Lighthouse is Google’s free, open-source tool for auditing a web page. It loads the page, scores it out of 100 for performance, accessibility, best practices and SEO, and lists specific issues to fix under each heading.
How Lighthouse works
You can run Lighthouse from the Lighthouse tab in Chrome DevTools, through PageSpeed Insights, from the command line, or automatically on every code change using Lighthouse CI. Each run loads the page under controlled conditions, by default emulating a mid-range phone on a throttled mobile connection, which makes its results lab data rather than a record of real visits.
The performance score is a weighted blend of several timings: First Contentful Paint, Speed Index, Largest Contentful Paint, Total Blocking Time and Cumulative Layout Shift. At the time of writing (October 2026), Total Blocking Time, Largest Contentful Paint and Cumulative Layout Shift carry most of the weight. Scores of 90 and above show green, 50 to 89 amber, and below 50 red.
The other three categories are collections of checks. Accessibility tests things such as colour contrast, missing alt text and form labels. Best practices covers security and browser issues such as HTTPS and console errors. SEO checks basics: a title and meta description, a crawlable page, valid links and a correct status code.
Why it matters
Lighthouse is the quickest free way to see what is slowing a page down and why, with each problem tied to an explanation and, often, an estimate of the time saved by fixing it. It runs on a staging site before launch, which makes it ideal for catching problems before visitors do.
Its accessibility checks are a useful starting point for UK businesses thinking about their duties under the Equality Act 2010, but automated tests catch only part of the picture. A manual review against WCAG is still needed. The same caution applies to SEO: a score of 100 there means the basics are present, not that the page is well optimised.
Common mistakes
- Chasing 100. Moving from 92 to 100 rarely changes anything a visitor notices. Fix what affects real users.
- Treating the score as a ranking factor. Google ranks using field data from real visitors, not Lighthouse scores.
- Running it with browser extensions active. Extensions add their own scripts; use a private window or PageSpeed Insights.
- Trusting one run. Scores vary between runs and machines, sometimes by ten points or more.
- Reporting the desktop score because it looks better, when most visitors use phones.
- Working down the list in order, rather than starting with the issues that hold up the main content.
How to act on it
Run Lighthouse on mobile settings in a private window, or through PageSpeed Insights, three to five times, and use the middle result. Start with the performance diagnostics tied to the slowest metric, usually the largest image or the scripts blocking the main thread, and ignore minor items until those are fixed.
Remember that Lighthouse tests only the single URL you give it. A homepage can score well while product, service or blog templates, which load different images and scripts, do not, so test one page of each template. Check field data in Search Console to confirm whether real visitors are affected, then use Lighthouse to test each fix on staging. For developers, Lighthouse CI can fail a release that makes performance worse. Turning a Lighthouse report into a prioritised list of fixes across a site’s templates is part of my technical SEO service.