Cumulative Layout Shift Signal Test
The Layout Shift Test isolates each individual layout-shift event during page load — what shifted, when, by how much and why — so you can fix the underlying cause element-by-element.
- Results in seconds
- Pass / fail + fix guidance
- No account required
The Layout Shift Test isolates each individual layout-shift event during page load — what shifted, when, by how much and why — so you can fix the underlying cause element-by-element. Where the CLS Test gives you a single composite score, this tool gives you the per-event detail needed for fast debugging.
What This Tool Checks
- Per-event shift score and timestamp
- Element selector for every shift
- Cause classification (image, font, ad, third-party)
- Shift score above the 0.05 individual-event threshold
- Cumulative impact during the first 5 seconds of load
Why It Matters for SEO
CLS is best fixed event-by-event. The composite score tells you "the page shifts too much" but not "this image moved 30 px when the font loaded at 1.2 s". This per-event view lets engineers fix layout instability in priority order, ship one change at a time and verify with the next test.
How to Fix It
Pick the highest-score event in this report and apply the fix the cause classification suggests. Re-test. Move to the next event. Iterate until cumulative score drops below 0.1.
How It Works
Headless Chrome subscribes to the Layout Instability API and records every shift event as it happens. Each event is enriched with the element selector, the rectangle that moved, the bytes / scripts loading at that moment, and a likely-cause classification.
Common Mistakes to Avoid
- Trying to fix CLS by guessing instead of measuring per event
- Treating only the largest shift while ignoring smaller events that aggregate
- Fixing one image and forgetting the same template ships dozens
- Late-loading third parties (ads, chat) injecting layout after first paint
Quick Checklist
- No single shift event above 0.05
- No shifts after second 3 of load
- Per-element fix logged for each remaining shift
- Cumulative score under 0.1
- Re-tested after every deploy
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The CLS test gives you the composite score. This tool breaks the score down into individual shift events so you can fix each cause one at a time.
A single event above 0.05 score is significant on its own. Many smaller events under 0.05 can also aggregate into a poor cumulative score.
No. The Layout Instability API explicitly excludes shifts within 500 ms of a user input, so accordions, expanding menus and intentional UI animations do not penalise your score.
Reserve space with min-height for any third-party container (ads, chat widgets, embeds). If the third party cannot be fully sized, defer it until after first paint or load it lazily.
Usually because of a late-loading third party, a font swap, or a "load more" pattern. The API only counts shifts within the first 5 seconds for the official metric, but later shifts still hurt UX.