Lighthouse

Lighthouse is a free, open-source tool from Google that audits a web page for performance, accessibility, best practices and basic SEO. It runs a simulated page load in a controlled environment and produces scores out of 100 with a list of suggested improvements.

Written by Paul Hoda, SEO consultant since 2005. Last updated 8 November 2026.

What Lighthouse checks

A Lighthouse report has four main categories:

  • Performance: how quickly the page loads and becomes usable, based on lab metrics.
  • Accessibility: automated checks such as colour contrast, form labels and image alt text. This overlaps with accessibility SEO.
  • Best Practices: security and code quality issues, such as using HTTPS and avoiding browser errors.
  • SEO: basic checks such as a title tag, a meta description, crawlable links, valid status codes and whether the page is blocked from indexing.

The performance metrics

The performance score is a weighted blend of several lab metrics: First Contentful Paint, Largest Contentful Paint, Total Blocking Time, Cumulative Layout Shift and Speed Index. Because a standard Lighthouse run does not involve a real person clicking, it cannot measure Interaction to Next Paint. Total Blocking Time is used as a lab proxy for responsiveness instead.

How to run it

  1. Chrome DevTools: open developer tools, choose the Lighthouse tab, select mobile or desktop and run the report.
  2. PageSpeed Insights: runs Lighthouse on Google’s servers and shows real-user data alongside it.
  3. Command line or Node: developers can run Lighthouse in scripts and build pipelines to catch regressions before release.
  4. SEO crawlers: some crawlers can run Lighthouse across many URLs at once.

When testing in DevTools, use an incognito window so browser extensions do not skew the results.

Lab scores vs real-world performance

Lighthouse produces lab data: one simulated load on one device profile. Results can change between runs because of network conditions, server load and the testing machine. Google uses field data from real visitors, not Lighthouse scores, when assessing Core Web Vitals for search. Treat Lighthouse as a diagnostic tool for finding problems, then confirm improvements with field data.

Its real strength is the detail beneath the scores. Each audit explains what was found, which resources are involved and roughly how much time a fix could save. This makes it easy to hand developers a clear list of issues, such as oversized images, render-blocking scripts or unused CSS, and to check the fix before it goes live.

Common mistakes

  • Treating a 100 SEO score as proof a page is well optimised. The SEO audit only covers technical basics, not content quality, links or search intent.
  • Comparing scores from different machines or settings.
  • Running one test and drawing conclusions. Run several and look at the median.
  • Fixing low-impact warnings before the issues that slow down the main content.

Key Takeaways

  • Lighthouse is a free Google tool that audits performance, accessibility, best practices and SEO.
  • Its performance score is based on simulated lab metrics, not real users.
  • It uses Total Blocking Time as a stand-in for responsiveness because it cannot measure INP.
  • A perfect SEO score only means technical basics are in place.
  • Use it to diagnose problems and confirm results with field data.

Related Terms

Browse more terms in the SEO Encyclopedia.