Cumulative Layout Shift
also called CLS
Definition
How much the page content moves around unexpectedly while it loads.
How it's calculated
Read from the product's weighted summary measure as a raw layout-shift value. Display rounds to three decimal places; grading buckets the raw value at 0.1 and 0.25. No vital is rescaled on the way out, the timing vitals are read and shown in seconds, and this one is read and shown as its native unitless score.
Scope, grain and dimensions
- Grain
- One number per date range, optionally split by device, or one row per page.
- Dimensions
- page · device
- Required filters
- date_range
- Aggregation
- The headline is the weighted summary measure read without a page dimension; page-level values feed distributions and worst-page evidence.
- Metric type
- score · unitless layout-shift score
Data sources
Where you'll see this
Named reports that normally include this metric.
Skills and analyses that use it
Skills carry the judgment; the analysis verbs do the reading.
Analysis verbs
site_health_cwv_check
Method rungs and levers
This sits on the bottom rung: if it reads badly, every measurement above it is unreliable.
levers L1 Technical & crawl health
Ask Quattr
- "What is our CLS?" Simulate this →
- "Which pages shift while loading?" Simulate this →
How to read it
Visual stability. Its numbers are orders of magnitude smaller than the timing vitals', which makes it the one most easily mistaken for the best of the three when they are compared naively, the weakest-vital logic exists precisely because a raw minimum across them would always pick this one.
Caveats, freshness and failure modes
Its numeric scale is orders of magnitude below the timing vitals, so raw numeric comparison across vitals is meaningless, comparison is done on grades instead.
Unitless. It carries no unit at all, where the timing vitals carry seconds, so a value quoted without its vital name is unreadable.
These are lab measurements, not field data from real visitors.
Collection cadence varies by site and by page.
- Freshness
- varies by site, priority URLs can run nightly; most pages far less often. The collection date rides on the card rather than being assumed.
Common failure modes
- Comparing a layout-shift value numerically against a timing vital to find the worst one.
Not the same as
The confusions that cause the most wrong decisions.
Cumulative Layout Shift Vital grade Compare definitions →
The number is a weighted average; the grade is the band. Score and verdict stay separate on every vital.
Watch this metric read in a real run
All runs →
Related metrics and workflows
Verification
A definition is the smallest part of this.
The measurement matters because something acts on it. Here is the rest of the showcase, in the order most people find useful.