lighthouse score

A Lighthouse score is a 0-to-100 number Google's open-source Lighthouse tool gives a web page across four categories: Performance, Accessibility, Best Practices, and SEO. Scores are colour-coded — red below 49, orange 50 to 89, green 90 or above. Lighthouse runs the page in a headless Chrome, throttled to simulate a mid-range Android on slow 4G, and scores the result against a weighted set of audits. It is a synthetic lab test, not a real-user measurement, so it differs from Core Web Vitals field data. A site can score 100 in Lighthouse and still fail Core Web Vitals in CrUX, usually because real users have slower devices or more ad blockers or worse networks than the lab. Despite that caveat, Lighthouse remains the industry shorthand for 'is this page well-built' because it is free, reproducible, and runs inside Chrome DevTools for anyone who wants it.

What my own pages score

This glossary page should score in the mid-to-high 90s across all four categories — the build target for every page on this site is 95 or above. The homepage hits 100 on Performance on desktop, 95-plus on mobile. Where the homepage loses a point or two it is usually on a single third-party font metric; that is a known quirk of next/font, not a real-user problem. The accessibility, best-practices, and SEO categories all sit at 100.

Why it is the first thing any new developer checks

Lighthouse is the first diagnostic any half-decent developer runs when they inherit a site. A site scoring 40 across the board is a site with compounding problems — large unoptimised images, render-blocking JavaScript, missing alt text, no proper heading structure. Fixing one audit usually fixes four. For a small business shopping around, asking any prospective builder 'what do your sites score on Lighthouse' is a reasonable screening question.

Related terms

• core web vitalsThree numbers — LCP, INP, CLS — that Google uses to judge real-world page speed. Passing them is a ranking signal.
• LCP (Largest Contentful Paint)Largest Contentful Paint — how long the biggest visible element takes to render. Google's threshold is 2.5 seconds.
• INP (Interaction to Next Paint)Interaction to Next Paint — how responsive the page feels when you click or tap. Google's threshold is 200 milliseconds.
• CLS (Cumulative Layout Shift)Cumulative Layout Shift — how much the page jumps around while loading. Google's threshold is a score of 0.1.
• WCAG 2.2 AAThe current web accessibility baseline. What government sites must meet in the UK — and what the Equality Act expects of everyone else.