Hop Substitution Calculator (Alpha Acid Adjustment)
Adjust the hop weight when substituting a variety or crop year with a different alpha acid percentage to keep the same bitterness — with hop storage (age) alpha-acid loss.
How it works
New weight = recipe weight × recipe %AA ÷ (your %AA × (1 − monthly loss)^months)
Bitterness scales with alpha acid, so when a recipe calls for hops at 12% and your packet says 9.5%, you need proportionally more to deliver the same alpha-acid units. Alpha acids also decay with time, oxygen and warmth — a hop with a high storage index can lose a third of its bittering power in a year at room temperature, while vacuum-packed frozen hops lose only a few percent — so the calculator discounts the packet figure by age and storage. The substitution keeps IBUs constant but not aroma: late and dry-hop additions are chosen for oils, not alpha acids, and should be substituted by flavour family rather than by this arithmetic.
Worked example
A recipe wants 30 g of 12% hops; yours are 9.5%, six months in the freezer (≈ 8.7% now): use 30 × 12 / 8.7 ≈ 41 g.
Frequently asked questions
What is the Hop Storage Index?
A lab measure of how fast a variety loses alpha acid; varieties with poor storage (Cascade, Saaz) lose alpha far faster than those with good storage (Magnum, Nugget) under the same conditions.
Should I substitute late additions by weight?
Yes — for flavour and aroma hops keep the weight roughly the same and choose a variety with a similar profile; the bitterness difference from late additions is small.
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This calculator is for educational purposes. Double-check important results before acting on them.