How Lighthouse scores your website (and what the numbers mean)

If you have ever run a website through Google's PageSpeed Insights, the scores you saw came from Lighthouse. It is an open-source tool built by Google that simulates a visit to your site on a mid-range phone over a slow connection, then grades it on four categories: Performance, Accessibility, SEO, and Best Practices. Each score runs from 0 to 100.

Lighthouse is the tool I use to audit every site I build and every site I evaluate. Understanding what the numbers actually measure — and what they do not — is the difference between knowing your site is fast and just hoping it is.

Performance: how fast the page loads

The Performance score is the one most people focus on, and it is the most volatile. It measures three Core Web Vitals — Largest Contentful Paint (how fast the main content appears), Interaction to Next Paint (how quickly the page responds to taps), and Cumulative Layout Shift (how much the content jumps around while loading) — plus a handful of secondary metrics like total page weight and time to first byte.

A Performance score of 90 or above is considered good. Between 50 and 90 needs improvement. Below 50 is poor. The score is simulated, which means it is a lab measurement — your real users on fast WiFi in Charlotte may have a better experience than your real users on a mid-range phone in a parking lot in Clover. But the simulation is deliberately weighted toward the slow end, because that is where the experience actually breaks down.

Accessibility: how usable the site is

The Accessibility score runs the same axe-core checks I use for WCAG compliance — color contrast, alt text, heading structure, keyboard navigation, ARIA labels. A score of 100 means no automated violations were found. It does not mean the site is fully WCAG-compliant (automated tools catch roughly 30-40% of issues), but a score below 90 means there are specific, fixable problems.

This score is the most directly correlated with legal risk. A low Accessibility score means the site has barriers that a person with a disability can point to in a demand letter.

SEO: how search-engine-friendly the site is

The SEO score checks whether the page has the basics search engines look for: a descriptive title, a meta description, a canonical URL, a mobile viewport tag, legible font sizes, and links that are crawlable. It does not measure where you actually rank — that depends on content, backlinks, and competition — but a low SEO score means the page has structural problems that hold its ranking back regardless of how good the content is.

Best Practices: the catch-all

The Best Practices score checks for things that are not strictly performance or SEO but affect quality and security: HTTPS, no browser console errors, no deprecated APIs, proper image formats. A low score here usually points to a site that was built and left to rot.

This site scores 100 on Performance, 100 on Accessibility, 100 on SEO, and 96 on Best Practices. Those are not vanity numbers — they are the same scores I run against your site, and they are re-runnable. If the numbers ever drop, the audit will show it.

What the scores do not tell you

A high Lighthouse score is necessary but not sufficient. A site can score 100 on Performance and still have content that does not answer the question a visitor came for. It can score 100 on SEO and still rank poorly because the content is thin. It can score 100 on Accessibility and still have a manual WCAG violation that the automated check cannot catch. The scores are a floor, not a ceiling — they tell you the site is built to a standard, not that it is doing its job.

The free audit runs Lighthouse against your existing site and sends you all four scores with an explanation in plain English. If a score is low, the report tells you why and what it would take to fix it.