TL;DR — Quick Summary
The CLS Score is a unitless number representing cumulative visual instability. It uses a 'session window' approach — shifts are grouped into windows, and the largest window becomes your CLS score. Good CLS is ≤ 0.1.
What is CLS Score?
The CLS Score is the final value of the Cumulative Layout Shift metric, calculated using session windows. Layout shifts are grouped into sessions with a maximum 5-second duration and no more than 1-second gap between shifts. The CLS Score is the maximum session window score (not the sum of all shifts). Each shift score = impact fraction × distance fraction.
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