CLS thresholds and scoring
| Rating | CLS |
|---|---|
| Good | 0.1 or less |
| Needs improvement | Over 0.1 and up to 0.25 |
| Poor | Over 0.25 |
web.dev measures CLS at the 75th percentile of page loads, split by mobile and desktop. It is one of three Core Web Vitals, with LCP for loading and INP for responsiveness.
A CLS of 0 means nothing moved on its own. Each unexpected shift adds to the score based on how much of the screen moved and how far it moved.
The score is not a total of every shift. web.dev groups shifts that happen less than 1 second apart into a session window, up to 5 seconds long. The page's CLS is the score of its worst window.
Shifts within 500 milliseconds of a click, tap, or key press are left out, since the visitor expects the page to change then. Scrolling, dragging, and pinch-zoom do not count as that kind of input.
What causes layout shift
web.dev lists the common causes. Each one shows up on ad landing pages:
- Images or videos without dimensions. The page does not know how much room to save, so text jumps when the photo loads. Set width and height on images.
- Ads, embeds, and widgets that resize after loading. Review widgets, map embeds, and chat bubbles.
- Content injected above existing content. A banner or cookie bar that pushes the form down after the page appears.
- Web fonts. The fallback font is a different size than the final font, so text reflows when the font arrives.
The simple fix for most of these is to save space. Give images and embeds a set size, keep banners from pushing content after load, and load fonts so the swap does not change the text size much.
Layout shift on a lead page is more than a score. If the quote form moves as someone reaches for it, they can tap the wrong thing.
A free ClickMend scan includes Core Web Vitals in the score (25%), using real-user CrUX field data when Google has it and lab data if not. See the landing page speed test guide for the rest of the report.
Frequently asked questions
Does a shift after someone taps a button count?
Usually not. web.dev says shifts within 500 milliseconds of a click, tap, or key press are left out. Scrolling and pinch-zoom do not get that pass.
Is CLS a time?
No. CLS is a score with no unit. Good is 0.1 or less, needs improvement is between 0.1 and 0.25, and poor is over 0.25.