Bitterness calculator (IBU, Tinseth method)

Bitterness is the first balance in a beer: it answers the sweetness of the malt. IBUs (International Bitterness Units) put a number on it, and let you compare a recipe with the benchmarks of its style: 20 to 30 IBU for a blonde, 40 to 70 for an IPA.

The calculation is mostly about staying in control when something changes. A hop never has the same alpha acid content from one harvest to the next: a Cascade at 5% and a Cascade at 8% do not give the same beer for the same weight. Recalculating the IBUs lets you adjust the grams and get the same bitterness back.

You do it when writing the recipe, then with every new bag of hops.

The tool applies the Tinseth method, the one most used by homebrewers. For each addition, it calculates the amount of alpha acids added, then the share actually turned into bitterness, called utilisation:

alpha acids (mg/L) = alpha (%) × weight (g) × 10 / volume (L)
utilisation = 1.65 × 0.000125^(SG − 1) × (1 − e^(−0.04 × minutes)) / 4.15
IBU = alpha acids × utilisation

Utilisation rises with boil time: a 60-minute addition extracts about four times more bitterness than the same one at 10 minutes. It drops when the wort is dense, because alpha acids dissolve less well in a sugar-laden wort.

Three special cases are handled. Pellets are counted as 10% more efficient than whole cones. A whirlpool addition below 100 °C is reduced according to the temperature. Dry hopping counts for 0 IBU.

• Tinseth slightly underestimates very short additions and overestimates above 100 IBU, where bitterness saturates.
• Perceived bitterness also depends on the sulphate in the water, the residual sugar and the carbonation: two beers at 40 IBU can seem very different.
• The calculation takes the alpha acid content from the label: poorly stored hops have lost some.