# What is Cumulative Layout Shift? Unexpected page movement

A web-performance metric for unexpected movement of visible page content, calculated from layout-shift events under its scoring rules.

An ad that pushes a paragraph down while someone reads is one possible source of layout shift. CLS measures qualifying unexpected movement; a screenshot of the finished page cannot show the movement that happened during loading.

## Finding the movement a reader sees

Imagine a reader reaches a paragraph just as an ad slot expands from 250 to 600 pixels tall. The paragraph moves down 350 pixels, potentially losing the reader’s place. Reserving room can prevent that particular movement, but the correct reservation depends on the accepted ad sizes and page layout.

## Why an ad slot needs a stable place

Reserve suitable space for ads before they load, then test small, large, late and absent creatives. A fixed container that works on desktop may waste space or break a phone layout. Use a performance trace or field report to measure the score, alongside watching the actual reading experience.

## Separate ad movement from other movement

Watch the same portion of the article while an ad appears. If the headline moves before the ad loads, check other elements too, such as an image whose space was not reserved. The timing helps narrow down the cause.

## Related terms

[Header bidding](https://digitalpublishingnews.com/dictionary/header-bidding/)

## Continue reading

[How to reduce article text movement caused by late-loading ads](https://digitalpublishingnews.com/articles/ad-slots-layout-shift/): It tests reserved ad space when an ad is late or the slot remains empty.

## Sources

https://web.dev/articles/optimize-cls

https://web.dev/articles/optimize-cls

Editorial method: https://digitalpublishingnews.com/how-we-use-ai/

Canonical: https://digitalpublishingnews.com/dictionary/cls/