What is a canonical tag?

The same page can often be reached at several addresses: with and without www, with tracking parameters, with a trailing slash or without, through a filter or a print view. To a search engine, those are different URLs with the same content. A canonical tag tells it which one you want indexed and shown in results.

What a canonical tag looks like

It is a link element in the page's head:

<link rel="canonical" href="https://example.com/running-shoes/">

Every version of the page (such as /running-shoes/?utm_source=newsletter or /running-shoes/?sort=price) carries the same canonical, pointing at the clean address.

Why canonical tags matter

Without a canonical, Google picks a version itself. It usually picks well, but not always, and duplicates cause real problems:

• Split signals: links and engagement spread across several versions instead of strengthening one.
• Wrong page in results: a parameter URL or an old address appears instead of the one you want.
• Wasted crawling: Google spends time on copies instead of new pages.

When to use a canonical tag

• Tracking and session parameters: ?utm_campaign=..., ?ref=....
• Sorting and filter pages that show the same products in a different order.
• Product variants at separate addresses where the content is almost identical.
• Syndicated content you've republished on another site, pointing back to the original.
• HTTP and HTTPS, www and non-www versions (though redirects are the stronger fix here).

Self-referencing canonicals

Every indexable page should carry a canonical pointing to itself. It looks redundant, but it protects the page when someone links to it with parameters attached, and it removes any doubt about which version is the main one.

Canonical vs redirect vs noindex

A canonical is a hint, not a command. Google may ignore it if other signals disagree, for example if your sitemap and internal links all point to a different version.

Common canonical mistakes

1. Every page canonicalised to the homepage. A theme or plugin bug that tells Google to drop the whole site.
2. Canonical pointing to a redirect or a 404. The target must return 200.
3. Relative or malformed URLs. Use the full absolute address, including https://.
4. Paginated pages all pointing to page 1. Page 2 of a category isn't a duplicate of page 1; give each page its own canonical.
5. Canonical and noindex together. These send mixed signals. Choose one.
6. More than one canonical tag on the page, often one from the theme and one from an SEO plugin.

How to check a page's canonical

Enter a page in the checker on this page. It shows the canonical in the HTML and the HTTP header, whether it points to itself, and whether the target address actually loads. In Google Search Console, the URL Inspection tool shows both the canonical you declared and the one Google chose.