Canonical tags help search engines understand which URL should be treated as the primary (preferred) version when multiple URLs serve the same, or very similar, content. This supports cleaner indexing, clearer reporting in Google Search Console, and helps consolidate ranking signals across duplicates.
Important note: Google treats canonicalisation as a set of signals, not a guarantee. In other words, Google may choose a different canonical if other signals conflict, or if the selected canonical appears to provide a better experience for searchers. Google: Consolidate duplicate URLs
Table of contents
- What is a canonical tag?
- Why use canonical tags?
- Canonicalisation signals (not just the tag)
- How to write a canonical tag
- Canonical tag best practices
- How to audit canonical tags
- Common duplicate URL scenarios
- Google Search Console: common canonical-related statuses
- Fixing canonical issues: a practical checklist
- FAQs
What is a canonical tag?
A canonical tag is an HTML element placed in the <head> of a page to indicate the preferred URL that should be treated as canonical.
<link rel="canonical" href="https://www.example.com/page/" />
Canonical tags are most useful when multiple URLs exist for the same content (exact duplicates) or near-duplicates. Google may still select a different canonical if other signals disagree. Google: canonical URL guidance
Why use canonical tags?
Canonical tags help reduce duplication-related problems such as:
- Indexing the “wrong” version of a page (for example, parameterised or filtered variants)
- Splitting ranking signals across multiple URL versions
- Noisy reporting in Search Console caused by multiple duplicates for the same content
Common causes of duplicates include session parameters, print-friendly pages, sorting/filter URLs, and protocol/host variations (HTTP vs HTTPS, www vs non-www). Canonical tags can help consolidate these, especially when redirects are not appropriate for every variant.
Canonicalisation signals (not just the tag)
Canonicalisation is not determined by a single factor. Google uses multiple signals and may choose the canonical that best aligns with those signals overall. The most common signals include:
- Redirects (often the strongest signal) when one URL should no longer be accessible as a separate version
- rel=”canonical” in the HTML
<head> - Canonical via HTTP header (useful for non-HTML files like PDFs)
- Internal links pointing consistently to the preferred URL
- Sitemaps listing the preferred URL (and omitting duplicates)
For Google’s official overview of canonical methods and signals, see: How to specify a canonical URL with rel=”canonical” and other methods.
How to write a canonical tag
A canonical tag uses a <link> element with:
rel="canonical"to identify it as the canonical link elementhrefto specify the canonical URL
<link rel="canonical" href="https://www.example.com/your-page/" />
Best practice is to use absolute URLs and ensure the canonical target returns a 200 OK status code and is indexable. Google: 5 common mistakes with rel=canonical
Canonical tag best practices
1) Use self-referencing canonicals
Include a canonical tag on the preferred page that points to itself. This removes ambiguity and supports consistency across templates.
2) Avoid mixed signals
- Avoid canonical loops (A → B and B → A)
- Avoid canonical chains (A → B → C)
- Avoid canonical targets that redirect
- Avoid canonical targets blocked by
robots.txtor set tonoindex
Google’s official list of common mistakes is helpful for QA and technical checklists: 5 common mistakes with rel=canonical.
3) Canonicals can apply to near-duplicates
Canonical tags can be appropriate for very similar pages where consolidation is desired (for example, tracking parameters or minor sorting variants). However, where a URL should not exist as a separate version at all, a redirect may be more appropriate.
4) Cross-domain canonicals (syndication and multi-site publishing)
When identical content is published across multiple domains (such as content syndication or franchise networks), cross-domain canonicals can signal the original source. This needs careful governance to avoid pointing canonicals incorrectly across distinct content versions.
How to audit canonical tags
Manual checks
- Open a page and view page source
- Search for
rel="canonical" - Confirm the canonical is in the
<head>, uses an absolute URL, and points to the correct preferred URL
Developer tools
- Inspect the page in browser dev tools
- Locate the
<link rel="canonical">element in the DOM
Crawler tools (scalable auditing)
Site crawlers can reveal canonical patterns at scale and help spot chains, loops, non-200 targets, and canonicals pointing to redirected or non-indexable pages.
