Lighthouse
Also called: google lighthouse, lighthouse audit, lighthouse score
Lighthouse is Google's free, open-source tool that runs an automated audit of a single web page and grades it 0 to 100 in four categories: Performance, Accessibility, Best Practices, and SEO. It reports lab data (one controlled test load) plus a list of specific fixes, and runs in Chrome DevTools, PageSpeed Insights, or a command-line tool.
What Lighthouse actually measures
Each of the four graded categories returns its own 0-to-100 score, so one page can score 98 on SEO and 41 on Performance. Those categories are Performance, Accessibility, Best Practices, and SEO. (Google removed the Progressive Web App category in Lighthouse 12, in 2024.) In 2026 Google added a fifth category, Agentic Browsing, which checks how ready a page is for autonomous AI agents. It is still under development, is reported as a pass/fail ratio rather than a 0-to-100 score, and does not affect your other four scores. You can run all of this from Chrome DevTools, the lighthouse npm CLI, or PageSpeed Insights with no install.
Two things trip people up. First, Lighthouse reports lab data: one simulated load on a fixed device and throttled network. That is why its LCP or CLS numbers often differ from the field (CrUX) data Google actually uses for the Core Web Vitals ranking signal. Treat Lighthouse as a diagnostic, not the scoreboard.
Second, the SEO category is a shallow hygiene check: does the page have a title, a meta description, valid hreflang, crawlable links, a viewport tag. A perfect 100 means nothing basic is broken. It does not predict rankings, and it says nothing about content quality or backlinks.
The Performance score is the one that maps to real work. It is a weighted blend of lab metrics including First Contentful Paint, Speed Index, Largest Contentful Paint, Total Blocking Time (the lab stand-in for interaction responsiveness), and Cumulative Layout Shift. The “Opportunities” section lists concrete fixes: defer unused JavaScript, serve images as WebP, eliminate render-blocking CSS.
How it affects your traffic
Lighthouse is usually the first place a technical problem shows up as a number you can act on. Its Opportunities and Diagnostics map almost one-to-one onto a technical SEO scope: render-blocking resources, oversized images, missing meta tags, uncrawlable links, weak mobile viewport. Fixing those raises the lab score, but the real targets are the field data behind your Core Web Vitals and the crawl budget behind your indexing. A technical SEO engagement is where Lighthouse flags get triaged by ranking impact and fixed in the code, not just logged in a report.
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