The Last PR update took place on the 28th of February 2008. The PR of any website is calculated with a formula :

PR(A)=(1-d)+(PR OF LINK PAGE 1*D/NO OF LINKS IN PAGE1)+ (PR OF LINK PAGE 2*D/NO OF LINKS IN PAGE2)+.....+(PR OF LINK PAGE P*D/NO OF LINKS IN PAGE P).

Where A,is your website
D is a damping factor =0.85
link page 1,2...,P are the pages that has a link pointing to your website and can be found out by conducting a search on Google with link:www.website.com.

Don't calculate it manually.Try the SEO calculator or check out other genuine online tools that help you to check your PR.