SiteVitals

Core Web Vitals, explained

Three numbers that sum up how a page feels to use: how fast the main content appears, how quickly it responds, and whether it stays still. Here is what each measures, where the lines are, and how Google collects them.

The three metrics

MetricQuestion it answersGoodPoor
LCPDid the main content show up quickly?≤ 2.5 s> 4 s
INPDid the page react when I tapped?≤ 200 ms> 500 ms
CLSDid things jump around under my finger?≤ 0.1> 0.25

A page passes when all three are good for at least 75% of visits, measured separately for mobile and desktop. Two supporting Web Vitals help diagnose the core ones: First Contentful Paint and Time to First Byte.

Why the 75th percentile

Averages hide slow experiences. Google instead takes the value that 75% of visits beat. That is strict enough to make you care about people on slower phones and networks, but not so strict that a handful of outliers on a train tunnel decide the result.

Move the slow share and watch p75

Median visit: 1.91 s. 75th-percentile visit (the one Google reports): 4.75 s → Poor

Each bar is one visit, sorted from fastest to slowest. The outlined bar is visit 75. Push the slow share past 25% and the p75 lands in the slow group, however fast your typical visit is.

Field data vs lab data

Field data (the Chrome UX Report) is what real Chrome users experienced over 28 days. It is what Google uses, and the only source of INP. Lab data (Lighthouse) is one simulated load, repeatable and full of diagnostics, but it can’t measure real interactions. It uses Total Blocking Time as a stand-in for INP.

The two often disagree. A page can score 45 in Lighthouse and still pass Core Web Vitals, because most real visitors have faster devices than the lab’s throttled phone. The reverse happens too, when a site’s audience is on slow networks or loads heavy personalised content that Lighthouse never sees.

How to check and improve them

  1. Run the Core Web Vitals checker for your key templates (home, product, article), not just the home page.
  2. Find the failing metric, then open its guide: each one lists the causes in order of how often they happen.
  3. Work through the optimization checklist, confirm with a lab test, then allow 28 days for the field data to catch up.

Questions people ask

What are the three Core Web Vitals?

Largest Contentful Paint (loading), Interaction to Next Paint (responsiveness) and Cumulative Layout Shift (visual stability). INP replaced First Input Delay in March 2024.

Are Core Web Vitals a ranking factor?

Yes, as part of Google’s page experience signals, but a modest one. Google has said relevant content can still outrank a faster page. Passing mainly protects you from losing ground on competitive queries and improves conversions.

What is the difference between Core Web Vitals and Web Vitals?

Web Vitals is Google’s wider set of user-experience metrics, including FCP and TTFB. The Core Web Vitals are the three that Google treats as essential for every page and uses in Search.

How often do Core Web Vitals update?

The Chrome UX Report behind them is a rolling 28-day window, updated daily in PageSpeed Insights. Search Console groups similar URLs and can take longer to reflect changes.