MetricsCLS
Cumulative Layout Shift
A unitless score for how much visible content unexpectedly moves around while the page loads and runs.
- Good
- <= 0.1
- Needs work
- 0.1 to 0.25
- Poor
- > 0.25
Field data (75th percentile)
CLS is the visual stability Core Web Vital. Every time an element shifts without a user action, the browser scores it by how much of the viewport moved and how far. CLS sums the worst burst of those shifts during the visit.
It is the metric behind the familiar frustration of tapping a button that jumps the instant a late banner or image loads. Unlike the time-based vitals it has no unit: lower is better, and 0 means nothing moved.
What moves it
- Images and embeds without width and height (or an aspect ratio).
- Ads, banners and iframes injected above existing content.
- Web fonts that reflow text when they swap in.
- Content inserted dynamically without reserved space.
Related terms
- CWVCore Web VitalsGoogle's set of three field metrics for loading (LCP), interactivity (INP) and visual stability (CLS) of a page.
- LCPLargest Contentful PaintThe time until the largest visible element in the viewport (image, video or text block) has rendered.
- INPInteraction to Next PaintHow long the page takes to visually respond after a user interacts, measured across the whole visit.
- FCPFirst Contentful PaintThe moment the browser renders the first piece of content: text, an image or a canvas.
- TTFBTime to First ByteThe time from the start of navigation until the browser receives the first byte of the response.
- FIDFirst Input Delay (retired)The delay between a visitor's first interaction and the browser starting to process it. Replaced by INP.
- TBTTotal Blocking TimeThe total time the main thread was blocked by long tasks between first paint and interactivity, measured in the lab.
- SISpeed IndexA lab metric for how quickly the visible parts of a page fill in during load, based on video capture.
- TTITime to Interactive (legacy)A lab metric for when a page has rendered and can reliably respond to input quickly.
- Page LoadThe classic load event: everything on the initial page, including images and subresources, has finished loading.
- Route LoadThe loading time of an in-app navigation in a single-page app, where no full page reload happens.
- Long Tasks & LoAFAny piece of JavaScript that occupies the main thread for more than 50 ms, blocking the page from responding.
- Experience ScoreA single 0 to 10 number fastmon derives from LCP, INP, CLS, FCP, TTFB, load time and error rate.
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