Monday, January 5, 2009

Google Pagerank Formula

David Callan had wrote a very good Google Ranking Tips and what interest me most is he did also write about the Google Pagerank Formula. A little math. The post really explain everything I want to know about Pagerank.

The Formula:

The damping factor:
The damping factor is the amount of your PR which you can actually pass on when you vote / link to another site. The damping factor is widely known to be .85, this is a little less than the linking pages own PR.

PR(A) = (1-d) + d(PR(t1)/C(t1) + ... + PR(tn)/C(tn))

In layman's terms PR(A) is the Pagerank boost your page A will get after being linked from someone else's site (t1). PR(t1) is the pagerank of the page which links to you and C(t1) is the amount of total links that (t1) has. It is important to know that a pages voting power is only .85 of that pages actual PR and this voting power gets spread out evenly between all sites it links to.

Imagine was linked by's link page which had a PR of 4 and 9 other links, here's how the formula should look like:

PR(ABC) = (1-.85) + .85*(4/10)
PR(ABC) = .15 + .85*(.4)
PR(ABC) = .15 + .34
PR(ABC) = .49

To sum up my site would get an injection of .49 PR after being linked from a page with a PR of four and 9 other links.

Let's say I was linked from a site with a PR of 8, double the previous example's amount, which had 15 other links, a total of 16 outbound links, my boost would be:

PR(ABC) = (1-.85) +.85*(8/16)
PR(ABC) = .15 + .85(.5)
PR(ABC) = .15 + .425
PR(ABC) = .575

The above two worked examples show that not only is the PR of the linking page important but what is also important is how many other sites are also linked to from that page.

I think we've had enough maths stuff for now, just remember that the name of the game is to get as many links from pages with high PR and few other links. The more of these links you get the more your PR will grow and the more your rankings will improve for your relevant keywords.


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