Bitterness calculation (IBU)
How many IBUs the hops give during the boil.
Beer bitterness is measured in IBU — international bitterness units. It depends not only on the amount of hops: in a dense wort a smaller share of the alpha acids isomerises, so the same addition gives different bitterness in a weak and in a strong beer.
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What IBU means
IBU is the amount of isohumulone in milligrams per litre of beer. Isohumulone forms from the alpha acids of hops during the boil: the longer the boil, the larger the share that dissolves and isomerises.
The IBU scale does not match perceived bitterness: 40 IBU in a dry beer with low final gravity tastes stronger than the same 40 IBU in a full-bodied, sweet one. Perception is affected by residual sugars, carbonation and serving temperature.
The Tinseth formula
The most common estimate is Glenn Tinseth’s formula. It calculates the share of alpha acids that will remain in the beer (utilisation) as the product of two corrections: one depends on wort gravity, the other on boil time.
Gravity = 1.65 · 0.000125^(OG − 1) Time = (1 − e^(−0.04 · min)) / 4.15 IBU = weight(g) · alpha(%) · gravity · time · 10 / volume(L)
Typical values
| Style | IBU |
|---|---|
| American light lager | 8–12 |
| Pilsner | 30–40 |
| Wheat beer | 8–15 |
| Pale ale | 30–45 |
| IPA | 40–70 |
| Double IPA | 60–100 |
| Stout | 30–50 |
What affects the calculation
- Alpha acid varies from harvest to harvest: take the value from the packet, not the average for the variety.
- In high-gravity wort utilisation falls — the gravity correction accounts for this.
- Hops added in the whirlpool or at packaging mostly give aroma: the formula counts the boil only.
- The result remains an estimate: a discrepancy of 10–15 % with a laboratory measurement is explained by differences in raw materials and equipment.