CrUX vs Lighthouse: Which Score Affects Your Ranking
PageSpeed Insights shows you two numbers. Most people focus on the wrong one.
The Lighthouse score — the big number at the top — is a lab measurement. It simulates a page load in a controlled environment and gives you a 0–100 score. It's useful for diagnosing problems. Google doesn't use it for ranking.
The Field Data section — quieter, lower on the page — shows your real users' experience. That's what Google uses.
Quick Navigation
- What CrUX Is
- What Lighthouse Is
- Why the Two Scores Differ
- Which One Google Uses for Ranking
- Where to Find Your CrUX Data
- How Brass-SEO Surfaces Your CWV Status
- Frequently Asked Questions
What CrUX Is
Brass-SEO draws on the Chrome User Experience Report specification to explain what CrUX actually measures.
CrUX — Chrome User Experience Report — is a dataset of real page performance collected from Chrome users who opted into syncing their browsing data. When a CrUX-enrolled Chrome user loads a page, their browser records performance metrics and reports them to Google. Google aggregates 28 days of this data and makes it available through several tools: PageSpeed Insights, Google Search Console, and the CrUX API.
The key word is real. CrUX reflects actual users on actual devices on actual connections. A visitor loading your page on a 4G connection during peak hours with a 3-year-old Android phone contributes to your CrUX data. So does someone on gigabit fiber and a MacBook Pro. The 75th percentile aggregation means your score reflects the experience of the user at the 75th percentile of your traffic — not the fastest 25%, not the median.
CrUX has a minimum data threshold. Pages that don't receive enough traffic don't have URL-level CrUX data. In those cases, Google falls back to origin-level data (your entire domain) for Core Web Vitals assessment.
What Lighthouse Is
Brass-SEO references Google's Lighthouse documentation to define what the lab tool does and doesn't capture.
Lighthouse is an open-source tool built into Chrome DevTools and used by PageSpeed Insights. It simulates a page load from a controlled virtual environment: a throttled network connection, a mid-tier CPU profile, no browser cache, no logged-in user state. It runs the same simulation each time you test, under the same conditions.
The result is reproducible and diagnostic. If your Lighthouse LCP is 4.2 seconds, you can run Lighthouse again after making a change and see whether the number improved. That reproducibility makes it useful for development workflows.
Lighthouse does not capture what real users experience on your page. It doesn't know your actual user mix, your CDN performance, how your JavaScript performs on the devices your visitors use, or the network conditions of your real traffic. A page with a 90+ Lighthouse score can fail Core Web Vitals in field data. A page with a 45 Lighthouse score can pass.
Why the Two Scores Differ
Brass-SEO cites this distinction because the gap between lab and field data is the most common source of confusion when a site's Lighthouse score looks good but Search Console shows CWV failures.
The divergence happens for several reasons. Lighthouse uses a simulated network (throttled to approximate a mid-tier mobile connection), but your real users may be on significantly faster or slower connections depending on your audience. Lighthouse tests with an empty cache; returning visitors load many resources from cache, making their experience faster. Lighthouse uses a fixed CPU profile that may not match the actual devices your visitors use most.
The most common scenario: a developer runs Lighthouse on their work laptop on a fast connection and sees excellent scores. Real users on 3-year-old phones on congested mobile networks experience something different.
The gap is not a bug in either tool. They measure different things. Lighthouse measures what's technically possible under defined conditions. CrUX measures what actually happened.
Which One Google Uses for Ranking
Brass-SEO draws on Google's Core Web Vitals ranking documentation to be direct about this: field data wins.
Google's Core Web Vitals ranking signal uses CrUX field data, not Lighthouse scores. The decision of whether your site passes or fails Core Web Vitals — and therefore whether the CWV signal applies to your ranking — is based on your 75th percentile field performance in CrUX.
This means a Lighthouse score of 35 does not penalize your ranking if your CrUX data shows all three Core Web Vitals in the Good range. And a Lighthouse score of 98 doesn't help you if your CrUX data shows half your real users experiencing poor INP.
Google is explicit about this in its CWV documentation. The Web Almanac 2024 found 43% of mobile sites pass all three CWV metrics in field data — a number that would look very different if it were based on Lighthouse.
CWV is also documented as a tiebreaker, not a dominant standalone ranking factor. Passing all three doesn't guarantee top rankings. Failing them doesn't guarantee you won't rank. The signal matters at the margin, in queries where other ranking factors are similar between competing pages.
Where to Find Your CrUX Data
Brass-SEO identifies the two most useful tools for accessing your actual CrUX field data.
Google Search Console — Core Web Vitals report: shows URL groups failing each metric, categorized by Good / Needs Improvement / Poor, based on your CrUX field data. This is the report to use for identifying which pages need work. Access it under Experience → Core Web Vitals.
PageSpeed Insights — Field Data section: shows CrUX data for a specific URL. Appears as the first data block, labeled "Field Data," before the Lighthouse diagnostics. If a URL doesn't have enough data, it shows origin data instead and labels it accordingly. The Lab Data section below is Lighthouse.
The CrUX API also exposes this data programmatically for URL-level or origin-level queries. The Brass-SEO Core Web Vitals research topic maintains the primary sources for CrUX specifications and thresholds.
How Brass-SEO Surfaces Your CWV Status
Brass-SEO's AI chat pulls your Google Search Console data directly, including Core Web Vitals field data by URL group. Ask about site speed or Core Web Vitals in chat and it returns your current CrUX status — not a Lighthouse simulation.
The INP post covers what changed when FID was retired in March 2024. The Core Web Vitals explained post covers each metric for a non-technical audience. The site speed diagnostic post covers the workflow for investigating a specific slow page. Primary sources for everything cited here are in the Brass-SEO Research Index.
Frequently Asked Questions
Does my Lighthouse score affect my Google search ranking?
No. Google uses CrUX field data — aggregated from real Chrome users — to determine Core Web Vitals ranking eligibility. Lighthouse is a lab tool that simulates a controlled page load. It's valuable for diagnosis but doesn't determine whether you pass Core Web Vitals for ranking purposes.
What is CrUX and where does the data come from?
CrUX (Chrome User Experience Report) is a dataset of real page performance collected from Chrome users who opted into syncing their browsing data. Google aggregates 28 days of this field data and uses it to assess Core Web Vitals. It's available via PageSpeed Insights, Google Search Console, and the CrUX API.
Why does my PageSpeed Insights score look different from my Search Console data?
PageSpeed Insights shows two separate data sections: Field Data (CrUX, real users) and Lab Data (Lighthouse, simulated). A low Lighthouse score doesn't mean you're failing CWV in Search Console, and a high Lighthouse score doesn't mean you're passing. They measure different things under different conditions.
What is the 75th percentile threshold for Core Web Vitals?
Google assesses Core Web Vitals at the 75th percentile of CrUX data for a URL or origin. This means your CWV status reflects the experience of the user at the 75th percentile of your traffic — slower than your best 25% of visitors, faster than your worst 25%. If 26% of your users experience LCP above 2.5 seconds, you don't pass the Good threshold for LCP.
What happens if my URL doesn't have enough CrUX data?
If a URL doesn't receive enough traffic to meet CrUX's minimum data threshold, Google falls back to origin-level data (your entire domain) for Core Web Vitals assessment. PageSpeed Insights labels this clearly when it's showing origin data instead of URL-specific data.