However, he reminds us of the cumulative nature of CLS:
“CLS is Cumulative Layout Shift, and that first word (cumulative) matters. We take all the individual shifts that happen within a short space of time (max 5 seconds) and sum them up to get the CLS score.”
Pollard explained the reasoning behind the 5-second measurement window:
“Originally we cumulated ALL the shifts, but that didn’t really measure the UX—especially for pages opened for a long time (think SPAs or email). Measuring all shifts meant, given enough, time even the best pages would fail!”
He also noted the theoretical maximum CLS score:
“Since each element can only shift when a frame is drawn and we have a 5 second cap and most devices run at 60fps, that gives a theoretical cap on CLS of 5 secs * 60 fps * 1.0 max shift = 300.”
Interpreting CLS Scores
Pollard addressed how to interpret CLS scores:
“… it helps to think of CLS as a percentage of movement. The good threshold of 0.1 means about the page moved 10%—which could mean the whole page moved 10%, or half the page moved 20%, or lots of little movements were equivalent to either of those.”
Regarding the specific threshold values, Pollard explained:
“So why is 0.1 ‘good’ and 0.25 ‘poor’? That’s explained here as was a combination of what we’d want (CLS = 0!) and what is achievable … 0.05 was actually achievable at the median, but for many sites it wouldn’t be, so went slightly higher.”
See also: How You Can Measure Core Web Vitals
Why This Matters
Pollard’s insights provide web developers and SEO professionals with a clearer understanding of measuring and optimizing for CLS.
As you work with CLS, keep these points in mind:
CLS is unitless and calculated from impact and distance fractions.
It’s cumulative, measuring shifts over a 5-second window.
The “good” threshold of 0.1 roughly equates to 10% of viewport movement.
CLS scores can exceed 1.0 due to multiple shifts adding up.
The thresholds (0.1 for “good”, 0.25 for “poor”) balance ideal performance with achievable goals.
With this insight, you can make adjustments to achieve Google’s threshold.
Featured Image: Piscine26/Shutterstock