Forum Moderators: Robert Charlton & goodroi
One point that may be of immediate help to you -- a "site" does not have PageRank - only a page (actually, a URL) has PageRank. When people speak informally about a site's PageRank, they are talking about the PR of the home page. We can understand PageRank as the probability that a surfer starting on any random page of the web and clicking on links randomly, will end up on the page being measured. But since probabilities range only between 0 and 1, just multiply that probability number by 10 and you have a decent understanding of the math.
The actual math for calculating PR is patented - and available from the USPTO [patft.uspto.gov].
We've got a reference thread on the topic: Google PR - PageRank FAQs [webmasterworld.com]
That thread is available from our Hot Topics area [webmasterworld.com] which is always pinned to the top of this forum's index page.