- Screaming Frog
- Sitebulb
- Ahrefs Site Audit
- Moz Pro
Using Screaming Frog to find duplicate content
- Go to Configuration → Content → Duplicates
- Enable Near Duplicates
- Set the similarity threshold (for example, 90%)
- Optional: define content areas in Configuration → Content → Area (include
<main>, exclude<header>and<footer>) - Start the crawl
- Run Crawl Analysis
- Use the filters: Exact Duplicates and Near Duplicates
Common duplicate URL scenarios
- HTTP vs HTTPS:
http://andhttps:// - www vs non-www:
www.example.comvsexample.com - Index files:
/vs/index.html - Session IDs:
?session=12345 - Sorting/filter parameters:
?sort=price-descor faceted navigation URLs - Print versions:
?print=true
Where one version should be the only accessible version (for example HTTPS-only, single host preference), redirects and internal linking consistency often form the core solution, with canonical tags used as a supporting signal. Google: consolidate duplicate URLs
Google Search Console: common canonical-related statuses
The Page indexing report (and URL Inspection) can surface canonical signals, including the difference between “user-declared canonical” and “Google-selected canonical”. Google recommends first checking whether the Google-selected canonical might be the better option for searchers, before forcing changes. Google: Fix canonicalization issues
Alternate page with proper canonical tag
This typically means the URL is not indexed because it correctly points to a canonical version. Action is only required if the canonical is incorrect.
Duplicate without user-selected canonical
This means Google detected duplicates and no canonical was provided.
Typical fix: implement a canonical tag pointing to the preferred version, and ensure internal links and sitemaps also support the preferred URL.
Duplicate, Google chose different canonical than user
This indicates a mismatch between the declared canonical and Google’s selected canonical.
Common causes: conflicting signals (redirects/internal links/sitemaps disagree), weak or thin canonical target, duplication that is too broad, or canonical targets that are not indexable (for example blocked, redirected, or noindex).
Google’s troubleshooting flow is here: Fix canonicalization issues
Fixing canonical issues: a practical checklist
1) Validate implementation
- Canonical tag is present and placed in the
<head> - Absolute URL is used
- Only one canonical tag exists per page
2) Validate the canonical target
- Canonical target returns 200 OK
- Canonical target is indexable (not blocked by
robots.txt, not set tonoindex) - Canonical target does not redirect (or at least does not redirect through chains)
3) Align signals across the site
- Internal links point consistently to the preferred URL
- Sitemaps list the preferred URL (and avoid listing duplicates)
- If one version should not exist, implement redirects (often preferred for protocol/host standardisation)
4) Use URL Inspection for confirmation
- Compare “user-declared canonical” vs “Google-selected canonical”
- Review “Crawled page” details for unexpected canonicals or conflicting tags
- After fixes, request reindexing where appropriate
Google’s guidance emphasises checking whether the Google-selected canonical is more appropriate for searchers, before enforcing changes. Google: canonicalisation troubleshooting
FAQs
What is a canonical URL?
A canonical URL is the preferred version of a page that search engines should prioritise when multiple URLs provide the same or very similar content.
When is a redirect better than a canonical tag?
Redirects are often the best option when a duplicate URL should not exist as a separate version (for example, HTTP to HTTPS, or non-www to www standardisation). Canonical tags are useful when multiple variants must remain accessible (for example, some parameterised URLs), but consolidation is still desired. Google: canonical methods and signals
Why does Google ignore a canonical tag?
Google may select a different canonical when signals conflict, when the canonical target is not indexable, when the canonical target redirects, or when Google believes another version provides a better experience for searchers. Google: fix canonicalisation issues
Are cross-domain canonical tags supported?
Yes. Cross-domain canonicals can be used for syndication or multi-site publishing to signal the original source URL, provided the pages are genuinely duplicates and the canonical target is accessible and indexable. Google: canonical specification methods
Can a canonical be specified for PDFs or non-HTML files?
Yes. Google supports specifying canonicals via an HTTP header, which can be useful for non-HTML resources. Google: canonical via HTTP header
What happens if multiple canonical tags exist on a page?
Multiple canonicals can confuse canonicalisation signals and reduce trust in the implementation. Google lists this as a common mistake and recommends a single, consistent canonical per page. Google: common mistakes with rel=canonical
