Cumulative Layout Shift
Cumulative Layout Shift (CLS) quantifies visual stability by calculating how much elements within the viewport move around during page load. It helps pinpoint how often unexpected movement occurs on your pages. CLS is a Core Web Vital metric.
How is Cumulative Layout Shift calculated?
Cumulative Layout Shift is the sum of the largest, unexpected layout shift scores from all of the unstable elements during the lifespan of a page load. The layout shift scores are reported via the Layout Instability API and are a product of:
- distance fraction: the distance any unstable element has moved in the frame
- impact fraction: the surface area of the viewport that’s affected by unstable elements between two frames
Learn more in Cumulative Layout Shift: Measure and Avoid Visual Instability and web.dev guide to Cumulative Layout Shift calculation.
What Cumulative Layout Shift measurement is fast?
Pages with Cumulative Layout Shift lower or equal than 0.1 can be considered as fast, and recordings above 0.25 are considered slow.
