Estimating Beer Bitterness

AAU Units

The simplest way to describe the bittering potential of a given hop sample is to use Alpha Acid Units (AAU) or Homebrew Bittering Units (HBU) (they're the same). For the sake of discussion we will use the term AAU, which is calculated as follows:

AAU = AA * W
where:

• AA = alpha acid % provided with the hops
• W = weight of the hops in ounces

AAU's are literally hundredths of an ounce, so the units would be ounces. This is a bit awkward, and they are usually discussed as if they are "unit-less". It is generally assumed that, when using AAU or HBU, the batch size is the standard homebrewing unit of 5 gallons. If a beer is said to have 10 AAU's of bitterness in it, and it is a 5 gallon batch, there would probably be no confusion. On the other hand, if it is a 10 gallon batch, there is actually half the AAU's per gallon when compared to the 5 gallon batch and the beer would be quite different. Another drawback to using AAU's is that they don't consider the utilization obtained from long, intermediate, or short boil times. Fudge factors are sometimes added but at best they offer a rough approximation.

Read More about hops.