Canonical Tag: Definition, Examples & Best Practices
Updated by Chima Mmeje — April 4, 2025.
What is a canonical tag?
A canonical tag, also known as “rel canonical,” is a method for informing search engines that a certain URL, referred to as the canonical URL, is the master copy of a page. This tag helps avoid issues related to identical or “duplicate” content found on multiple URLs.
Essentially, the canonical tag directs search engines to display your chosen version of a URL in search results, clearly indicating which version of a page should be indexed as the preferred one.
How does it work?
The canonical tag functions by being placed in the HTML head of a webpage. When you add this tag to a page, you're essentially guiding search engines by specifying the "authoritative" version of that page.
Here’s how it works in practice:
Placement: You insert a <link> element with the attribute rel="canonical" into the <head> section of the duplicate pages’ HTML.
Identification: This link element points to the URL you've designated as the canonical or preferred version.
Crawling and indexing: When search engines crawl your website, they recognize the canonical tag and understand that the content at the URL specified in the tag should be considered the primary version.
By using the canonical tag, search engines consolidate signals such as links to all the duplicates, into the canonical URL. This process ensures that the preferred URL maintains its search ranking potential and visibility, avoiding dilution through multiple similar pages.
Why does canonicalization matter for duplicate pages?
Duplicate content presents complex challenges. When search engines encounter numerous URLs hosting identical or very similar content, several SEO problems can arise concerning duplicate pages.
Here’s why canonicalization is crucial:
Avoid missed content: If search crawlers are overwhelmed by duplicate content, they might overlook some of your unique content, which could have otherwise added value to your search presence.
Prevent dilution of ranking power: Extensive duplication can dilute your website’s ranking capability across cannibalized pages rather than consolidating it into one authoritative page.
Ensure correct URL recognition: Without canonicalization, even if your content ranks well, search engines might display the wrong URL as the “original” in search results.
By employing canonicalization, you can direct search engines to recognize and index the preferred version of a page. Also, effective canonicalization optimizes crawl budgets and improves a page's ranking in Google Search by consolidating inbound links from duplicate content.
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The challenge of URL variation in SEO
URL variations pose significant SEO challenges. Here’s how these differences impact search visibility:
Search engines can access your pages in different ways
You might wonder, “Why would anyone duplicate a page?” and might mistakenly think that canonicalization isn’t something you need to worry about.
However, while we often perceive a page, like your homepage, as a single concept, search engines see each unique URL as a separate page.
For example, search engines might access your homepage through various URLs:
- http://www.example.com
- https://www.example.com
- http://example.com
- http://example.com/index.php
- http://example.com/index.php?r…
To us, these URLs represent one page. To a search engine, each URL is considered a unique “page.” Even in this simple example, there are five different versions of the homepage, showing how easily duplicates can occur.
Using canonical URLs helps manage these variations by consolidating link equity and improving search rankings.
Websites and CMS generating multiple URLs for the same content
Modern content management systems (CMS) and dynamic, code-driven websites further complicate matters.
Many sites automatically generate multiple URLs for the same content. They add tags, allowing multiple pathways to the same content, and appending URL parameters for searches. You could have thousands of duplicate URLs on your site without even realizing it.
Search engines won't recognize the variations
Search engines may not recognize these variations as the same page, leading to issues with duplicate content. This misunderstanding can dilute your SEO efforts across multiple pages rather than consolidating them into one authoritative page.
How to implement canonical tags for SEO
Implementing canonical tags correctly is essential for directing search engines to treat certain pages as duplicates and to index only the preferred version.
Basic steps to implement canonical tags include:
Identify duplicate content
Start by identifying pages that have similar or identical content where a canonical tag would be beneficial. You can find this using the Moz Site Crawl feature.
Log in to your Moz Pro account and click On-Demand Crawl on the left side of the dashboard. Enter your website URL and the tool will scan your entire site and generate a report that includes issues it finds.
