How to Use This Tool
Apply the CLS session-window arithmetic to geometry measured in browser tooling. Estimate added and total Cumulative Layout Shift from user-supplied impact fraction, distance fraction, event count and baseline CLS.
The decision this tool supports
A visible jump is not enough to estimate CLS: the affected viewport share and movement distance both control the event score. This page keeps the decision bounded to estimated total cls and the supporting outputs shown beside it. Layout Shift Risk does not import an account, infer a market rate, or silently substitute an industry average.
Inputs and units
The Layout Shift Risk calculation uses Impact fraction per shift, Distance fraction per shift, Comparable shifts in window, Existing CLS in window. Keep all money values in one currency and all time, distance, mass, energy or volume entries in the unit printed beside the field. Mixing Layout Shift Risk scopes can produce a plausible number with the wrong meaning.
- Impact fraction per shift is entered in 0 to 1.
- Distance fraction per shift is entered in 0 to 1.
- Comparable shifts in window is entered in events.
- Existing CLS in window is entered in score.
Formula and worked check
Layout-shift score = impact fraction × distance fraction; modeled added CLS sums the entered comparable shifts and adds the baseline. Three shifts with 0.25 impact and 0.10 distance add 0.075 to a 0.04 baseline, producing estimated total CLS of 0.115. The Layout Shift Risk default is an executable known-answer case, not a benchmark or recommendation. Change one input and verify that the direction of estimated total cls still matches the stated relationship.
How to interpret the result
Use measured fractions from DevTools or field diagnostics; the estimate helps triage a scenario but cannot reproduce session-window grouping from aggregate guesses. The additional Layout Shift Risk outputs expose the denominator, comparison, capacity or reverse value needed to audit the primary result instead of presenting one unexplained number.
Assumptions
- Every modeled event has the same supplied fractions.
- Events belong to the same CLS session window.
- User-input or recent-navigation exclusions have already been handled.
Save the Layout Shift Risk input values and date with any material decision. A later Layout Shift Risk rerun is reproducible only when the same assumptions and units are available.
Limitations and safety boundary
It does not observe a page, apply exclusion rules, group real session windows or replace CrUX, Lighthouse or PerformanceObserver measurements. Layout Shift Risk is an estimate and cannot replace a contract, local code, licensed professional, calibrated measurement, lender statement or platform report where one governs the decision.
Source and privacy
The Layout Shift Risk definition or rule was checked against Google web.dev — Cumulative Layout Shift on 2026-08-26. Recheck Google web.dev — Cumulative Layout Shift when a specification or policy behind Layout Shift Risk can change. Layout Shift Risk arithmetic runs in this browser tab; ecech does not receive the values through a calculation API.
Sources & assumptions
Tool Spec v2 · verified 2026-08-26. Platform rules and fees can change; the editable inputs remain authoritative for your account.
Official references
- Google web.dev — Cumulative Layout Shift (checked 2026-08-26)
Model assumptions
- Every modeled event has the same supplied fractions.
- Events belong to the same CLS session window.
- User-input or recent-navigation exclusions have already been handled.
- It does not observe a page, apply exclusion rules, group real session windows or replace CrUX, Lighthouse or PerformanceObserver measurements.
