The same content often lives at more than one URL: with and without tracking parameters, under two categories, in a print version. The canonical tag is how you tell search engines which one is the real page, so ranking signals are consolidated there and that's the URL that shows in results.

Key takeaways from this guide

What it looks like

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

It goes in the <head> of the page. Use an absolute URL, with the exact protocol and host you want indexed.

When to use a canonical

• URL parameters: /shoes?utm_source=whatsapp should canonicalise to /shoes.
• Sorting and filtering: /shoes?sort=price shows the same products in a different order.
• A product in several categories: /men/running/shoe-x and /sale/shoe-x.
• Printer-friendly or alternate versions of the same article.
• Syndicated content: when your article is republished elsewhere, the copy can carry a canonical pointing to your original.

Self-referencing canonicals

Every normal, indexable page should include a canonical pointing to itself. It looks redundant, but it protects you: if someone links to the page with a parameter attached, or the page is reachable via an alternate path, the canonical clarifies which URL is the original. Most CMSs and SEO plugins do this automatically.

Canonical vs redirect vs noindex

See our duplicate content guide for the bigger picture.

Why Google sometimes ignores your canonical

The canonical is a strong hint, not an instruction. Google weighs it with other signals and may choose a different URL if they conflict. Typical reasons:

• The pages aren't actually duplicates. Canonicalising page two of a category to page one, or a product to its category, contradicts the content.
• Internal links point to the other version. If your menu links to /shoes/ but the canonical says /shoes, signals conflict.
• The sitemap lists the other version.
• The canonical URL redirects, 404s or is noindexed.
• Multiple canonical tags on one page, often from a theme and a plugin both adding one. Google may then ignore both.

In Search Console's URL inspection, compare "User-declared canonical" with "Google-selected canonical". If they differ, look for one of the conflicts above.

Mistakes that cause real damage

1. Every page canonicalised to the homepage, usually a template bug. Google may drop inner pages from the index.
2. Paginated pages canonicalised to page one. Pages 2, 3, 4 contain different products or posts; give each a self-referencing canonical.
3. Relative or malformed URLs, such as href="shoes" or a missing protocol.
4. Canonical to http on an https site, left over from a migration.
5. Canonical in the body instead of the head, which Google ignores.
6. Cross-language canonicals: the Hindi version canonicalised to the English one. Use hreflang for language versions, with self-referencing canonicals on each.

Checking canonicals

Our canonical URL checker shows the canonical declared on any page, flags multiple tags, and checks whether the target returns 200. Run it on one example of each template: home, category, filtered category, product, article. Template bugs show up immediately.

HTTP header canonicals

For non-HTML files such as PDFs, you can send the canonical as an HTTP header:

Link: <https://example.com/guides/gst-guide>; rel="canonical"

Useful when the same PDF is available at several URLs, or when you want an HTML page to be the canonical for a PDF version. You can see headers with the HTTP headers checker.

A worked example: an online shoe shop

A single product, "Trail Runner X", can be reached in several ways on a typical shop:

And the canonical target itself must return a 200 status, not a redirect or 404. If you're unsure what status a URL returns, our guide to HTTP status codes explains each one and how to check.

Quick rules to remember

• One canonical per page, in the head, absolute URL.
• Self-referencing on every indexable page.
• Point only to a truly equivalent page that returns 200.
• Keep links, sitemap and canonical pointing to the same version.