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.
AAUs are the easiest to calculate, but most brewers use IBUs. For normal strength beers you can approximate the IBUs in Tinseth simply by multiplying the AAUs by 3.5. Using the above example 4 AAUs x 3.5 = 14 IBUs.
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
First Wort Hopping
The preceived bitterness expressed in a ratio of IBUs to gravity. This is frequently seen expressed as BU/GU. That is Bitterness Units/Gravity Units. This is used as a comparator and for checking balance (or lack thereof) in a recipe. The Gravity Units are the decimal portion of the original gravity. A 1.060 OG wort would have a GU of 60. If this recipe called for 30 IBUs, then the ratio would be 30/60 or 0.5 It is with this method that a brewer can estimate whether bitterness or malt will dominate the flavor profile.