How to use it
- Choose the formula and the units: grams and litres or ounces and US gallons.
- Enter the batch volume that goes into the fermenter and the gravity of the wort during the boil, such as 1.050.
- For each hop addition enter the weight, the alpha acid percentage from the packet and the minutes it boils. Set the weight to 0 for unused rows.
- Read the total IBU and the share of each addition.
The formulas
Tinseth: IBU = U × (AA × grams × 1,000 ÷ litres), where the utilisation U = 1.65 × 0.000125^(G − 1) × (1 − e^(−0.04 × minutes)) ÷ 4.15 and AA is the alpha acid fraction.
Rager: utilisation U = (18.11 + 13.86 × tanh((minutes − 31.32) ÷ 18.27)) %, and IBU = grams × U × AA × 1,000 ÷ (litres × (1 + GA)), with the gravity adjustment GA = (G − 1.050) ÷ 0.2 above 1.050 and 0 otherwise.
Ounces and gallons are converted to grams and litres first (1 oz = 28.35 g, 1 US gal = 3.785 L).
Worked example
20 L at 1.050 with 30 g of 10 % hops for 60 minutes and 20 g of 5 % for 15 minutes, Tinseth:
- gravity factor 1.65 × 0.000125^0.05 = 1.0528;
- 60 min: time factor (1 − e^−2.4) ÷ 4.15 = 0.2191; alpha acids 0.1 × 30 × 1,000 ÷ 20 = 150 mg/L; 1.0528 × 0.2191 × 150 = 34.6 IBU;
- 15 min: (1 − e^−0.6) ÷ 4.15 = 0.1087; 0.05 × 20 × 1,000 ÷ 20 = 50 mg/L; 1.0528 × 0.1087 × 50 = 5.7 IBU;
- total 40.3 IBU. Rager gives 50.3 IBU for the same recipe.
Tips
- Alpha acids vary from crop to crop: use the figure printed on your packet, not a typical value.
- Hops lose alpha acids in storage, so old hops give less than calculated.
- Use the gravity during the boil, which is lower than the original gravity if you boil a larger volume down.