How CLS is Measured
CLS measures the sum total of all unexpected layout shift scores occurring during the entire lifespan of a page. Each individual layout shift score is calculated by multiplying the impact fraction (how much of the viewport shifted) by the distance fraction (how far the element moved). A good CLS score is 0.1 or less. A score between 0.1 and 0.25 needs improvement, and anything above 0.25 is considered poor. Google measures CLS using real-world Chrome user data collected through the Chrome User Experience Report (CrUX).
Common Causes of High CLS
Most CLS problems stem from resources loading without reserved space:
- Images without specified width and height attributes
- Ads, embeds, or iframes without reserved dimensions
- Dynamically injected content above existing page content
- Web fonts causing text to reflow (FOUT — Flash of Unstyled Text)
- Animations or transitions that trigger layout changes
Why It Matters for SEO
CLS is one of three Core Web Vitals that Google uses as ranking signals through the Page Experience update. Beyond rankings, high CLS creates a genuinely poor user experience — accidental clicks on ads or wrong buttons frustrate users and damage trust. Improving CLS benefits both SEO and conversion rates, as a stable page layout keeps users focused and in control of their interactions. Setting explicit size attributes on all images and ads is often the fastest path to CLS improvements.