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Monday, September 20, 2010

How to Google PageRank Calculate:

Google's PageRank (PR) is one of the most sought after, and yet misunderstood, web page attributes. PageRank, named after one of the founders of the Google search engine, Larry Page, was the innovative foundation that the Google search engine was built on.

How is PageRank calculated:

In simplest terms PR is calculated by the sharing of PR from all the IBL links to your page. This is not strictly accurate because Google also uses the internal links within a site in the calculation of PR. Each link to a page carries with it and passes PR value to the target page. The PR points or value passed depend on the PR value of the page they come from, and the total outbound links from the page. It is generally agreed that a page will only pass about 85% of its value to the page it links to. So a PR5 page with a single outbound link will pass 85% of the value of a PR5 page to the page it links to.
But virtually no page has only a single link -- remember internal links are also used in the total outbound link count -- so the value passed to any page is 85% of the PR, divided by the total number of outbound links.
The question now becomes what is the PR point value of the different PR levels. Most observers believe that the relationship between PR levels is logarithmic rather than linear. In other words PR5 is not worth 25% more than a PR4, but may be worth 4 to 6 times more.
It is also understood that a PR value is not a single number, but is in fact a range of values. So not all PR6s are equal. As the chart below shows a PR6 maybe just on the upper boundary of a PR5 or it maybe just short of the entry point for a PR7.
The chart that follows shows the range for each PR value. It also shows how much PR value or PR points a page with 50 outbound links will pass depending on of its own PR rank. From this I have calculated the number of links required from each value of PageRank necessary for a page to attain a desired page rank.
The assumptions and the mathematics:
For those who are interested I have used logarithmic values of base 5.5. In other words the value range for a PR1 lies between 5.5 to the power of 1 and 5.5 to the power of 1.99, and PR2 lies between the value of 5.5 to the power of 2 and 5.5 to the power of 2.99 etc. The rest of the chart is fairly straight forward. It assumes that there are 50 links per page and that 85% of the PR value is passed to the recipient page.
The number of links required to attain any ranking is based on the median value of the donor page and the entry threshold of the desired PR value. In other words to achieve a PR5 you need 5,033 points and the average points available from a PR6 page with 50 links is 1507.
The chart was calculated with an Excel spreadsheet and it can be downloaded if anyone wants to play with the calculations and assumptions. It might be interesting to work with a different base number for the logarithmic calculation. And it is also interesting to see the impact of more or less outbound links from a page.
Nobody knows for sure how Google calculates PR. I have shown this chart to a number of knowledgeable people and they have all agreed that my calculations look reasonable. One SEO guru from a major firm said the results were very similar to independent research that his firm had conducted.
So take it or leave it. It is probably a fair reflection of how PR is passed and accumulated.
The lesson that can be drawn from this explanation and chart is that if you want to increase your PR you need a few links from pages with equivalent or higher PR, or a great many links from sites with lower PR.


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