Lighthouse Score Calculator
See exactly how each metric contributes to your performance score.
Import from PageSpeed Insights
Understanding the Metrics
Lighthouse performance score is a weighted average of these five key metrics. Improving any of them will raise your overall score.
First Contentful Paint
Time until first text or image is painted
Speed Index
How quickly content visually populates
Largest Contentful Paint
Time until largest content element is visible
Total Blocking Time
Sum of blocking time for long tasks
Cumulative Layout Shift
Movement of visible elements during load
Frequently Asked Questions
How does Lighthouse calculate the performance score?
Lighthouse calculates performance score as a weighted average of six metrics: First Contentful Paint (10%), Speed Index (10%), Largest Contentful Paint (25%), Total Blocking Time (30%), and Cumulative Layout Shift (25%). Each metric is scored 0-100 using log-normal curves based on real-world data from HTTPArchive.
Why do my Lighthouse scores vary between runs?
Lighthouse scores can vary 5-10 points between runs due to: network conditions, server response time variations, third-party script loading, background processes on your device, and variability in JavaScript execution. Run multiple tests and use the median score for accuracy.
What is a good Lighthouse performance score?
A score of 90-100 is considered good (green), 50-89 needs improvement (orange), and 0-49 is poor (red). Focus on reaching 90+ but don't obsess over perfect 100—the last few points have diminishing returns and normal variance makes it impractical.
Which metrics have the biggest impact on Lighthouse score?
Total Blocking Time (30%) and Largest Contentful Paint (25%) together account for 55% of your score. CLS adds another 25%. To improve your score quickly, focus on reducing JavaScript execution time (TBT) and optimizing your largest above-the-fold element (LCP).
Why is my PageSpeed Insights score different from Chrome DevTools?
PageSpeed Insights tests from Google's servers with a simulated mobile device, while Chrome DevTools uses your local machine and network. PSI also shows field data from real Chrome users (CrUX) which reflects actual user experience and may differ significantly from lab results.
Optimize Your Real-World Scores
Lighthouse is a lab test. For production sites, use our free tools to check your real Chrome User Experience data.