Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) measures visual instability on a page. An unexpected movement might occur when an image loads without reserved space or a banner pushes a button downward just as someone tries to tap it. The current metric evaluates layout shifts in session windows rather than simply adding every shift over an unlimited page lifetime. A lower CLS generally means a steadier interface.

Web performance guidance treats a CLS score of 0.1 or less as good, evaluated at the 75th percentile of page visits. A score above 0.25 is considered poor. CLS is separate from Largest Contentful Paint (LCP), which measures loading performance, and Interaction to Next Paint (INP), which measures responsiveness.

To improve CLS, specify dimensions or aspect ratios for images and embeds, reserve space for dynamic content, and avoid injecting unexpected elements above existing content. Compare field data across mobile and desktop. A test score from one run does not represent every visitor's experience.