Canonical tag checker

What a canonical tag is

The same page is often reachable at several addresses — with and without www, with tracking parameters, sorted differently. A canonical tag, <link rel="canonical" href="…">, tells Google which one you want shown and credited. Google calls it "a strong signal", alongside redirects, though it may still choose a different address if the other signals disagree (Google: consolidate duplicate URLs).

What this checker looks for

• In the head. Google reads the canonical only from the page's <head>. One in the body is ignored outright.
• Exactly one. Several that disagree may all be ignored.
• A full address. Google supports relative paths but says it does not recommend them, because they "can cause problems in the long run (for example, if you unintentionally allow your testing site to be crawled)".
• No # fragment. Google says not to use one in a canonical.
• Pointing at a page that loads. We fetch the canonical address: a redirect or a 404 there means the tag is ignored.
• No chains or loops. If the canonical page names yet another page as canonical, we follow it (up to 3 pages) to see where it ends.

Google also accepts a canonical sent in the Link response header. Our audit reads only the HTML tag; this checker reads both, and says which it found.

Next: follow the redirects, or check every tag in the head.