There are two different methods used to calculate hop
        bitterness. The default IBU
        calculation uses the Rager method, from Norm Pyle's "Hops
        FAQ" <http://realbeer.com/hops/FAQ.html>. The other
        method is the Tinseth method, from Glen Tinseth's "Hop
        Page" <http://realbeer.com/hops/>. To change to the
        Tinseth method, open up the Configure... dialog and check the
        Use Tinseth bitterness
        calculation box. The utilization table for
        the Rager method can be edited by changing the appropriate
        area of the qbrewdata file.
      
Please see the above links for details on these formulae, as they can be a bit complex.