Glossary
What is CLS (Cumulative Layout Shift)?
Also called: Cumulative Layout Shift, layout shift
CLS (Cumulative Layout Shift) is the Core Web Vital that scores how much visible content jumps around while a page loads. 0.1 or less is good.
हिंदी में पढ़ेंCumulative Layout Shift measures visual stability. Each unexpected shift scores impact fraction × distance fraction: the share of the screen covered by the moving content, before and after, times how far it moved as a share of the screen’s larger side. A CLS of 0.1 or less is good and over 0.25 is poor, measured at the 75th percentile of page loads. Shifts within 500 ms of a tap or click do not count.
Ads are a common cause. An ad that loads late above the text pushes the article down just as the reader starts reading, and one tall ad can take a page from good to poor on its own.
Google’s guidance is to reserve each slot’s space in CSS, with min-height, before the ad tag runs, not with JavaScript afterwards. For multi-size slots, use your fill data to choose the size to reserve, and do not shrink the slot for a smaller creative. Then the ad fills a box that is already there, and nothing moves.
Example
On an 800-pixel phone screen, text covers 650 pixels before and after a 300×250 ad pushes it down 250 pixels. Impact is 650 ÷ 800 = 0.8125 and distance is 250 ÷ 800 = 0.3125. 0.8125 × 0.3125 ≈ 0.254: poor, from a single ad.
Why it matters to publishers
Layout shift annoys readers and hurts one of Google's Core Web Vitals. Reserving slot height is one of the cheapest speed fixes for ad-heavy pages.