Cumulative Layout Shift (CLS)

A layout shift happens when an element that was already visible changes position between two frames. Each shift scores impact fraction × distance fraction: the share of the viewport the moving elements touched, before and after, times the distance they moved as a share of the viewport.

Worked example: an element covering half the viewport moves down by a quarter of it. Together its old and new positions cover 75% of the viewport, so the impact fraction is 0.75; the distance fraction is 0.25. The shift scores 0.75 × 0.25 = 0.1875 — on its own, well past the 0.1 line.

Shifts are grouped into session windows: shifts less than 1 second apart, up to 5 seconds in all. CLS is the score of the worst window. Shifts within 500 ms of a click, tap or key press don't count — the visitor expected those.

/lighthouse returns cumulative-layout-shift in metrics, unitless, with its 0–1 score; it carries 25% of the Lighthouse 13 performance score. A lab run only sees shifts during load — shifts that happen as a visitor scrolls show up in field data only. With include_audits: "true" the audits list the elements that moved.

To see a shift rather than measure it, capture the page at two moments with /screenshot — once as it is, once with a delay in page_options — and compare.