Cumulative Layout Shift (CLS)
Measures unexpected visual movement observed during the current synthetic page load.
Free CLS test
Test Cumulative Layout Shift in a real browser.
Measure the CLS observed during a controlled Chromium page load and keep the result in context with page weight, JavaScript, rendering and the rest of the SiteState performance report.
Free · No signup required · Real Chromium lab test
What this tool measures
Visual stability inside the complete page-load report
Largest Contentful Paint
Shows main-content rendering timing so layout stability can be reviewed alongside load speed.
Total Blocking Time
Adds main-thread blocking context when JavaScript and rendering work are heavy.
Page weight
Shows transferred bytes and resource count for the current browser run.
JavaScript CPU
Shows sampled browser CPU and third-party script impact from the same page load.
Resource analysis
Identifies high-impact resources even when they are not directly responsible for every layout shift.
Interpret the result
CLS can vary when the page itself is dynamic.
Layout shifts are often caused by content that changes after the first render, such as images without reserved space, late fonts, ads, embeds or injected interface elements. Dynamic content can also make one lab run different from another.
The SiteState CLS result is therefore a measurement of the current controlled page load, not a guarantee that every real visitor sees the same value. Repeated synthetic monitoring is useful for spotting persistent changes and regressions.
Methodology
CLS is measured during a controlled Chromium lab run. SiteState currently reports the aggregate CLS value from that run; it does not yet provide element-by-element layout-shift replay.
FAQ
Common questions about this speed test
What is CLS?
Cumulative Layout Shift measures unexpected visual movement during a page session. A layout that moves while loading can make the page feel unstable even if it loads quickly.
Why can CLS change between tests?
Dynamic ads, fonts, injected content, embeds, responsive layouts and other conditional content can change which layout shifts occur on each load.
Does SiteState show which exact element shifted?
Not yet. The current free tool reports the aggregate CLS from the lab run and the wider performance report. Element-level layout-shift replay is not part of the current tool.
Can I monitor CLS over time?
Yes. SiteState can keep compatible synthetic browser metrics under continuous monitoring so sustained changes become visible as performance regressions.
Related free tools
Test another part of website speed
Continuous performance
Need more than one test? Website performance monitoring keeps compatible browser metrics and regression history over time. Performance.
A one-off speed test is only a snapshot.
SiteState can keep the same website under continuous monitoring and detect when performance changes.