Lighthouse Performance Score
The Lighthouse performance score is a weighted composite of six metrics. Here's exactly how it's calculated and what it actually means for your site.
Google Lighthouse produces a performance score from 0 to 100 based on a weighted combination of six lab metrics: First Contentful Paint (10%), Speed Index (10%), Largest Contentful Paint (25%), Total Blocking Time (30%), and Cumulative Layout Shift (25%). These weightings change with Lighthouse versions.
The performance score is a useful summary but doesn't directly equal Core Web Vitals scores or Google rankings. Google's ranking signal uses CrUX field data for LCP, CLS, and INP — not the Lighthouse score. A 90 performance score can coexist with 'Needs Improvement' CrUX field data if real users have worse conditions than the lab.
TBT (Total Blocking Time) carries the highest weight (30%) in the score and is the primary driver of scores below 90. Most sites with scores in the 60–80 range have high TBT from excessive JavaScript execution. Reducing TBT is typically the highest-leverage improvement for the overall score.
FAQ
What is a good Lighthouse performance score?
90–100 is 'Good'. 50–89 is 'Needs Improvement'. Below 50 is 'Poor'. However, the score is less important than the individual CWV metrics (LCP, CLS, INP) which are the actual Google ranking signals.
Does the Lighthouse score affect Google rankings?
Not directly. Google doesn't use the Lighthouse score for rankings — it uses CrUX field data for LCP, CLS, and INP specifically. The Lighthouse score is useful for identifying performance issues, not as a ranking indicator.
Why do I have a high Lighthouse score but poor CrUX data?
The Lighthouse score is a lab measurement under controlled conditions. CrUX field data reflects real users on diverse devices and networks. If your real users are on slower devices or connections, CrUX data can be significantly worse than lab scores.
Which metric has the highest weight in the Lighthouse score?
Total Blocking Time (TBT) carries the highest weight at 30%, followed by LCP (25%) and CLS (25%). Improving TBT — usually by reducing JavaScript execution — has the biggest impact on the overall score.
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