How Google Rates Websites

The system Google actually uses

Google's website quality measurement isn't a black box. The whole system is public and documented:

1. Lighthouse — an open-source audit engine that scores any URL across four categories: Performance, Accessibility, Best Practices, SEO
2. PageSpeed Insights — a hosted version of Lighthouse plus real-user data
3. Chrome User Experience Report (CrUX) — anonymized real-user performance data from millions of Chrome users
4. Core Web Vitals — three specific metrics (LCP, CLS, INP) extracted from CrUX
5. Page Experience signal — Core Web Vitals plus a few other criteria, used as a ranking factor

How the four Lighthouse categories work

Performance (0-100)

Weighted score based on six metrics:

Metric	Weight
LCP — Largest Contentful Paint	25%
TBT — Total Blocking Time	30%
CLS — Cumulative Layout Shift	25%
Speed Index	10%
FCP — First Contentful Paint	10%

90+ = Good, 50-89 = Needs Improvement, below 50 = Poor.

SEO (0-100)

Technical on-page checks only — titles, meta descriptions, viewport, crawlability, structured data. Does NOT measure content quality, keywords, or backlinks.

Accessibility (0-100)

Roughly 40 automated WCAG checks. A single failure can drop the score 10+ points. Not a substitute for manual accessibility testing.

Best Practices (0-100)

About 15 checks: HTTPS, no console errors, no deprecated APIs, no vulnerable JS libraries, correct image aspect ratios, passive scroll listeners.

Lab vs field data — the most important distinction

There are two ways to measure performance:

• Lab data: Lighthouse runs a single synthetic test in a controlled environment. Same input always gives roughly the same output. Fast, reproducible, great for debugging.
• Field data: CrUX collects real Chrome user metrics over a rolling 28-day window. Reflects how your site actually performs across thousands of real devices, networks, and locations.

Google ranks on field data. You optimize using lab data because field data updates slowly. If your lab numbers are good, your field numbers will eventually follow.

The Core Web Vitals thresholds Google uses

A page passes Core Web Vitals if the 75th percentile of real users hits all three thresholds:

Metric	Good	Needs Improvement	Poor
LCP	≤ 2.5s	≤ 4s	> 4s
CLS	≤ 0.1	≤ 0.25	> 0.25
INP	≤ 200ms	≤ 500ms	> 500ms

"75th percentile" means three out of four of your real users have to experience the page faster than the Good threshold. That's why a fast laptop test isn't enough — you need to be fast for the median user on their phone.

How much does this affect rankings?

Page Experience is a tiebreaker, not a primary signal. Google has said clearly that relevance still beats speed. But:

• For two pages that match a query equally well, the faster one wins
• For competitive queries with many similar pages, Core Web Vitals matter more
• Failing Core Web Vitals badly can suppress an otherwise good page

What this means for you

1. Treat Lighthouse as your dashboard, not your goal. A 100 score is nice but pointless if your content isn't relevant.
2. Optimize for the 75th percentile, not the median. Test on slow phones, slow networks.
3. Watch field data, not just lab data. PageSpeed Insights and Search Console both show CrUX numbers.
4. Fix the highest-impact issues first. The Improvement Checklist in any RateMySite.io report ranks these for you.

Run your site through the audit below to see exactly where you stand on every metric Google cares about.