Canonicalization in SEO is the process of choosing one preferred URL when the same or very similar content is reachable under several URLs. The preferred one is the canonical URL. Search engines then show that URL in results and combine the signals of the duplicates on it. The word has other meanings in computing, such as normalizing data formats. This entry covers URL canonicalization for search.
How does Google choose a canonical URL?
Google evaluates several signals and picks the canonical itself. A site can send signals, and they differ in strength:
- Redirects are a strong signal that the redirect target should be canonical.
- rel=“canonical” is a strong signal. It works as a
linkelement in the head or as an HTTP header, which suits PDFs. - Sitemap inclusion is a weak signal. URLs listed in a sitemap are more likely to become canonical.
Google also prefers HTTPS over HTTP, and prefers URLs that belong to an hreflang cluster. Google makes the final choice itself. None of these methods is required, and without a declaration Google picks the version it considers best for users. When signals conflict, the selected canonical can differ from the declared one.
Example
These URLs show the same product page:
https://example.com/shoes/trail-pro
https://example.com/shoes/trail-pro?utm_source=newsletter
https://www.example.com/shoes/trail-pro/
The preferred version is set with a self-referencing canonical on the page and consistent internal links:
<link rel="canonical" href="https://example.com/shoes/trail-pro" />
Common mistakes
- Relative URLs in the canonical. Google recommends absolute URLs.
- Conflicting signals. A canonical to URL A while the sitemap lists URL B, or internal links to a third variant.
- noindex, robots.txt or the removals tool for duplicates. Google advises against using any of them to choose a canonical. noindex blocks the page from Search completely, and the removals tool hides all versions of a URL.
- A URL fragment as canonical. Google generally does not support fragments.
| Method | Prevents crawling? | Keeps the URL out of Search? | What Google says | Typical use |
|---|---|---|---|---|
robots.txt Disallow | yes | no, the URL can be indexed without its content if other pages link to it | used mainly to avoid overloading the server with requests; not a mechanism for keeping a page out of Google | Limiting crawler load, excluding areas from being fetched |
noindex in the robots meta tag | no, Google has to be able to fetch the page | yes | only works if the page isn't blocked by robots.txt; Google then drops the page even if other sites link to it | HTML pages that stay reachable but shouldn't show up in search results |
noindex in the X-Robots-Tag HTTP header | no, Google has to be able to fetch the URL | yes | any robots meta tag rule can also be sent as a header; same prerequisite as the meta tag | Files without an HTML head, such as PDFs, images or videos |
rel="canonical" | no | not directly; a strong signal for which of several duplicate URLs becomes canonical | helps consolidate signals such as links into the preferred URL; don't use robots.txt or noindex for canonicalization | Duplicate and very similar pages, for example from parameters |
| Status code 410 (or 404) | no, but Google gradually fetches the URL less often | yes, a previously indexed URL is removed | treats 404 and 410 the same way | Page permanently removed with no replacement |
| Password protection (login) | yes, crawlers can't reach the content | yes | one of the ways Google lists for permanent removal; authorised users see the page, crawlers don't | Private or confidential content |
| Search Console: Removals | no, Google can keep crawling the URL | hides the URL from search results for about six months | temporary only; 404/410, password protection or noindex make it permanent | Urgent cases, until a permanent measure takes effect |
robots.txt and noindex cancel each other out. If a URL is blocked by robots.txt, Google never sees the noindex, and the URL can still appear in results. Google doesn't support noindex inside the robots.txt file itself. Sources: Google Search Central: Introduction to robots.txt (last updated 2025-12-10); Google Search Central: Block Search indexing with noindex (last updated 2025-12-10); Google Search Central: Robots meta tag, data-nosnippet, and X-Robots-Tag specifications (last updated 2026-03-24); Google Search Central: How to specify a canonical URL (last updated 2026-07-10); Google Crawling Infrastructure: How HTTP status codes affect Google's crawlers (last updated 2026-02-04); Google Search Central: Remove information from Google (last updated 2025-12-10); Search Console Help: Removals and SafeSearch reports tool. Last checked: .
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