A canonical tag (rel="canonical") tells search engines which version of a page is the “master” copy when multiple URLs show the same or very similar content. It’s placed in the <head> of a page’s HTML and points to the preferred URL.
Why this matters: Many sites unintentionally create duplicate content — the same product page accessible via /product?color=red and /product?color=blue, or a page reachable both with and without a trailing slash. Without a canonical tag, search engines have to guess which version to index and rank, which can split ranking signals across duplicates instead of consolidating them into one strong page.
Common use cases: E-commerce filter/sort URL parameters, print-friendly page versions, syndicated content republished on partner sites, and www vs. non-www or HTTP vs. HTTPS variants.
Common mistakes: Pointing a canonical tag to a page that then redirects elsewhere (creates a loop), setting canonicals to a completely unrelated page, or having conflicting signals — like a canonical tag pointing to Page A while the sitemap lists Page B as the primary URL.
Self-referencing canonicals — where a page’s canonical tag simply points to itself — are considered good practice on most pages, even when there’s no duplicate, since it removes any ambiguity.
How to add a canonical tag
A canonical tag is a single line in the <head> of the page:
<link rel="canonical" href="https://example.com/product/" />Always use the full absolute URL, including the protocol and the exact version of the domain you want indexed. Relative URLs work in theory, but they are a common source of mistakes after migrations and staging launches. If you can't edit the HTML (for PDFs, for example), you can send the same signal through an HTTP header: Link: <https://example.com/file.pdf>; rel="canonical".
Canonical tag vs 301 redirect vs noindex
These three tools are often confused, but they solve different problems:
- 301 redirect: use it when the duplicate URL should not exist for users at all, such as an old URL after a site migration. Visitors and crawlers are both sent to the new page.
- Canonical tag: use it when both URLs need to stay accessible for users (a filtered product list, a tracking URL, a print version) but only one should be indexed.
- noindex: use it when a page should not appear in search at all, and there is no better version to consolidate its signals into.
Never combine noindex with a canonical pointing to another page. The two signals contradict each other, and Google may ignore both.
A canonical is a hint, not a command
Google treats rel="canonical" as a strong signal, not a directive. It also looks at redirects, internal links, sitemap URLs, HTTPS vs HTTP and how similar the pages really are. If those signals disagree with your tag, Google may choose a different canonical. You can see this in Search Console: open the URL Inspection tool and compare the "User-declared canonical" with the "Google-selected canonical". In the Page indexing report, the status "Duplicate, Google chose different canonical than user" lists every page where this happened.
To make your choice stick, keep every signal consistent: link internally to the canonical URL, list only canonical URLs in your XML sitemap, and avoid pointing canonicals at pages that redirect, return 404 or are blocked in robots.txt.
Canonical tags on ecommerce and paginated pages
On ecommerce sites, filter and sort parameters (?color=red, ?sort=price) usually canonicalise to the main category URL. The exception is a filtered page that targets real search demand, such as "red running shoes". That page may deserve its own self-referencing canonical, a unique title and indexable content.
For paginated series (/blog/page/2/), each page should normally have a self-referencing canonical. Pointing every page in the series to page one tells Google to ignore the products or articles that only appear on later pages.
How to audit your canonical tags
- Crawl the site with a tool such as Screaming Frog and export the canonical column.
- Flag pages with a missing canonical, more than one canonical tag, or a canonical pointing to a non-200 URL.
- Check that canonical URLs match the URLs in your sitemap and internal links.
- Review the "Google chose different canonical" list in Search Console every month and fix the conflicting signals.
Done well, canonical tags are invisible: Google indexes the URL you want, ranking signals consolidate on one page, and duplicate URLs stop competing with each other.
