Cumulative Layout Shift Test
Measure CLS against the 0.1 threshold — how much the page jumps around while it loads.
- 100% free
- No sign-up
What is Cumulative Layout Shift Test?
Cumulative Layout Shift measures how much visible content moves while a page is loading, and it is the vital that most directly matches an experience everyone recognises: reaching for a button just as an advert loads above it and pushing the page down.
It is a unitless score — good is 0.1 or under, poor is above 0.25 — computed from how much of the viewport moved and how far. The causes are nearly always the same handful: images without width and height, web fonts swapping and reflowing text, content injected above what is already rendered, and embeds that arrive without reserved space.
How to use Cumulative Layout Shift Test
- 1
Test the page
CLS accumulates across the whole load, so it is measured per page and per session.
- 2
Set dimensions on every image
This is the single biggest cause. Width and height attributes let the browser reserve the space before the file arrives.
- 3
Reserve space for anything injected
Adverts, banners, embeds and consent bars all push content down if they arrive without a placeholder.
- 4
Check your font loading
font-display: swap trades an invisible-text period for a reflow when the real font arrives. Matching fallback metrics keeps the reflow small.
Everything this tool
does for you
- CLS against the 0.1 and 0.25 thresholds
- Mobile and desktop results
- Supporting load metrics
- No account and no queue
- Free, no signup, no daily limit
Further reading
Related free tools
Frequently asked questions
Beyond this tool
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- CLS on mobile and desktop
- Google’s 0.1 / 0.25 thresholds applied
- Explains the usual causes in order of likelihood
- Supporting metrics included
- Free, no account, no daily limit