To calculate the Page Rank for a page, all of its inbound links are taken into account. These are links from within the site and links from outside the site.

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

       That's the equation that calculates a page's Page Rank. It's the original one that was published when Page Rank was being developed, and it is probable that Google uses a variation of it but they aren't telling us what it is. It doesn't matter though, as this equation is good enough.

        In the equation 't1 - tn' are pages linking to page X, 'O' is the number of outbound links that a page has and 'd' is a damping factor, usually set to 0.85.

         We can think of it in a simpler way:-

a page's Page Rank = 0.15 + 0.85 * (a "share" of the Page Rank of every page that links to it)

"share" = the linking page's Page Rank divided by the number of outbound links on the page.

        A page "votes" an amount of Page Rank onto each page that it links to. The amount of Page Rank that it has to vote with is a little less than its own Page Rank value (its own value * 0.85). This value is shared equally between all the pages that it links to.