How the Lighthouse performance score works
The score is a weighted mix of five lab metrics measured in a throttled Chrome. It is not what your visitors see, but it is the fastest way to find what slows them down.
Step by step, with screenshots: Explaining a getReport score to a client (or your boss) →
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What it is
The Lighthouse performance score is a number from 0 to 100 that summarises how fast a page loads in a lab test. Lighthouse loads the page once on a simulated mid-range phone with a slow 4G connection, measures five metrics and turns them into one weighted score.
The weights are Total Blocking Time 30 %, Largest Contentful Paint 25 %, Cumulative Layout Shift 25 %, First Contentful Paint 10 % and Speed Index 10 %. 90 to 100 is green (good), 50 to 89 orange (needs improvement) and 0 to 49 red (poor). Google does not rank on this score; it uses real-user Core Web Vitals. Treat a green score as a sign the page is in good shape and the metric breakdown as your to-do list.
What a passing site looks like
- Lighthouse performance score: passesinfo onlyeffort M
Lighthouse performance score
Why it matters. This is the number Google's own tools show for your page. It combines five of the lab metrics below (all except TTFB), weighted the way Lighthouse weights them. 90 and above is green.
- Work through the failed metrics and opportunities below in order of points; the score follows.
Questions people ask
What is a good Lighthouse score?
90 or above is good and shows green; 50 to 89 needs improvement and below 50 is poor. On mobile, where Lighthouse simulates a slow phone, 90 or more is hard for pages with lots of scripts, so many well-run sites sit in the 70s and 80s. Aim for green, but judge success by your real-user Core Web Vitals.
How is the Lighthouse performance score calculated?
Each of the five metrics is converted to a 0 to 100 score on a curve based on real website data, then combined with fixed weights: TBT 30 %, LCP 25 %, CLS 25 %, FCP 10 % and Speed Index 10 %. Because of the curve, a saving on a slow metric moves the score more than the same saving on a metric that is already fast. Google publishes a scoring calculator that shows the effect.
Does the Lighthouse score affect SEO?
Not directly. Google does not use the Lighthouse score for ranking; it uses Core Web Vitals measured on real Chrome users as part of its page experience signals. The two are related, because the fixes that raise the Lighthouse score usually improve real LCP, INP and CLS too. Content and relevance still matter far more than speed.
How do I improve my Lighthouse performance score?
Work on the heaviest-weighted metrics first: Total Blocking Time (30 %), Largest Contentful Paint (25 %) and Cumulative Layout Shift (25 %). Cut and defer JavaScript, compress and resize the hero image, and reserve space for images and ads. The opportunities in the report are ranked by estimated savings, so the top items move the score most.