The exact details are lengthy, and far beyond what I am capable of dissecting. But the basic formula is as follows:
PR(A) = (1-d) + d (PR(T1)/C(T1) + ….. + PR (Tn)/C(Tn))
PR(A) is the PageRank of a particular page (A) - not a website as a whole.
1-d is the dampening factor, as explained below.
PR(T1) is the PageRank of the page that links to our (A) page, and C(T1) is the number of links contained on that same page.
The formula is repeated throughout every single page that contains a link to this (A) page.
Two important points to take into account. First of all, if you're thinking that the formula would in practice be an infinite loop, then you're correct. This is the very nature of the web itself, and is also why Google has introduced the so called dampening factor.
The second point concerns the way that PageRank is awarded by one page to another. The generally accepted means of understanding this is to consider that a given page has, according to its own PageRank, a certain amount of voting power. If the page in question links to five other pages, then each of the pages being linked to receive their PageRank "award" of one fifth of the original page's voting power. It's also worth noting that the number of links on a page includes a website's internal links.
As the formula shows, PageRank works as a multiplier of a site's overall value, so Google has made sure that link farms have their own value of zero - which means that a link from them counts for nothing, quite literally.
Having links to your web pages on sites with a low page rank and a large number of links means that the benefits are quite effectively minimized to zero. But this will not detract from your current PageRank at all.
Of far greater importance is the PageRank of each of these pages, and how many links appear on them.
Finding out a page's PageRank couldn't be simpler. Follow the link to Services and Tools from the Google home page, and find the Google Toolbar. After installing the software, a bar appears at the top of the browser showing a value for each page you're visiting. Hold the mouse over the bar, and you'll be told the page's PageRank - a score out of ten. As already mentioned, this figure is little more than a representation of a page's actual PageRank.
Not surprisingly, very few pages score ten out of ten, and those that do include the likes of Microsoft, Yahoo, Google itself, AltaVista, Adobe, AOL, Mozilla.org and others. In other words we're looking at the biggest of the biggest websites - and not something that most of us could ever hope to achieve!
Of course, there is a simple reason that search engines and directories have such a high PageRank. Not only do they link to a huge, ever-growing list of sites and pages, but more importantly, a truly staggering number of these sites and pages link back to them. When you consider the importance of reciprocal linking, you start to understand why they do so well. With Adobe, you only need to consider the sheer number of web pages out there that link to a PDF file (with links to Adobe for their frëe reader software), and you will see why they have achieved such a high number.
A nine out of ten score still puts you within a very small minority of the web. Should you be able to achieve this high a PageRank, you'll be rubbing shoulders with the likes of MSN, BBC News, Winzip and Internet.com. We're talking about the web's upper classes - not really attainable for the majority of normal website owners.
Eight out of ten starts bringing you to the "reachable" web. You'll find sites such as CNN, TuCows, Simtel, the Association of Shareware Professionals, the Shareware Industry Conference site and Lockergnome. A PageRank of seven is starting to appear reasonably attainable, as long as we're willing to work hard on the content and reputation of your site. The sevens include companies such as D-Link, MSNBC, CNET's Download.com and our very own SharewarePromotions.com.
Download Pagerank from this link to Google