When the crawl is complete, click Crawl Report and scroll down to Pages Crawled. Under Issue Type select duplicate content to view all duplicate pages on your website.
Choose the preferred URL
Decide which version of the content you want search engines to index. This URL will serve as the canonical URL.
Evaluate which version of the page has accumulated the most SEO value. This includes factors like:
- Inbound links
- Keyword ranking
- Search volume
- Page authority
- Historical traffic
Preferably, the URL with the highest performance metrics should be chosen as the canonical version to maximize the benefit of consolidated ranking signals.
Add the canonical tag
Insert the canonical tag in the <head> section of the HTML of the duplicate pages. The tag should look like this:
<link rel="canonical" href="http://www.example.com/preferred-url"/>.
Canonicalization on specific platforms
Implementing canonicalization can vary depending on the platform you use. Here’s how it works on some popular platforms:
Wix automatically adds a self-referential canonical tag to every page on a website, ensuring that each page is recognized as its own canonical version. Additionally, Wix allows users to customize the canonical tag for each page, providing flexibility for more complex SEO needs.
WordPress offers several plugins that can assist with canonicalization, such as Yoast SEO and All in One SEO Pack. These plugins make it easy to add and manage canonical tags, helping you avoid duplicate content issues and improve your site’s search engine rankings.
Shopify allows users to specify a canonical URL for each product page, ensuring that search engines recognize the preferred version of each page. Shopify also has a built-in canonicalization feature that can be enabled in the settings, simplifying the process for store owners.
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Canonical tag best practices
Effective use of canonical tags is crucial for optimizing your website’s SEO. Here are the best practices to follow when implementing canonical tags:
Use self-referential canonical tags
It's beneficial to have canonical tags point to your own URL. For instance, if URL X is deemed the canonical version among duplicates X, Y, and Z, it’s effective to include a self-referential canonical tag on URL X. This practice is essential to reinforce to search engines which URL is canonical, even on the canonical page itself.
Proactively canonicalize your homepage
Since homepages often face duplication through various URL forms, proactively placing a canonical tag on your homepage template is a preventive measure against potential issues. This ensures that variations in how people link to your homepage don’t create duplicate content problems.
Regularly check dynamic canonical tags
On e-commerce and CMS-driven sites, ensure that your canonical tags aren’t dynamically generating incorrect versions. Periodic checks can prevent errors that might cause each URL variant to mistakenly have its own canonical tag, thus defeating the purpose of canonicalization.
Avoid mixed signals
Ensure consistency in your canonicalization signals to search engines:
- Avoid circular or conflicting canonicalization, such as canonicalizing A to B and then B back to A.
- Avoid canonical tag chains, where A points to B, B points to C, and so forth. These can confuse search engines and dilute the clarity of canonical signals.
Be cautious with near-duplicates
While canonical tags are typically used for exact duplicates, they can also be cautiously applied to very similar pages.
If the variations are minor, such as differences in currency, location, or slight product variations, canonical tags can be appropriate.
However, if pages differ significantly, search engines might ignore the canonical tags due to the perceived relevance of the unique content on each page.
Cross-domain canonicalization
If you manage multiple sites with overlapping content, such as publishing the same article across several domains, using cross-domain canonical tags can concentrate your SEO efforts on a single site.
This approach should align with your overall business and SEO strategy since it prevents the non-canonical sites from ranking.
Test canonical tags
It’s crucial to test your canonical tags to ensure they are working correctly and that search engines are recognizing the canonical URL of each page. Use tools like the MozBar or site audit software to verify your implementation.
Advanced canonicalization techniques
While canonical tags are a common method for specifying the preferred version of a web page, there are several advanced canonicalization methods that can also be effective:
rel=”canonical” HTTP headers
Use rel=”canonical” header in the HTTP response of a web page, which specifies the canonical URL of the page, especially for non-HTML content like PDFs.
Sitemaps
Use sitemaps to declare the canonical URL for each page, aiding search engines in understanding site structure and preferred URLs.
