What is a canonical tag?
A canonical tag tells search engines which URL is the master copy of a page when the same or very similar content is available at more than one address. Google then concentrates ranking signals such as links and relevance on that one URL instead of spreading them across duplicates.
Duplicates appear far more often than most site owners realise. Common causes include:
- Tracking and filter parameters, such as
?utm_source=or?colour=blue. - HTTP and HTTPS, or www and non-www versions that both resolve.
- Trailing slash and uppercase variations of the same path.
- Products listed under several category paths.
- Syndicated or printer-friendly versions of articles.
Canonical tag or 301 redirect?
Use a 301 redirect when the duplicate URL has no reason to exist for visitors, for example after a migration or when merging two pages. Use a canonical tag when both versions need to stay accessible, such as a filtered product listing that customers use but which you do not want indexed separately.
Canonical mistakes that cost rankings
- Canonicalising to a redirect, 404 or noindexed page. The target must be live, indexable and return a 200 status.
- Multiple canonical tags. Plugins and themes sometimes add a second tag. When they disagree, Google may ignore both.
- Pointing paginated pages at page one. Page two of a category has different products on it. It should normally canonicalise to itself.
- Relative URLs. Always use the full URL, including
https://. - Mixed signals. If your sitemap lists one URL, internal links use another and the canonical names a third, Google will decide for itself.
To check canonicals across many pages at once, run a list of URLs through the Bulk SEO Checker. It flags missing, self-referencing and cross-pointing canonicals in one pass.