Lighthouse Score Calculator
See exactly how each metric contributes to your performance score.
How Lighthouse Calculates Your Performance Score
Lighthouse calculates your performance score using a weighted average of five key metrics. Each metric is scored from 0-100 using a log-normal distribution based on real-world performance data from the HTTP Archive.
The scoring algorithm converts raw metric values (like milliseconds or unitless shift scores) into a 0-100 score using statistical curves. A metric value at the 10th percentile of all sites scores 90 (good), while the median scores 50. This means your score reflects how your site compares to others on the web.
v10+ Score Weights
80% of your score comes from TBT, LCP, and CLS.
The Log-Normal Scoring Algorithm
Lighthouse doesn't use simple linear scoring. Instead, it applies a log-normal distribution to convert raw metric values into scores. This statistical approach means:
- Diminishing returns — Improving from 95 to 100 requires more effort than 50 to 55
- Based on real data — Curves calibrated against millions of real websites
- Percentile-based — Score of 90 means better than 90% of sites
Each metric has two key control points: the p10 value (scores 90) and the median (scores 50).
Core Web Vitals
Google's essential metrics for page experience ranking
Core Web Vitals are Google's official metrics that affect your search rankings. As of March 2024, they consist of LCP, CLS, and INP (which replaced FID). To pass, 75% of page visits must meet the "good" threshold for all three.
Largest Contentful Paint
Time for the largest visible element to render—when users perceive the page as "loaded."
Cumulative Layout Shift
Visual stability—how much elements move unexpectedly as the page loads.
Interaction to Next Paint
Responsiveness—delay between user interactions and visual response. Lighthouse uses TBT as a proxy.
Other Performance Metrics
Additional metrics that affect your Lighthouse score
Beyond Core Web Vitals, Lighthouse measures additional metrics. TBT carries the highest weight (30%) because it directly correlates with how responsive your page feels.
Main thread blocking time >50ms. Lab proxy for INP.
First text or image rendered.
How quickly visible content is populated.
Server response time—not scored but delays all metrics.
Understanding Score Thresholds
Lighthouse assigns color-coded ratings based on your final weighted score.
01How 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.
02Why 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.
03What 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.
04Which 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).
05Why 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.
Test Your Entire Site
Your homepage might score 100, but what about the rest? Scan every page to find issues site-wide.