Redirects
Use 301 redirects to permanently move old URLs to the preferred ones, clearly signaling to search engines which URL should be seen as canonical.
Hreflang annotations
For multilingual or multi-regional sites, hreflang annotations help specify the appropriate version of content for different languages or regions, effectively managing canonicalization for diverse audiences.
How to audit your canonical tags for SEO
When auditing your canonical tags, there are a number of things worth checking for optimal SEO performance.
Here's a checklist:
- Does the page have a canonical tag?
- Does the canonical point to the right page?
- Are the pages crawlable and indexable?
A common mistake is to point the canonical at a URL that is either blocked by robots.txt, or is set to "noindex". This can send mixed and confusing signals to search engines. A few common ways to inspect and audit your canonical tags are below.
View-source
In most browsers, you can right-click to view-source, or simply type it into the address bar, like this: view-source:https://moz.com/learn/seo/cano... the source code, search for canonical tag in the <head>. If present, it should look like this:
Use the MozBar
The MozBar is a free SEO toolbar that will easily show you the canonical tag on any given page. After installation, simply hit the Page Analysis tab, then click on "General Attributes" to view any canonical information.
Audit in bulk with software solutions
Most SEO site audit software allows you to audit canonical tags in bulk. Moz Site Crawl checks for missing canonical tags, and can do so for 100s of thousand of pages at a time.
Follow the same process we discussed for finding duplicate pages. You’ll find pages with missing canonical tags in your Crawl Report, under Issue Type.
Canonical tags vs. 301 redirects
A common SEO question is whether canonical tags pass link equity (PageRank, Authority, etc.) like 301 redirects.
While canonical tags generally help consolidate ranking signals, they work differently from redirects and should be used based on the intended outcome.
Understanding the difference
301 redirects means the page has permanently moved to a new URL. When Page A is redirected to Page B, search engines and human visitors are automatically taken to Page B. Page A no longer exists as an independent page.
Canonical tags indicate that multiple URLs contain the same or similar content but allow both pages to remain accessible. If Page A has a canonical tag pointing to Page B, search engines recognize Page B as the preferred version, but visitors can still access both pages.
When to use each
Use a 301 redirect when you want to completely remove Page A and consolidate its value into Page B. This is best for outdated or merged content where you want a seamless user experience.
Use a canonical tag when duplicate or similar content needs to exist for users, but you want search engines to recognize a preferred version. This applies to product variations, tracking parameters, and syndicated content.
Choosing the right solution ensures both search engines and users experience your site the way you intend while preserving SEO value.
Summing up
Canonical tags help search engines identify the preferred version of a page, preventing duplicate content issues. Implementing them correctly ensures better indexing, stronger rankings, and a more efficient crawl budget.
To get the best results:
- Implement canonical tags correctly to signal the preferred URL
- Regularly audit your site to catch and fix duplicate content issues
- Choose between canonical tags and 301 redirects based on the intended outcome
Identify pages with missing canonical tags
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FAQs about canonical tags
What is canonical tag code in HTML?
A canonical tag is an HTML element placed in the <head> section of a webpage to indicate the preferred version of a URL. It looks like this:
<link rel="canonical" href="https://www.example.com/preferred-page/">
What is a canonical URL example?
A canonical URL is the preferred version of a webpage that search engines should index. For example, if both https://example.com/shoes and https://example.com/shoes?color=red exist, the canonical URL might be:
<link rel="canonical" href="https://example.com/shoes">
Do canonical tags affect SEO?
Yes, canonical tags help consolidate ranking signals and prevent duplicate content from diluting SEO value. They guide search engines to index the correct page, improving search visibility.
How to fix canonical tags?
To fix canonical issues, identify duplicate pages using tools like Moz Pro’s Site Crawl, ensure each page has a correct canonical tag, avoid conflicting signals, and regularly audit your tags to maintain accuracy.
Identify pages with missing canonical tags
in Moz Pro Site Crawl
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