Alpha Acid Units is a quick way for brewers to estimate how much bitterness hop additions will add to their beer. It is calculated by multiplying the amount of hops added (in ounces) by the alpha acid content of the hops. The drawback to this is that it takes neither boiling time nor wort gravity into account, both of which affect the utilization (conversion of alpha-acids to iso-alpha acids) of the hops. For example, if you added 1 oz of 4% alpha acid Fuggles, you would have 4 AAUs (4 x 1) if you added 1.5 oz of 13.25% Simcoe you would get 19.875 AAUs. The AAU format is helpful for sharing recipes but does not give an accurate prediction of final bitterness.
Glenn Tinseth developed a method to calculate bitterness in beer based on his research.
Glenn found that many factors affected the conversion of alpha-acids to iso-alpha-acids (boil time, wort gravity, volume of wort in boil)
Glenn developed the following formula to calculate bitterness in IBUs:
IBU = (U * ozs hops * 7490)/Volume (in gallons) U represents the utilization of the hops (conversion to iso-alpha-acids) based on boil time and wort gravity. U = bigness factor * boil time factor