Hop freshness calculator
Enter the alpha acid from the label, the variety or its hop storage index (HSI), the harvest date, how the hops were stored and your recipe amount. You get the alpha acid that is likely left, a freshness rating and how much to use for the same bitterness.
Hop
From the package or harvest report
Age is counted from this date.
Bittering addition, written for the label alpha acid
Storage since then
Alpha acid today
–
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How alpha acid loss is calculated
Alpha acids oxidize slowly after harvest, so a hop rated at 12% AA on the label may only bring 10% to your boil a year later. How fast that happens depends on the variety and on how warm and how airtight the hops were kept.
The calculator uses the hop storage index (HSI) model from Mark Garetz’s Using Hops. Here the HSI is the share of alpha acid a variety loses in six months (180 days) at room temperature (20 °C / 68 °F) in open air. The loss is exponential: with an HSI of 40, six months loose at room temperature leave 60%, twelve months leave 60% of that, about 36%.
Colder or better sealed storage slows the clock. Garetz’s factors scale the rate of loss: it halves for every 15 °C (27 °F) colder, so hops age at 0.44 times the rate in the fridge (about 2 °C), 0.17 times in the freezer (−18 °C) and 1.26 times in a warm room (25 °C). Packaging multiplies that by 1 for loose hops, 0.75 for a closed bag with air inside (also a pack that was opened and closed again) and 0.5 for vacuum sealed or nitrogen flushed packs.
AA_now = AA × (1 − HSI / 100) ^ (days / 180 × f_temp × f_pack)
f_temp = 2 ^ ((T − 20 °C) / 15 °C)
- AA
- Alpha acid on the label or harvest report (%)
- HSI
- Hop storage index: % lost in 6 months at 20 °C in open air
- days
- Age in days since harvest or packaging
- f_temp
- Temperature factor: halves every 15 °C colder (frozen 0.17 to warm 1.26)
- f_pack
- Packaging factor (vacuum 0.5, closed bag 0.75, loose 1)
Loose hops at room temperature (1 × 1) are the HSI baseline. Age is counted from the harvest or packaging date.
How temperature and packaging affect hop freshness
Temperature is the biggest lever. In the model, hops age more than twice as fast in the fridge as in the freezer, and more than twice as fast again at room temperature. A year in a warm cupboard can cost more alpha acid than five years in a freezer.
Oxygen is the second. Vacuum-sealed or nitrogen-flushed packs keep the hops away from air. A closed bag with air inside, or one that has been opened and pressed shut again, ages about 1.5 times as fast, and loose hops in an open container twice as fast.
The table shows the alpha acid left after twelve months for a hop with an average HSI of 40.
| Storage | Remaining |
|---|---|
| Freezer, vacuum sealed | 91% |
| Freezer, opened and resealed | 87% |
| Fridge, vacuum sealed | 80% |
| Fridge, opened and resealed | 71% |
| Room temperature, vacuum sealed | 60% |
| Room temperature, loose | 36% |
Adjusting bittering hops for old hops
Bitterness scales with the alpha acid you add. If your hops have lost some of it, use more of them in proportion: multiply the recipe amount by the label alpha acid and divide by the estimated current alpha acid. Enter the recipe amount and the calculator shows the amount for the same bitterness directly.
Example: the recipe calls for 1 oz (28 g) at 12.5% AA, and your six-month-old hops from the fridge are estimated at 11.0%. 1 oz × 12.5 / 11.0 gives about 1.14 oz (32 g) for the same IBU.
- The adjustment matters most for the bittering addition at the start of the boil, where most of the IBU comes from.
- Late, whirlpool and dry hop additions are about aroma oils, which also fade with age. More old hops will not bring back a fresh aroma.
- Simplest route: enter the current alpha acid in your recipe and let the IBU calculation work out the amount.
amount_new = amount × AA / AA_now
- AA
- Alpha acid the recipe was written for
- AA_now
- Estimated alpha acid of your hops today
The result is an estimate. When the lot has a fresh lab analysis, use that alpha acid instead.
Hop storage index by variety and storage tips
Varieties age at very different rates. Simcoe and Magnum hold their alpha acid well, while Saaz, Hallertau, Tettnang and Cluster lose it quickly. If your variety is not in the list, the calculator uses an HSI of 40, or you can enter your own value.
- Keep hops in the freezer at about −18 °C (0 °F), not in the fridge door.
- Leave packs sealed until brew day. Vacuum seal or squeeze the air out of opened bags before they go back in.
- Keep hops dark: light speeds up aging, so opaque foil bags are better than clear jars.
- Buy amounts you will brew within a season and check the harvest year on the pack.
| Variety | HSI |
|---|---|
| Simcoe | 20% |
| Magnum | 25% |
| Mosaic | 30% |
| Citra | 38% |
| Centennial | 38% |
| Cascade | 48% |
| Saaz | 55% |
| Cluster | 65% |
Built into every BrewForge recipe
The hop freshness calculator is part of the calculator toolbox in BrewForge. Your hop inventory keeps the harvest year and alpha acid of every lot, and recipes calculate IBU from the alpha acid you enter. Put in the current value of an aged hop and the amount for your target bitterness follows.
- Each hop lot can carry its own alpha acid, so this year’s and last year’s Saaz stay apart
- Harvest year stored with every hop lot in your inventory
- Scale hop amounts to a target IBU in the recipe
Use this calculator via API
The same calculation is also a free JSON API: no account, no key, one GET request. Handy for scripts, spreadsheets and AI assistants that should use exact brewing math instead of estimating it.
Add units=imperial (or one unit parameter per value) for °F, gallons and pounds. Every answer lists the inputs, the units and the formula it used.
FAQ
Questions & answers
What does the hop storage index (HSI) mean?
In this calculator, the HSI is the percentage of alpha acid a hop variety loses in six months at room temperature (20 °C / 68 °F) in open air. A low HSI like 20% means the variety stores well, a high HSI like 55% means it loses alpha acid quickly.
How long do hops last in the freezer?
Vacuum-sealed and frozen, hops keep most of their alpha acid for years. For a hop with an HSI of 40, the model estimates about 91% after one year and 84% after two years in the freezer. Aroma fades faster than alpha acid, so use aroma hops within a year or two.
Can I still brew with old hops?
Usually yes, for bittering. Old hops that still smell like hops can bitter a beer fine if you add more of them. If they smell cheesy or sweaty, the alpha and beta acids have oxidized and the hops are best thrown out.
How much more of an old hop do I need?
Multiply the recipe amount by the alpha acid the recipe assumes and divide by the estimated current alpha acid. If the hops have dropped from 12.5% to 11.0%, use about 14% more for the same bitterness.
My supplier lists an HSI of 0.30. What do I enter?
That number is a lab measurement (a ratio of light absorbance) that describes the condition of that lot when it was tested, not the six-month loss percentage this calculator uses. The two cannot be converted directly. Pick your variety in the list or keep the default instead. A low lab value means the hops were fresh when tested.
Are old hops fine for dry hopping?
Alpha acids hardly matter in a dry hop, but the aroma oils do, and they fade with age just like alpha acids. Old hops give a flatter, sometimes grassy or cheesy aroma. Use your freshest hops for dry hopping and late additions.
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