Lesson 17 / 25
Lighthouse scoring and audits
Understand Lighthouse scoring methodology and use audit results to prioritize optimizations.
Performance score calculation
The Lighthouse performance score is a weighted blend of lab metrics: Total Blocking Time (30%), LCP (25%), CLS (25%), FCP (10%) and Speed Index (10%). Scores of 90–100 are good, 50–89 need improvement, and below 50 is poor. Weights change between Lighthouse versions, so check the current calculator.
Audit opportunities and diagnostics
Lighthouse lists specific issues: unoptimized images, unused JavaScript, slow server response, missing cache headers. Prioritize high-impact suggestions. Diagnostic metrics show underlying bottlenecks like unused CSS or render-blocking resources.
Interpreting Lighthouse results
Focus on opportunities section, not just score. Test multiple times—results vary. Throttling affects measurements. Compare baseline vs. improved. Set incremental goals (e.g., 50 → 70 → 90).
Quick check
Quick check: What is the weight of LCP in Lighthouse scoring?
- 15%
- 25%
- 10%
- 50%
Answer
25% — LCP carries a 25% weight in the current Lighthouse performance score.