Docs

The Accessibility index

How the 0–100 number is worked out, how to read changes in it, and what it doesn't mean.

The Accessibility index is a number from 0 to 100 that summarizes how well your site meets the WCAG criteria that apply to it, on the pages and date in the report.

How it’s worked out

  1. Find the criteria that apply. WCAG 2.2 AA has 55 success criteria. Some don’t apply to every site: a site with no video has nothing to test for captions. Those are left out, not counted as passes.
  2. Give each criterion a status. From the automated results and the testing by a person, each applicable criterion is met (scores 1), partly met (0.5) or not met (0).
  3. Weight by level. Level A criteria cover the most basic barriers, so each counts 1.5. Level AA criteria count 1.
  4. Average. The index is the weighted share of the maximum possible score, times 100.

Criteria that still need human verification, waiting on a person’s judgment, aren’t guessed at. They’re listed separately and left out of the index until someone has checked them.

Reading changes

  • The index went up. Issues were fixed, or a criterion that was waiting on a person’s review was checked and met.
  • The index went down. Something new broke (a new page, plugin or content), or a person found a problem the scans couldn’t see.
  • A marked point on the chart. The testing rules changed between those two reports: a new engine version or a scoring update. A change at a marked point may come from the rules, not your site. The report lists what changed.

What it doesn’t mean

The index is not a compliance percentage. An index of 85 doesn’t mean “85% compliant”, and 100 isn’t a certificate.

An index of 100 means every applicable criterion we tested was met, on the pages tested, on that date. That’s a strong, well-documented position. But no tool or audit can certify a website as compliant with the ADA or any other law, and some barriers only show up when people use your site with their own assistive technology.

Why not just count errors?

Error counts swing wildly with page count: one broken button in a shared header is “one error” on a one-page site and “fifty errors” on a fifty-page site. The index is built on criteria, not raw counts, so it stays comparable as your site grows. Reports still give you every count, grouped by root cause.