SEO Tools
Lighthouse Score Calculator
Calculate the Google Lighthouse Performance score from FCP, Speed Index, LCP, TBT, and CLS using the official log-normal weights for mobile and desktop.
Lighthouse score calculator
Form factor
Lighthouse uses different scoring curves for mobile (Moto G Power, slow 4G) and desktop. Pick the audit profile you are estimating.
Field metrics
Enter values from a Lighthouse run, a PageSpeed Insights report, the Chrome User Experience Report, or a synthetic monitoring tool. Times are in milliseconds; CLS is a unitless decimal (0 to ~1).
First Contentful Paint FCP
Time from navigation start to the first text or image painted to the screen.
Hit 90+ at <= 1,800 ms; below 50 above > 3,000 ms.
Speed Index SI
How quickly the contents of a page are visibly populated, measured frame by frame.
Hit 90+ at <= 3,387 ms; below 50 above > 5,800 ms.
Largest Contentful Paint LCP
Render time of the largest text block or image in the viewport. A Core Web Vital.
Hit 90+ at <= 2,500 ms; below 50 above > 4,000 ms.
Total Blocking Time TBT
Total time the main thread was blocked long enough to delay input response.
Hit 90+ at <= 200 ms; below 50 above > 600 ms.
Cumulative Layout Shift CLS
Sum of layout shift scores during the page lifecycle. A Core Web Vital.
Hit 90+ at <= 0.100; below 50 above > 0.250.
Lighthouse Performance (mobile)
Good
Weighted average of FCP (10%), Speed Index (10%), LCP (25%), TBT (30%), and CLS (25%). Lighthouse rounds the final score to the nearest integer and colors it green at 90 and above, amber from 50 to 89, and red below 50.
Score breakdown
Each metric contributes to the final score in proportion to its weight. The column on the right shows how many points each metric actually added.
| Metric | Value | Score | Weight | Contribution |
|---|---|---|---|---|
First Contentful Paint FCP | 1.80 s | 90 | 10% | 9.0 pts |
Speed Index SI | 3.40 s | 90 | 10% | 9.0 pts |
Largest Contentful Paint LCP | 2.50 s | 90 | 25% | 22.5 pts |
Total Blocking Time TBT | 200 ms | 90 | 30% | 27.0 pts |
Cumulative Layout Shift CLS | 0.100 | 90 | 25% | 22.5 pts |
Final | Sum of weighted contributions, rounded to nearest integer | 90 / 100 | ||
| Total weight tracked: 100% | ||||
Scoring curves for mobile
The Lighthouse log-normal curve is anchored at two control points: the p10 value (the 10th percentile of public sites, scores 0.9) and the median value (scores 0.5). The curve below those is steep, the curve above is gentle, which is why a small regression near the target threshold can drop a metric score sharply.
| Metric | p10 (= score 0.9) | Median (= score 0.5) | Score 0.5 cap |
|---|---|---|---|
| FCP | 1,800 ms | 3,000 ms | 3,000 ms |
| SI | 3,387 ms | 5,800 ms | 5,800 ms |
| LCP | 2,500 ms | 4,000 ms | 4,000 ms |
| TBT | 200 ms | 600 ms | 600 ms |
| CLS | 0.100 | 0.250 | 0.250 |
Values shown are from Lighthouse 10 and 11, the same release line PageSpeed Insights uses today. Older Lighthouse releases (8, 9) used slightly different weights and slightly different control points.
Why your real Lighthouse score may differ
- Lighthouse runs a fresh page load on a throttled environment. Small variations in network and CPU between runs cause the score to shift by a handful of points even with identical code.
- Field data (real-user CrUX) and lab data (synthetic Lighthouse) do not always match. Lighthouse uses lab thresholds; the Core Web Vitals report uses 75th-percentile field thresholds.
- PageSpeed Insights blends a lab Lighthouse run with CrUX field data. The big number at the top is the lab score, not the field one, even though both appear on the same page.
- Speed Index can be hard to predict without a recording. If you only have FCP and LCP from a synthetic monitor, estimate Speed Index as roughly the geometric mean of the two.
Tips for pushing the score up
- TBT (30%) is the heaviest weight. Cut long tasks: code-split, defer non-critical JavaScript, and move heavy work to a worker or to the server.
- LCP (25%) and CLS (25%) tie for second. For LCP, preload the hero image and avoid render-blocking CSS. For CLS, reserve space for images, ads, and embeds.
- FCP and SI (10% each) improve together when you reduce server response time, ship less CSS, and avoid render-blocking resources at the top of the page.
- On desktop the thresholds are much tighter (under one second for p10 LCP). A site that scores 95 on mobile can still drop to 60 on desktop if the same render-blocking work is in the critical path.
How to use
- Pick Mobile or Desktop. Lighthouse uses different scoring curves for each, and the same metric value gets a very different score on the two profiles.
- Paste or type each metric value. Times go in milliseconds; CLS is a unitless decimal between 0 and roughly 1.
- Read the per-metric score in the input row and the colored band: green at 90 and above, amber from 50 to 89, red below 50.
- Check the breakdown table to see how many points each metric contributed to the final score, weighted by importance.
- Use the helper line under each input to find the value that hits a 90 score and the value above which the metric drops below 50.
- Click Copy report to paste a one-line summary into a PR comment, a Slack thread, or a performance tracking sheet.
About this tool
Lighthouse Score Calculator estimates the Google Lighthouse Performance score from the five metrics that drive it: First Contentful Paint (10%), Speed Index (10%), Largest Contentful Paint (25%), Total Blocking Time (30%), and Cumulative Layout Shift (25%). It uses the same scoring math Lighthouse uses internally: a log-normal curve anchored at two control points, the p10 value of public sites (which scores 0.9) and the median value (which scores 0.5). Type your field values, pick mobile or desktop (the two profiles use very different control points), and the tool computes a per-metric score with the same green-amber-red color bands Lighthouse uses, plus a weighted final score from 0 to 100. The breakdown table shows exactly how each metric contributes in points, so it is easy to see where the biggest wins live. A scoring-curve reference shows the p10 and median anchors for the current Lighthouse 10 and 11 release line that PageSpeed Insights ships with today. Useful when you have field data from the Chrome User Experience Report or from a synthetic monitor and want to know what Lighthouse would report, when you are setting performance budgets and need a concrete target value for each metric, or when you are reviewing a PR and want to estimate the score delta before running a fresh audit. Everything runs in your browser; no metrics are uploaded.
Free to use. Works in your browser. No signup, no login.
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