Google’s Web Performance Developer Advocate, Barry Pollard, has clarified how Cumulative Layout Shift (CLS) is measured.
CLS quantifies how much unexpected layout shift occurs when a person browses your site.
This metric matters to SEO as it’s one of Google’s Core Web Vitals. Pages with low CLS scores provide a more stable experience, potentially leading to better search visibility.
How is it measured? Pollard addressed this question in a thread on X.
For Core Web Vitals what is CLS measured in? Why is 0.1 considered not good and 0.25 bad, and what do those numbers represent?
I’ve had 3 separate conversations on this with various people in last 24 hours so figured it’s time for another deep dive thread to explain…
🧵 1/12 pic.twitter.com/zZoTur6Ad4
— Barry Pollard (@tunetheweb) October 10, 2024
Understanding CLS Measurement
Pollard began by explaining the nature of CLS measurement:
“CLS is ‘unitless’ unlike LCP and INP which are measured in seconds/milliseconds.”
He further clarified:
“Each layout shift is calculated by multipyling two percentages or fractions together: What moved (impact fraction) How much it moved (distance fraction).”
This calculation method helps quantify the severity of layout shifts.
As Pollard explained:
“The whole viewport moves all the way down – that’s worse than just half the view port moving all the way down. The whole viewport moving down a little? That’s not as bad as the whole viewport moving down a lot.”
Worse Case Scenario
Pollard described the worst-case scenario for a single layout shift:
“The maximum layout shift is if 100% of the viewport (impact fraction = 1.0) is moved one full viewport down (distance fraction = 1.0).
This gives a layout shift score of 1.0 and is basically the worst type of shift.”