Lighthouse Score Estimator

About the Lighthouse Score Estimator

Google Lighthouse calculates a performance score from 0 to 100 using a weighted combination of five metrics: First Contentful Paint (FCP), Speed Index (SI), Largest Contentful Paint (LCP), Total Blocking Time (TBT), and Cumulative Layout Shift (CLS). Understanding the weights helps you prioritize which metrics to improve for maximum score impact.

This calculator converts your raw metric values into sub-scores using log-normal distributions (matching Lighthouse's scoring methodology) and combines them using official weights: FCP (10%), SI (10%), LCP (25%), TBT (30%), and CLS (25%). TBT and LCP together account for 55% of the total score.

Use this estimator to predict how metric improvements will affect your overall Lighthouse score, helping prioritize optimization work for maximum impact.

When This Page Helps

Lighthouse scores influence both SEO rankings and stakeholder perception. Understanding how individual metrics contribute to the overall score helps you prioritize the improvements that will have the greatest impact on your score.

How to Use the Inputs

1. Run a Lighthouse audit or check PageSpeed Insights for your current metric values.
2. Enter FCP in seconds (First Contentful Paint).
3. Enter Speed Index in seconds.
4. Enter LCP in seconds (Largest Contentful Paint).
5. Enter TBT in milliseconds (Total Blocking Time).
6. Enter CLS as a unitless score.
7. Review the estimated overall performance score.

Example Calculation

Tips & Best Practices

• TBT (30% weight) is the single most impactful metric — prioritize reducing main-thread blocking.
• LCP and CLS together account for 50% — optimize these for half the score.
• Lab scores (Lighthouse) differ from field scores (CrUX) due to testing conditions.
• Run Lighthouse in incognito mode to avoid extension interference.
• Lighthouse scores vary between runs; average 3–5 runs for reliable results.
• Focus on reducing JavaScript execution time to improve both TBT and SI.

Lighthouse Scoring Methodology

Lighthouse converts raw metric values into 0–100 sub-scores using log-normal distribution curves. Each metric has a median value (score = 50) and a point-of-diminishing-returns (score ≈ 90). Values better than the target curve receive higher sub-scores.

Weight Distribution

The scoring weights reflect the relative importance of each metric for user experience. TBT at 30% reflects the critical importance of interactivity. LCP and CLS at 25% each capture loading and stability. FCP and SI at 10% each capture early loading signals.

Score Ranges

Lighthouse scores: 0–49 = Poor (red), 50–89 = Needs Improvement (orange), 90–100 = Good (green). The scoring is intentionally difficult — reaching 90+ requires systematic optimization across all metrics.

Practical Optimization Strategy

Start by identifying your lowest-scoring metrics. Use the weight distribution to calculate which improvement will yield the most score points. For example, reducing TBT from 600ms to 200ms (30% weight improvement) will typically yield more score points than reducing FCP by the same relative amount (10% weight).

Frequently Asked Questions

• Are Lighthouse scores exact?

  No. Lighthouse scores are estimates based on simulated conditions. They vary between runs due to network variability, CPU load, and other factors. It gives an approximation to help with prioritization, not an exact prediction.

• What changed in Lighthouse 10?

  Lighthouse 10 updated scoring weights. The current weights are: FCP 10%, SI 10%, LCP 25%, TBT 30%, CLS 25%. Previous versions had different weights. TBT and LCP are the most impactful metrics in the current version.

• What is Total Blocking Time?

  TBT measures the total time between FCP and Time to Interactive (TTI) where the main thread was blocked for more than 50ms. It is a proxy for interactivity — lower TBT means the page responds faster to user input.

• Why does Speed Index matter if it's only 10%?

  While SI has low weight, it captures perceived visual loading progress. Optimizing SI often correlates with improving other metrics because it reflects overall loading efficiency. It measures how quickly content is visually populated.

• How do I improve my Lighthouse score the fastest?

  Target TBT first (30% weight): reduce JavaScript, code-split, defer non-critical scripts. Then LCP (25%): optimize hero images, preload key resources. Then CLS (25%): set image dimensions, avoid late-loading elements.

• Is 100 achievable?

  Yes, but only for very simple pages. Complex applications with third-party scripts, dynamic content, and rich interactivity typically score 60–90. A score of 90+ is considered excellent. Focus on user experience, not chasing a perfect score.