What a canonical tag does
When the same content is reachable at several URLs, such as with and without www, with tracking parameters, or sorted differently, search engines have to choose one to index. <link rel="canonical" href="…"> tells them which one you prefer. Ranking signals from the duplicates are then consolidated onto that URL.
It is a strong hint, not a command. Google may pick a different canonical if your signals disagree, for example if the canonical points at a page that redirects, or if internal links and the sitemap all use another version.
What this checker tests
- Present or missing. Without a canonical, Google picks one for you.
- Self-referencing. The most common correct setup: each page names itself. Small differences like a trailing slash are tolerated.
- Pointing elsewhere. Correct for real duplicates, such as a printer view or a filtered product list. A mistake on a page you want ranked, and one of the quieter ways to lose a page from search.
- Multiple canonicals. Two different canonicals, often from a theme and a plugin, make Google ignore both.
- Relative URLs.
href="/pricing"works in theory but breaks easily. Use absolute URLs. - http vs https and www. A canonical on http:// for an https page, or on the other www variant, sends mixed signals.
- Cross-domain. Legitimate for syndicated articles, harmful by accident.
- Link header. Canonicals can also be sent as an HTTP header, which is how PDFs declare them. The checker reads both and flags disagreement.
- The target itself. We request the canonical URL and warn if it redirects or returns an error.
Canonicals and redirects
If a URL should never be visited, a 301 redirect is stronger than a canonical. Use canonicals for duplicates that need to stay reachable. The redirect checker shows a full redirect chain, and the SEO checker puts the canonical in context with the rest of the page.