Google's PageRank Algorithm

PageRank is Google's method of giving a numerical value to the number and quality of the links pointing to a web page. It was the original foundation for Google's approach to ranking, as expressed in the doctoral thesis by the company's founders Larry Page and Sergey Brin when they attended Stanford University. Their theory was that the quality of any webpage could, in part, be evaluated in terms of the number and quality of the links from other webpages that point to it. The theory equated each link as a "vote" for the page being linked to. But instead of a straight numerical count of these links, as other search engines had tried, PageRank also considered the quality of each link.

The original PageRank formula: For any Page A which is linked to by pages T1 through Tn, the PageRank of Page A is determined by the following equation:

PR(A) = (1-d) + d (PR(T1)/C(T1) + ... + PR(Tn)/C(Tn))

where PR(A) is the PageRank of page "A", PR(Ti) is the PageRank of page "Ti", C(Ti) is the total number of links on page "Ti7", and d is a damping factor.

As you can see in the formula above, the quality of a link is based on the PageRank score of the page on which the link originates divided by the number of links on that same page. So, a link from a page with a modest PageRank score but which contains relatively few links can pass on more PageRank value than a link from a page with a relatively higher PageRank score that contains many links. Each web page's PageRank score is then the approximate sum of the PageRank value of the links pointing to it. The exact formula Google uses for determining PageRank has changed significantly in recent years, but for webmasters this is still the best way to think of PageRank in terms of their own websites. Since the actual formula results in a number with a much higher level of precision than the simple 0 to 10 scale shown in the Google Toolbar, Google must periodically adjust the scale by which their internal PageRank value is converted for display in the Toolbar. This is why you'll sometimes see the Toolbar PageRank score of your pages fall after an update, even though the links pointing to that page appear to have remained unchanged and with no changes in rankings.

The value of PageRank in affecting the ranking of a particular page is still quite high, but not as high as it used to be, which has caused many people to recklessly speculate that PageRank is irrelevant or worthless. Within the past couple of years, Google has been giving increasing value to the text used within the link's anchor text - that is, the text inserted between the and tags in the HTML code. But since you still need a link to get the value of the anchor text boost, which would also contribute to a page's PageRank score, the two are very connected in the ranking potential of every web page. So, while PageRank and anchor text are distinct ranking factors used by Google, the simple fact remains that you can't have one without the other.

What confuses many new webmasters is the PageRank display in the Google Toolbar for Microsoft Internet Explorer and Firefox which has two limitations that you need to be aware of. First, as noted above, the Toolbar is only capable of displaying whole numbers from 0 to 10. Google's internal PageRank system has a much finer degree of gradation. It is widely thought that the Toolbar PageRank scale is exponential. That is, each step is not equal. So, the difference in the value of each step is progressively higher. The second limitation is that the PageRank scores displayed in the Google Toolbar is taken from a database that is only updated every 3-4 months. Google's internal PageRank database is updated continuously as they crawl the web and discover new links. This is one reason why you see Google's rankings change so often and why you can't simply look at the Toolbar PageRank scores of different pages and determine why one page outranks another without considering many other important ranking factors. Note, too, that all of the online tools for displaying PageRank get their information from this same Toolbar database. The only difference is that some of them allow you to check the information on more than one of Google's datacenters, so you can sometimes see if the information is in the process of being updated. But only Google has access to their internal PageRank data, which is the information that really matters in this regard.

One last thing you need to know is that Google is constantly changing their methods of counting or ignoring links and how PageRank is calculated. While the basic premise of the original formula still forms the foundation of PageRank, the evolution of the web and the need to apply an ever-increasing level of sophistication in determining the quality of individual links and pages means that the effectiveness of simpler methods of building links as a shortcut to better rankings will continue to diminish. Link exchanges, text link ads, and directory submissions are currently in Google's crosshairs. So your link building efforts should focus on getting more natural links from quality websites whose main topic is related to yours.