What Is a Canonical Tag?
Canonical tags are link elements in a page’s HTML head that tell search engines which URL is the preferred version when the same or very similar content exists at several addresses. They consolidate signals, such as links, on one URL. Google treats a canonical tag as a strong hint, not a command.
| Method | Strength of signal | When to use |
|---|---|---|
| 301 redirect | Strong | The duplicate page is no longer needed |
| rel=”canonical” link element | Strong | Both URLs must stay accessible |
| rel=”canonical” HTTP header | Strong | Non-HTML files, such as PDFs |
| Sitemap inclusion | Weak | Supporting signal |
| Consistent internal linking | Supporting | Always |
The strengths are those described in Google’s guide to consolidating duplicate URLs. The methods can be combined, which strengthens the signal.
Why Do Canonical Tags Matter?
They matter because one page often exists at many URLs, and search engines must pick one to index.
Without guidance:
- Signals are split. Links to different versions are not combined.
- The wrong URL may appear in results, such as one with tracking parameters.
- Crawling is wasted on duplicates.
- Reporting is confused, with metrics spread across versions.
A canonical tag says: “treat this URL as the main one.”
Where Do Duplicate URLs Come From?
Google lists common causes:
- Tracking parameters, such as ?utm_source= or ?gclid=.
- Sorting and filtering on category pages.
- The same content reachable by several paths, such as a product in two categories.
- HTTP and HTTPS, or www and non-www, versions.
- Trailing slash and capitalisation variants.
- Print and mobile versions.
- Session IDs.
- Content syndicated to other sites.
- Regional versions with identical text.
Most sites have some of these without anyone having created duplicates deliberately.
How Do You Add a Canonical Tag?
Place a link element in the head of the page:
- A link element with rel=”canonical” and an href containing the full preferred URL, for example https://www.example.com/shoes/red-boot/.
Rules from Google’s guidance:
- Use absolute URLs, not relative paths.
- Put it in the head. A canonical in the body is ignored.
- One per page. Multiple conflicting canonicals are likely to be ignored.
- Put it in the HTML source. If JavaScript must inject it, make sure nothing changes it afterwards.
- Point to an indexable page that returns a 200 status.
- Do not use URL fragments as canonicals.
Most content management systems and SEO plugins add canonical tags automatically. Check what they output.
Should Every Page Have a Self-Referencing Canonical?
Yes, it is good practice. Google recommends that the canonical page include a canonical link to itself.
A self-referencing canonical protects against duplicates you did not create, such as someone linking to your page with tracking parameters added. The page then declares its clean URL as the preferred one.
Check that the self-reference is exact: same protocol, same host, same trailing slash, same case.
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See our SEO servicesIs a Canonical Tag a Directive or a Hint?
A hint. Google considers it together with other signals and chooses the canonical it judges best. It may ignore your tag when:
- The pages are not similar enough. A canonical between different content is not credible.
- Signals conflict. Internal links, sitemaps and redirects point to another URL.
- The target is not indexable: it redirects, returns an error or carries noindex.
- There are several canonical tags on the page.
- HTTP is declared when an HTTPS version exists; Google generally prefers HTTPS.
To see which canonical Google chose, use the URL Inspection tool. It shows the “user-declared canonical” and the “Google-selected canonical”. When they differ, find the conflicting signal.
What Is the Difference Between a Canonical and a Redirect?
| Canonical tag | 301 redirect | |
|---|---|---|
| Visitor experience | Stays on the URL they opened | Sent to the new URL |
| Both URLs accessible? | Yes | No; the old one forwards |
| Signal | Strong hint | Strong; also moves users |
| Use when | Duplicates must remain reachable | The old URL is no longer needed |
| Example | A product with filter parameters | An old page replaced by a new one |
Use a redirect when you can. Use a canonical when the duplicate has to keep working for users, such as sorted or filtered views. See our guide to 301 redirects.
How Do Canonicals Interact with Other Signals?
- Noindex. Do not combine noindex with a canonical pointing elsewhere; they give conflicting instructions. Google advises against using noindex to choose a canonical within a site.
- robots.txt. Do not block duplicate URLs you want consolidated. Google cannot read a canonical on a page it may not crawl.
- Hreflang. Each language version should have a canonical to itself, or to a page in the same language.
- Pagination. Each page in a series should canonicalise to itself, not to page one.
- Sitemaps. List only canonical URLs.
- Internal links. Link to the canonical URL, not to duplicates.
Consistency across all of these is what makes Google follow your preference.
How Do You Handle Common Cases?
| Case | Approach |
|---|---|
| Tracking parameters | Self-referencing canonical on the clean URL |
| Product in several categories | One product URL; canonical from any path variants |
| Filter and sort URLs | Canonical to the main category, or block from crawling |
| HTTP and HTTPS | 301 redirect to HTTPS; canonical on HTTPS |
| www and non-www | 301 redirect to one; canonical to match |
| Printer-friendly pages | Canonical to the main page |
| Syndicated articles | Ask the republisher to canonical to your original, or to use noindex |
| PDFs duplicating a web page | Canonical HTTP header to the web page |
| Near-identical regional pages | Hreflang; self-canonicals; add real local differences |
What Are the Common Mistakes?
- Every page canonicalising to the home page. Often a template error; it can remove pages from the index.
- Canonical to a redirected or 404 URL.
- Relative URLs that resolve incorrectly.
- Canonical pointing to the staging domain after launch.
- Paginated pages canonicalised to page one, hiding items on later pages.
- Two canonical tags from two plugins.
- Canonical plus noindex to a different URL.
- Canonicalising pages that are not duplicates, hoping to pass value.
- Protocol or slash mismatch in a self-reference.
- Internal links pointing to non-canonical versions.
How Do You Audit Canonicals?
- Crawl the site, and export each URL with its canonical. See our guide to using an SEO crawler.
- Filter for: missing canonicals, canonicals to non-200 URLs, non-self canonicals on pages that should rank, and multiple canonicals.
- Open the Page indexing report. Review “Duplicate without user-selected canonical” and “Duplicate, Google chose different canonical than user”.
- Inspect important URLs individually.
- Check the sitemap contains only canonical URLs.
Repeat after any platform change or redesign. Our guide to duplicate content covers causes and fixes more widely.
Frequently Asked Questions
Is a canonical tag a directive or a hint?
A hint, though a strong one. Google weighs it with other signals, such as redirects, internal links and sitemaps, and selects the URL it considers best. If the pages are not similar or your signals conflict, it may choose a different canonical from the one you declared.
Should every page have a self-referencing canonical?
Yes. Google recommends it. A self-referencing canonical tells search engines the clean address of the page, which protects against duplicates created by tracking parameters or alternative paths. Make sure it matches the real URL exactly, including protocol, host and trailing slash.
What is the difference between a canonical and a redirect?
A redirect sends both visitors and search engines to another URL, so the original is no longer shown. A canonical tag leaves both URLs accessible and only tells search engines which one to index. Use a redirect when the duplicate is not needed, and a canonical when it is.
How do I check which canonical Google chose?
Paste the URL into the URL Inspection tool in Search Console. Under page indexing, it shows the user-declared canonical and the Google-selected canonical. If they differ, look for conflicting signals such as internal links, sitemap entries or redirects pointing to another version.
Can I use a canonical tag across different domains?
Yes. Cross-domain canonicals are supported and are used for syndicated content, where a republished article points to the original. As with any canonical, it is a hint, and the content must be the same or very similar for search engines to honour it.
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