Free Tool · Analyzer

Core Web Vitals Checker

Real Core Web Vitals from Google PageSpeed Insights — LCP, INP, CLS, FCP, TTFB. Lab + field data, mobile + desktop.

Enter a URL and click Run audit. We'll measure LCP, INP, CLS, FCP, TTFB on Google's servers and surface the top performance opportunities.

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How to use

How to use the Core Web Vitals Checker

Test mobile first — it's what Google ranks on
Google switched to mobile-first indexing in 2019. The mobile version of your site is the version they evaluate for ranking. A site that scores 95 on desktop but 35 on mobile won't rank well even on desktop searches. Always run the mobile audit first, fix what's broken there, and then check desktop.
Field data is the source of truth — lab data is a useful proxy
Lab data (Lighthouse) is reproducible but synthetic — it runs once on Google's servers with a simulated network and CPU. Field data (CrUX) is real Chrome users in the wild over the past 28 days, at the 75th percentile. When the two disagree, trust field. Field is what Google uses for ranking.
If a URL has no field data, that's normal — not a problem
CrUX only includes URLs with enough real Chrome traffic to be statistically meaningful (~thousands of page views over 28 days). New pages, deep pages, and low-traffic sites legitimately have no field data. We fall back to origin-level CrUX (the whole domain) and label it as such — that's still useful, just less specific.
INP replaced FID in March 2024 — check Interaction to Next Paint
First Input Delay (FID) measured the delay before the first user interaction. INP measures the worst response time across the entire page lifecycle — it's a much harder bar. Sites that previously scored 'good' on FID are often only 'needs improvement' on INP. The fix is the same: less main-thread JavaScript.
LCP issues are usually one of three things
Largest Contentful Paint is almost always: (1) an oversized hero image (fix: WebP, lazy-load, responsive sizes); (2) render-blocking JS in <head> (fix: defer, async, code-split); (3) slow server TTFB (fix: edge caching, faster DB queries, CDN). Run this audit, look at what Lighthouse identifies as the LCP element, and address it directly.
Top opportunities are sorted by time saved — fix the biggest one first
The Opportunities section ranks Lighthouse-identified fixes by estimated time savings. A fix that saves 4.2 s should be addressed before five fixes that save 0.3 s each. Knock out the highest-impact item, re-run the audit, and confirm the improvement before moving on.

Frequently asked questions

About the Core Web Vitals Checker

Core Web Vitals are Google's three official user-experience metrics used as ranking signals. LCP (Largest Contentful Paint) measures loading speed — target 2.5 s or less. INP (Interaction to Next Paint) measures responsiveness — target 200 ms or less. CLS (Cumulative Layout Shift) measures visual stability — target 0.1 or less. Pages that meet all three thresholds at the 75th percentile of real users qualify as 'passing' Core Web Vitals.

Google's PageSpeed Insights API. Lab data (LCP, FCP, CLS, TBT, Speed Index, Performance score) comes from a single Lighthouse simulation Google runs on their servers when you request the audit. Field data (LCP, INP, CLS, FCP, TTFB) comes from the Chrome User Experience Report (CrUX) — anonymized real-user measurements from Chrome users who have opted into syncing browsing history. Both are the same primary sources Google uses for its own ranking algorithm.

Lighthouse runs a full simulated page load on Google's servers — fetching every resource, executing all the JavaScript, measuring paint timings. That typically takes 20–40 seconds, occasionally up to 60. Don't refresh the page or close the tab while it's running. We set a 70-second total timeout to accommodate slow responses.

Common reasons: (1) Lab uses a simulated 'slow 4G' network and a mid-tier mobile CPU; your real users may be on faster connections (lab worse than field) or slower (lab better). (2) Lab measures one cold load; field includes warm/cached loads. (3) Lab measures a single moment; field is the 75th-percentile across 28 days. When they disagree significantly, trust field — that's what Google uses for ranking.

Yes — but with limits. Core Web Vitals are a confirmed ranking signal, but a tiebreaker, not a primary one. A page with 'poor' CWV won't beat a similar page with 'good' CWV at the same content quality. But great CWV alone won't outrank a more authoritative competitor. Treat it as: getting from 'poor' to 'good' is high-leverage; going from 'good' to 'great' has diminishing returns.

Not for casual use — the underlying PageSpeed Insights API is free with a quota of about 1 request per second per source IP without a key. We do support an API key on the server (set GOOGLE_PSI_API_KEY in env) which raises our quota to 25,000 requests/day, but you don't need to provide one as a user.

Absolutely — the audit works on any public URL. Common workflow: run your URL, run a top-3 competitor's URL, compare. Pay attention to lab Performance score AND field LCP/INP — these are the metrics that most often correlate with ranking gaps. If your competitor has dramatically faster CWV, that's a tractable area to invest in.

URLs you submit are forwarded to Google's PageSpeed Insights API. We cache responses for 1 hour at our edge to smooth bursty traffic and respect Google's quota — that means if multiple people audit the same URL within an hour, they get the same result. We don't log or store URLs beyond this cache. The audit is read-only — nothing on your site is modified.

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