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Core Web Vitals Explained

Google will complete an algorithm update by the end of August 2021 that will incorporate new user experience (UX) metrics into its page experience ranking. The “page experience update” was first announced in November 2020 with a proposed launch of May but was pushed back in April to a mid-June 2021 rollout.

This Google algorithm update will introduce three Core Web Vitals metrics as ranking signals.

Google says the gradual summer rollout is like “adding a flavoring to a food you're preparing. Rather than add the flavor all at once into the mix, we'll be slowly adding it all over this time period.”

Publishers and site owners are always leery anytime the Google algorithm is tinkered with, but Google assures that its “new flavoring” will not give you indigestion.

“Sites generally should not expect drastic changes,” said Google in an April update. “In addition, because we're doing this as a gradual rollout, we will be able to monitor for any unexpected or unintended issues.

Page experience is just one of the many factors that play into the Google algorithm mix.

“This isn’t the first time that UX signals have been used by Google – site speed and mobile usability have been ranking factors for some time – but this marks the introduction of new UX metrics,” writes Graham Charlton for Pi Datametrics.

Core Web Vitals: Measuring Great User Experiences

Core Web Vitals are part of the Web Vitals initiative that Google hopes will cut through the clutter of web metrics and tools and give insight into what qualifies as great user experiences on the web.

“Site owners should not have to be performance gurus in order to understand the quality of experience they are delivering to their users. The Web Vitals initiative aims to simplify the landscape, and help sites focus on the metrics that matter most, the Core Web Vitals,” wrote Philip Walton on

Core Web Vitals metrics are expected to evolve over time, but currently real-world user experience is focused on three aspects:

  • Loading
  • Interactivity
  • Visual Stability

“In a nutshell, it’s a way for Google to assess the user experience of your site through the use of a selection of measurable criteria,” writes Charlton.

Core Web Vitals Criteria: LCP, FID and CLS

Look, we know the world has enough acronyms, but Core Web Vitals are measured by LCP, FID and CLS:

  • Largest Contentful Paint (LCP): Measures loading performance. To provide a good UX, LCP should occur within 2.5 seconds of when the page first starts loading.

    “LCP is an important, user-centric metric for measuring perceived load speed because it marks the point in the page load timeline when the page’s main content has likely loaded – a fast LCP helps reassure the user that the page is useful,” Walton writes.

  • First Input Delay (FID): Measures interactivity. To provide a good UX, pages should have an FID of 100 milliseconds or less.

    “FID is an important, user-centric metric for measuring load responsiveness because it quantifies the experience users feel when trying to interact with unresponsive pages – a low FID helps ensure that the page is usable,” Walton writes.

  • Cumulative Layout Shift (CLS): Measures visual stability. TO provide a good UX, pages should maintain a CLS of 0.1 or less.

“CLS is an important, user-centric metric for measuring visual stability because it helps quantify how often users experience unexpected layout shifts – a low CLS helps ensure that the page is delightful,” writes Walton and Milica Mihajlija.

How to Achieve Passing Marks on Core Web Vitals

Obviously, how your pages perform on these new UX metrics will affect your rankings.

Breaking the three metrics into good, needs improvement and poor results:

  • LCP: Good is 2.5 seconds or less; Need Improvement is between 2.5 seconds and 4 seconds; and Poor is over 4 seconds.
  • FID: Good is 100 ms or less; Needs improvement is between 100 and 300 ms; and Poor is over 300 ms.
  • CLS: Good is a score of 0.1 or less; Needs improvement is scores between 0.1 and 0.25; Poor is a score over 0.25.

Publishers and site owners will have some work to do to score “good” on all three metrics as a recent Reddico study found that only one in three UK top 500 retail brands scored “good” on LCP and CLS.

We know one thing for sure, the ingredients that make up the Google algorithm will always be shifting.  

Carl Hendy summarizing the Reddico study said: “Core Web Vitals are an evolving benchmark with which to grade website usability. Today’s “Poor” could be tomorrow’s “Good” and vice versa. Some of the metrics might be removed and others could be added. They will always reflect what Google considers important to the user, however, a slow initial page load will never come into fashion, and a hard-to-navigate page will not become the new “easy”.

If we can help you get to Good (or better) with your website, please reach out.