URL Canonicalization: What is it? And How Does it Affect SEO?

URL canonicalization is one of the most fundamental search engine optimization (SEO) techniques, and one every webmaster needs to understand if they want to be successful at promoting their site.  If a site’s canonicalization issues are not addressed then the site can never reach its maximum potential from an SEO perspective.

What is a Canonical URL?

There should be one and ONLY one URL that can be used to render any given page on your site called the canonical (or preferred) form of that page’s URL.  The reasons why you should only allow browsers and search engine crawlers to reference a web page with a single URL is obvious once you understand a little about search engines and how they work.

Why are URL Canonicalization Issues Bad for SEO?

Before going any further, you need to understand that Google and other search engines rank URLs. They do NOT rank web pages. There is a difference.

If you allow a web page to be rendered with multiple URLs  then over time, the lack of URL canonicalization will lead to two issues that are problematic for search engine optimization:

• Duplicate content
• Split PageRank or “link juice”

Whoa! Duplicate Content? Split Link Juice? Greek?

The best way to explain how URL canonicalization issues cause duplicate content and split PageRank/link juice is probably to just show you an example.

So here goes…

Assume your site’s home web page is index.html. Then the following URLs might all render your home page in a browser:

• http://example.com/
• http://example.com/index.html
• http://www.example.com/
• http://www.example.com/index.html

This may seem normal or harmless to the untrained eye, but it is a problem.

Now imagine 40 different web sites link to your home page.  For example, maybe:

• Sites 1-10 link to http://www.example.com/,
• Sites 11-20 link to http://www.example.com/index.html,
• Sites 21-30 link to http://www.example.com/, and
• Sites 31-40 link to http://www.example.com/index.html.

Since search engines follow links to discover new URLs they might want to index, the search engines will likely have the exact same content from your home page associated with all four of the above different URLs in their index. 

One URL will get flagged as the original copy of your home page’s content and be favored slightly at Google.  And the other three URLs will be flagged as containing duplicate versions of your home page’s content.  But you have no way of knowing which one is considered the original.  This is not desirable from an SEO perspective.

Also,in the above example the search engines will NOT see your home page as having 40 inbound links.  Instead, they will see the above URLs for your home page as four different web pages, each with 10 inbound links. 

This is what I mean when I say “split PageRank” or “split link juice”.  Because links carry so much weight in the ranking algorithms at all search engines, this is very bad from an SEO perspective.

How to Fix URL Canonicalization Issues

There are a few ways to correct URL canonicalization issues:

• 301 redirect all non-canonical URLs to their canonical form
• Use the <link rel=”canonical”> element to specify a canonical URL
• Use webmaster tools

The absolute best way to correct your site’s URL canonicalization problems so that they are fixed across all search engines is to use a 301 redirect.  This method has been the recommended method since URL canonicalization became an SEO term.

Using the example above, you would first select one of the URLs as the canonical URL.  I typically pick http://www.example.com/ as my canonical URL.  It does not matter which you pick.  It is just important that you have one prefered URL.

Once you have determined the canonical URL, you then 301 redirect all requests for the other non-canonical URLs to the canonical form.

For example, if http://www.example.com/ were chosen as the canonical URL then you would need to  implement the following 301 redirects:

The 301 redirects will tell the search engines (and browsers) that the other three “pages” have been permanently moved to http://www.example.com/.  This causes the search engines to pass credit for all inbound links to those three pages over to the canonical URL.  Google will remove the other three non-canonical versions of your URLs from their index eliminating the duplicate content.

Now the search engines will see your home page as one URL (the canonical URL), and that canonical URL will get credit for all 40 inbound links. 

Again, using 301 redirects to correct URL canonicalization issues is always the preferred method.

The alternate methods of using the <link rel=”canonical”> element is not recognized by all search engines.  Google, Microsoft, and Yahoo! all “said” they would support it when the new rel=”canonical” attribute was introduced a couple of years ago, but it appears that Google is the only one who has actually implemented code to support it .

The third method I mentioned was using Google Webmaster Tools to specify a preferred domain (www vs non-www).  But this only fixes some of your canonicalization issues (and only for Google).  You will still have issues with default documents like index.html.

You can learn more about the URL canonicalization and 301 redirecting web pages at CanonicalSEO.com.