What is a canonical URL and when should you use one?

Short answer
A canonical URL is the preferred version of a web page that you want search engines to index and rank when multiple URLs serve the same or very similar content. You declare it using a link rel='canonical' tag in the HTML head. Search engines consolidate ranking signals (backlinks, authority) to the canonical URL instead of splitting them across duplicates. Use canonicals for paginated content, URL parameter variations, HTTP/HTTPS duplicates, and syndicated content.

Duplicate content is one of the most common crawl budget wasters on large sites. E-commerce sites are especially susceptible: a product page accessible via /products/blue-widget, /products/blue-widget?color=blue, /products/blue-widget?ref=homepage, and www.example.com/products/blue-widget are four separate URLs serving identical content. Without canonicalisation, Googlebot wastes crawl budget visiting all four, and any backlinks pointing to those variants spread PageRank thin instead of concentrating it on one authoritative URL.

The canonical tag is a hint, not a directive. Google respects it in the vast majority of cases but will override it if it detects the canonical is set to a page with clearly different content (a common CMS bug). Strong canonicalisation signals include: the canonical URL being internally linked, included in the sitemap, and not blocked by robots.txt.

Self-referencing canonicals - a page pointing to itself as canonical - are considered best practice on all pages, not just duplicate pages. They prevent accidental duplication if another party links to a parameter variant of your URL.

For syndicated content (guest posts, press releases published on multiple sites), always ask the publishing partner to include a canonical back to your original URL. This ensures you retain the SEO credit for content you created, even when it legitimately lives on external domains.

Canonicals work alongside, not instead of, 301 redirects. If a URL will never receive direct traffic, redirect it permanently. Canonicals are the right tool when you need both URLs to remain accessible but want search engines to consolidate signals to one.

Reviewed by Rameez Majeed · Last reviewed