Google Lighthouse is a free auditing tool from Google that grades a web page and tells you where it falls short. You point it at a URL and it returns a score out of 100 in four areas, performance, accessibility, SEO and best practices, plus a list of concrete things to fix. It is built right into Chrome, so any developer can run it in seconds.
Think of it as a health check-up for a page. A doctor measures a few key numbers, flags what looks off and hands you a list of things to work on. Lighthouse does the same: it does not just say a page is slow, it points to the oversized image or the blocking script that is causing it, so you know where to start.
Its performance section leans heavily on Core Web Vitals, the same speed and stability signals Google uses in search. Lighthouse is also the engine behind PageSpeed Insights, which runs the same audit through a web page instead of the browser.
One thing to watch is that the score swings from run to run. Lighthouse tests on a simulated slow connection and throttled CPU, so a busy laptop or a noisy network can drag a result down by ten points with no change to the actual page. Run it a few times, or in an incognito window with extensions off, before you trust a single number. The accessibility checks are useful but only catch part of the picture, since things like clear labels or sensible reading order still need a human eye. The takeaway is to treat the score as a guide, fix the issues that affect real users, and not obsess over the last few points.
At TopDevs we run Lighthouse on every build and act on its findings, so a site ships fast and accessible rather than scoring well only on paper.
