Canonical URL
What is a canonical URL?
A canonical URL is an HTML tag that tells search engines which version of a page is the "original" and should be indexed. It's used when the same or very similar content is available at multiple URLs — a situation far more common than one might think.
What does the canonical tag look like?
<link rel="canonical" href="https://example.com/article" />
This tag is placed in the <head> section of an HTML page. It tells Google: "this is the preferred URL for this content — index this one, not the other variants."
The canonical URL can also be specified in the HTTP response header (Link: <URL>; rel="canonical") — useful for PDFs and other resources without a <head> section.
When to use canonical?
A canonical URL is essential wherever the same content is available at multiple addresses. The most common scenarios:
- URL parameters —
/products?sort=priceand/products?sort=namehave the same content, differing only in sorting - www/non-www versions —
www.example.comvsexample.com - HTTP/HTTPS —
http://vshttps://(although this should be resolved with a 301 redirect) - Trailing slash —
/articlevs/article/ - Content syndication — when the same article appears on multiple sites (e.g., a reprint on an industry portal)
- Mobile versions —
m.example.comvsexample.com(less common with responsive design) - Pagination —
/blog?page=1,/blog?page=2may have a canonical pointing to the main category page - Product variants — the same product in different colors under different URLs
Why is canonical important?
Without a canonical URL, Google has to decide on its own which page variant to index — and it often chooses incorrectly. The consequences of missing canonicalization can be serious — which is why it's a key item in every SEO audit:
Duplicate content issues
- Authority dilution — backlinks pointing to different URL variants don't aggregate; instead of one strong page, you have several weak ones
- SERP cannibalization — multiple versions of the same content compete against each other in search results
- Wasting crawl budget — Google wastes time crawling duplicates instead of discovering new content
- Inaccurate analytics — traffic split across multiple URLs makes analysis difficult
Benefits of proper canonicalization
- Authority consolidation — all ranking signals go to a single, preferred URL
- Clean data in Google Search Console — one URL per piece of content, easy analysis
- Control over the index — you decide what Google indexes
- Better crawlability — the crawler uses the budget more efficiently
How to properly implement canonical?
Implementation rules
- Every page should have a canonical — even if it points to itself (self-referencing canonical). You can find the full list of elements to verify in the technical SEO checklist
- Canonical must point to the HTTPS version — always use a full, absolute URL
- Canonical should point to a 200 page — not to a redirect or a 404 page
- One canonical per page — multiple canonical tags on a single page are ignored
- Consistency with other signals — canonical should align with sitemap, hreflangs, and internal linking
Common mistakes
- Canonical to a noindex page — conflicting signals; Google may ignore both
- Canonical chains — A -> B -> C; Google may not follow the chain
- Canonical to a page with different content — Google ignores the canonical if the content significantly differs
- Missing canonical on pagination pages — leads to indexing of hundreds of
/page/2,/page/3pages
Canonical vs 301 redirect
Canonical is a "suggestion" for Google — the search engine may ignore it. A 301 redirect is a "directive" — the browser and crawler are physically redirected. When possible, prefer a 301 redirect over canonical — it's a stronger signal.
| Situation | Solution |
|---|---|
| One piece of content, one correct URL, the rest to be removed | 301 redirect |
| One piece of content, multiple URLs must exist (parameters, sorting) | Canonical |
| Content syndication on an external site | Canonical |
| Domain migration | 301 redirect + canonical |
Related terms
- Duplicate content — the problem that canonical solves
- 301 Redirect — permanent redirect, an alternative to canonical
- Crawlability — the ability of crawlers to traverse a website
- Indexing — the process of recording a page in Google's index
- Crawl budget — crawl budget, wasted by duplicates