Google PageRank calculation
Google's most mysterious ranking influencer, the Google
PageRank (PR), has been the topic of many discussions and observations.
The way the Google PageRank calculation works is still not 100%
clear today, but I thought it might be useful to write the concept
of the original mechanism down in plain English and show it using
a simple example.
The Google PageRank (PR) is calculated for every webpage
that exists in Google's database. It's real value varies from 0,15
to infinite, but for representation purposes it is converted to
a value between 0 and 10 (from low to high). The calculation of
the PR for a page is based on the quantity and quality of webpages
that contain links to that page.
According to Sergey Brin and Lawrence (Larry) Page, Co-founders
of Google, the PR of a webpage is calculated using this formula:
PR(A) = (1 - d) + d * SUM ((PR(I->A)/C(I))
- PR(A) is the PageRank of your page A.
- d is the damping factor, usually set to 0,85.
- PR(I->A) is the PageRank of page I containing a link to
- C(I) is the number of links off page I.
- PR(I->A)/C(I) is a PR-value page A receives from page I.
- SUM (PR(I->A)/C(I)) is the sum of all PR-values page A receives
from pages with links to page A..
In other words: The PR of a page is determined
by the PR of every page I that has a link to page A. For every page
I that points to page A, the PR of page I is devided by the number
of links from page I. These values are cumulated and multiplied
by 0,85. Finally 0,15 is added to this result, and this number represents
the PR of page A.
To illustrate how this calculation works we defined the
following simplified example.
In this example we use an imaginary Google database containing
only four (4) pages that all have an initial PR value of 1 (the
above formula needs an assumed start PR value for every page). These
four pages have the linkstructure as shown in the first picture.
In practice Google calculates the PageRank for billions of pages.
Our model is simplified to show the evaluation of a PageRank.
To determine the PR for pages A to D the formula has
to be executed several times. For every calculation we use the PR
results of the former calculation. In this example the final PageRanks are found after 20 iterations. The next picture shows the
calculated PR for every page.
Have a look at this spreadsheet
to find the calculation model.
Dmoz.org and Yahoo
Finally I would like to mention two special sites that
influence the Google PageRank of your webpage. Inclusions in the
Open Directory Project and Yahoo! directories (www.dmoz.org
and www.yahoo.com) seem to offer your
site an extra boost in the PageRank calculation. A clear explanation
for their influence is unknown to me, but it is obvious that a top-10
listing shows many sites that are included in the Google Directory.
And this Google directory is a copy of the Dmoz.org database. Therefore
I recommend you to get listed in both directories.
That's about it. Many things are know and many things
are unknown about the internals of Google. I guess we have to accept
that. However, I trust you can use this information to get a step
ahead in your plans to receive a higher Google ranking.
In case you have any comments or additions, please feel
free to send them in.
PS: Use the Google
toolbar to determine the PR of webpages.