The Hop Substitution AA% Adjuster works out how much of a replacement hop you need so your beer keeps the same bitterness when the variety in your recipe is out of stock or has a different alpha acid percentage than the original called for. It is the quickest way to stay on target for IBU when you swap hops.
How it works
Bitterness in beer comes from alpha acids, and the bittering contribution of a hop addition is proportional to the weight multiplied by the alpha acid percentage. That product is sometimes called alpha acid units. To keep bitterness constant when substituting, you hold the alpha acid units constant:
New weight = original weight * (original AA% / substitute AA%)If the substitute is stronger (higher AA%), you need less of it. If it is milder (lower AA%), you need more. The calculator also shows the weight ratio and the difference from the original so you can sanity-check the swap at a glance.
Worked example
Your recipe calls for 28 g of a hop at 5.5% alpha acid, but the variety you
have on hand is 7.2% alpha acid:
- New weight = 28 * (5.5 / 7.2)
- = 28 * 0.764
- = about
21.4 g
Using 21.4 g of the 7.2% hop delivers the same bitterness as 28 g of the 5.5% original. If instead your substitute were weaker, say 4.0% AA, you would need 28 * (5.5 / 4.0) = about 38.5 g.
Tips and notes
This calculation matches bitterness only. Flavour and aroma are properties of the specific variety, so always pick a substitute with a similar character first, then scale the weight here. Because alpha acid percentage varies by harvest year, use the figure printed on the actual package you have rather than a generic average, or your bitterness will drift. For late and whirlpool additions where bitterness is minor, you can keep the weight roughly the same and adjust to taste, since aroma rather than alpha acids is doing the work.