How is the Lighthouse Performance Score Calculated?
The Lighthouse Performance score is a single number from 0 to 100 that summarises a page's loading performance. It is a weighted aggregate of five lab metrics: Largest Contentful Paint (LCP), Total Blocking Time (TBT), Cumulative Layout Shift (CLS), First Contentful Paint (FCP), and Speed Index.
What Lighthouse Score Measures
The score is not a direct measurement — it is a composite derived from applying log-normal distribution curves to each metric value, assigning a score of 0–1 to each, and weighting them. The score reflects how a page compares to the web as a whole using data from HTTP Archive. A score of 90+ is in the top ~17% of all pages.
What is a Good Lighthouse Score Score?
What Causes Poor Lighthouse Score Scores?
- ✗LCP (25% weight) — slow loading of the largest element; the single biggest score driver
- ✗TBT (30% weight) — the largest contributor to the score; high TBT from JavaScript execution tanks the score fastest
- ✗CLS (25% weight) — layout instability; unexpected shifts cause significant score loss
- ✗FCP (10% weight) — time to first content; contributes less than LCP/TBT/CLS but still important
- ✗Speed Index (10% weight) — how fast the visible area fills; least impactful individually
How to Fix Lighthouse Score
- ✓Focus on TBT first (30% weight) — reduce JavaScript bundle sizes, defer third-party scripts, avoid long tasks
- ✓Then fix LCP (25% weight) — preload the LCP image, switch to WebP, improve TTFB
- ✓Then fix CLS (25% weight) — set explicit image dimensions, reserve space for dynamic content
- ✓FCP and Speed Index often improve automatically when TBT and LCP are fixed
Check Your Lighthouse Score Score
Run a free audit at speedexporter.com to see your performance score broken down by metric, with each metric's exact contribution to your score and a prioritized list of fixes.
Run a free Lighthouse audit →Frequently Asked Questions
What is a good Lighthouse score?
A score of 90–100 is considered good (green). 50–89 is needs improvement (amber). 0–49 is poor (red). For e-commerce and content sites where performance directly impacts conversions and SEO, aim for 85+ on mobile.
Does Lighthouse score directly affect Google rankings?
The Lighthouse score itself is not a ranking factor. Google's Page Experience ranking signal uses the Core Web Vitals (LCP, INP, CLS) measured from real Chrome users (CrUX), not lab Lighthouse data. However, improving your Lighthouse score is the best way to improve your Core Web Vitals.
Why does my Lighthouse score vary between runs?
Lighthouse runs in an emulated environment and can be affected by server variability, CDN fluctuations, and network conditions. Score variance of 5-10 points between runs is normal. Run 3+ tests and average them for a reliable baseline.
Which metric has the highest impact on Lighthouse score?
Total Blocking Time (TBT) has the highest weight at 30%. After that, LCP and CLS are tied at 25% each. Focus on TBT first — reducing JavaScript execution time will have the fastest impact on your score.