Everybody is using it, but (almost) nobody really knows how it works. Google PageRank is probably one of the most important algorithms ever developed for the Web. With billions of existing pages and millions of pages generated every day, the search issue in the Web is more complex than you probably think it is. PageRank, only one of hundreds of factors used by Google to determine best search results, helps to keep our search clean and efficient. But how is it actually done? How does Google PageRank work, which factors do have an impact on it and which don’t? And what do we really know about PageRank?

In this article we put the facts straight.

Over the last weeks we’ve done an extensive research and selected dozens of facts and suggestions about PageRank, which seem to be true in practice. Besides, we’ve collected academic papers related to the issue - such as scientific proposals for better search results (such as Topic-Sensitive PageRank); you’ll also find references to mathematical background of PageRank as well as 16 useful PageRank tools you can use to analyze und track the ranking of your web-projects.

Update: we’d like to apologize for some misleading facts we’ve initially included in this article. We’ve re-checked the sources and inaccurate or incomplete data. The .pdf-file won’t contain any mistakes. Thanks to all the readers who’ve pointed us to the mistakes (particularly Dan Grossman and Reuben Yau).

• Update: We are going to publish the .pdf-version of this post soon, so subscribe to our RSS-feed to keep track on our next posts.
• You don’t have to read the whole article. Most important facts are selected in the beginning of the post as a brief summary.
• You might be interested in reading our article Google AdSense: Facts, FAQs and Tools, which should provide you with the most important facts, tools and resources about Google AdSense.
• Update (28.07.2007): Spanish version of this article is available (thanks, Juan Manuel Lemus).

Summary: How Does PageRank Work?

1. PageRank is only one of numerous methods Google uses to determine a page’s relevance or importance.
2. Google interprets a link from page A to page B as a vote, by page A, for page B. Google looks not only at the sheer volume of votes; among 100 other aspects it also analyzes the page that casts the vote. However, these aspects don’t count, when PageRank is calculated.
3. PageRank is based on incoming links, but not just on the number of them - relevance and quality are important (in terms of the PageRank of sites, which link to a given site).
4. PR(A) = (1-d) + d(PR(t1)/C(t1) + … + PR(tn)/C(tn)). That’s the equation that calculates a page’s PageRank.
5. Not all links weight the same when it comes to PR.
6. If you had a web page with a PR8 and had 1 link on it, the site linked to would get a fair amount of PR value. But, if you had 100 links on that page, each individual link would only get a fraction of the value.
7. Bad incoming links don’t have impact on Page Rank.
8. Ranking popularity considers site age, backlink relevancy and backlink duration. PageRank doesn’t.
9. Content is not taken into account when PageRank is calculated.
10. PageRank does not rank web sites as a whole, but is determined for each page individually.
11. Each inbound link is important to the overall total. Except banned sites, which don’t count.
12. PageRank values don’t range from 0 to 10. PageRank is a floating-point number.
13. Each Page Rank level is progressively harder to reach. PageRank is believed to be calculated on a logarithmic scale.
14. Google calculates pages PRs permanently, but we see the update once every few months (Google Toolbar).

Summary: Impact on Google PageRank

1. Frequent content updates don’t improve Page Rank automatically. Content is not part of the PR calculation.
2. High Page Rank doesn’t mean high search ranking.
3. DMOZ and Yahoo! Listings don’t improve Page Rank automatically.
4. .edu and .gov-sites don’t improve Page Rank automatically.
5. Sub-directories don’t necessarily have a lower Page Rank than root-directories.
6. Wikipedia links don’t improve PageRank automatically (update: but pages which extract information from Wikipedia might improve PageRank).
7. Links marked with nofollow-attribute don’t contribute to Google PageRank.
8. Efficient internal onsite linking has an impact on PageRank.
9. Related high ranked web-sites count stronger. But: “a page with high PageRank may actually pass you less if it has more links, because it’s spread too thin.”
10. Links from and to high quality related sites have an impact on Page Rank.
11. Multiple votes to one link from the same page cost as much as a single vote.