Page Rating (PR) shows the strength of URL's overall internal backlink profile. It is measured on a logarithmic scale from 1 to 100, with the latter being the strongest.
To calculate PR, we consider:
The higher the PR of a page, the more “link juice” is transferred to linked URLs.
The target URL splits its rating equally amongst the URLs it links to. So: a PR-10 URL which links to three other URLs can influence the PR more than a PR-80 URL which links to many other URLs.
Through repeated calculation, we get a rating for each URL. The final PR value is then obtained by scaling a computed, absolute value into the 0-100 range.
Tip: Think of PR as a relative term in the sense that it's dependent on not only the number of high-PR URLs that link to you, but also on how many other URLs these high-PR URLs link to.
Do note that it is necessary for the crawl to be completed in order to get accurate PR as PR is only computed after post-processing.