I have a recipe I'm going to try soon that expresses the hop additions in AAUs (Alpha Acid Units) instead of ounces. The guy at the homebrew store told me that the Alpha Acid percentage of a particular hop species can vary from one crop to the next, so you should calculate the amount of hops to use for any recipe based on the AA percentage.
Example:
I bought a two ounce package of Northern Brewer pelletized hops that's rated at 9.3% alpha acid, and a two ounce package of pelletized Cascade hops that are rated at 6.4%. The recipe I'm going to use on my next brew calls for 6 AAUs of Northern Brewer at two different times during the boil, plus 4 AAUs of Cascade later on.
The HBS guy said Alpha Acid times ounces = AAU's (or HBU's)
so solving for ounces, it's AAUs divided by AA = ounces.
6 AAU's of Northern Brewer at 9.3% AA works out as 6/9.3 = .645 ounces for each of the two additions of this hop.
4 AAU's of Cascade at 6.4% AA works out as 4/6.4 = .625 ounces
How can a recipe simply specify the hops in ounces, given that AA% can vary from crop to crop, and yield consistent results?