An Alpha Acid Unit (AAU), also called Homebrew Bitterness Units (HBU) is a measure of hops used - typically for a 5 gallon batch. It can be calculated simply by multiplying the percent alpha acid of the hops used by the number of ounces of hops. For example 3 ounces of 5% alpha hops would be equivalent to 15 AAUs.
- AAUs = (hops_alpha_percent * hops_weight_oz)
Because AAU's do not take into account important factors such as the batch size, boil time and specific gravity of the wort, AAU's have largely been replaced by International Bitterness Units which provide a much more accurate picture of the bitterness of a particular brew.