Lighthouse is Google's free auditing tool that scores a page out of 100 on performance, accessibility, best practices and SEO, and tells you exactly what to fix.
Lighthouse is Google's free auditing tool that scores a page out of 100 on performance, accessibility, best practices and SEO, and tells you exactly what to fix. It is built into Chrome's developer tools, and the same engine powers PageSpeed Insights and a command-line version you can wire into a build pipeline.
It works by loading your page in a controlled, simulated environment, a throttled connection and CPU meant to stand in for a mid-range mobile phone, then grading what it sees. The performance score is not a single measurement but a weighted blend of lab metrics, including Largest Contentful Paint, Total Blocking Time and Cumulative Layout Shift. The key thing to understand is that this is a lab test on a simulated device, not a reading from your real visitors.
Read the score as a starting point, not a report card on your business. The genuine value is lower down, in the diagnostics and opportunities, where Lighthouse lists specific, actionable fixes: unoptimised images, render-blocking scripts, missing alt text, and so on. The accessibility, best-practices and SEO audits in particular are close to a free technical SEO checklist. Work the list, not the number.
Two honest caveats. First, the performance score is a weighted average that can swing by several points between runs on the same page, so take the median of a few runs and do not agonise over a couple of points. Second, Lighthouse is a lab tool and does not always agree with the Core Web Vitals field data Google actually ranks on. A perfect 100 is a nice trophy but not the goal in itself, and it is very achievable on a well-built site, as I show in my write-up on scoring 100 on Lighthouse without a framework.
Key points
- A free Google tool that scores a page out of 100 across performance, accessibility, best practices and SEO.
- Built into Chrome DevTools and also powers PageSpeed Insights and a CLI version.
- It is a lab test on a simulated mid-range phone, not a reading from your real visitors.
- The performance score is a weighted blend of lab metrics like LCP, Total Blocking Time and CLS.
- The real value is in the diagnostics and opportunities, work the list, not the number.
- Scores wobble run to run, so take the median of a few and do not chase the last two points.
Frequently asked questions
Common questions about lighthouse.
No, and I would gently talk you out of treating it as the target. There is a real difference between a page that scores in the 90s and one stuck in the 50s, that gap represents genuine speed your visitors will feel. But the climb from 95 to 100 is often a lot of effort for a change nobody notices. I aim for a healthy, comfortable score with clean diagnostics, then spend the remaining energy on content and conversion. A perfect 100 is achievable and satisfying, but it is a nice-to-have, not the point.
Because Lighthouse simulates a throttled device and network, and that simulation has natural variance, plus anything else running on your machine or network at that moment nudges the result. It is completely normal to see the performance score bounce several points between back-to-back runs on an unchanged page. The fix is to stop reading single runs, take the median of three to five, ideally in an incognito window with extensions off, and judge changes against that. PageSpeed Insights, which runs on Google's own infrastructure, is steadier than your local DevTools.
Not directly, and this trips a lot of people up. Google does not rank you on your Lighthouse number, it ranks on Core Web Vitals, which come from real Chrome visitors rather than Lighthouse's lab simulation. The two are related but not the same, so it is quite possible to score well in Lighthouse and still fail the field data, or vice versa. Use Lighthouse to find and fix problems, then confirm the win in the real-visitor data, because that is the version Google actually counts.
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