IBU (Rager) Calculator

How it is calculated

The Rager utilisation is a smooth curve of boil time t in minutes, followed by a gravity adjustment: utilisation = (18.11 + 13.86 × tanh((t − 31.32) / 18.27)) / 100; GA = (OG − 1.050) / 0.2 if OG is above 1.050, otherwise 0; IBU = (oz × 7489 × AA/100 × utilisation) / (gallons × (1 + GA)). Hop weight is in ounces, AA is alpha acid in percent (up to 100), volume is in US gallons, and OG is the wort gravity as a specific gravity such as 1.050 (accepted from 0.980 to 1.200). The 7489 factor converts ounces of alpha acid per gallon into mg/L. The curve levels off near 32 % for long boils, and a hop added at flameout (0 minutes) counts as zero utilisation, so it gives 0.0 IBU. Boil time can be 0 to 1,440 minutes. The result shows IBU to one decimal and the utilisation used. It handles one hop addition, so add several additions together yourself.

Common mistakes

• Entering the hop weight in grams. The field is ounces, so 28 g must be entered as about 1.
• Entering the volume in litres rather than US gallons. A 19 L batch typed as 19 gives 8.5 IBU instead of about 32.
• Typing the gravity as 1050 or 50. The tool expects a specific gravity such as 1.050, and values outside 0.980 to 1.200 give no result.
• Entering the alpha acid as a fraction (0.07) rather than a percent. The tool accepts it and returns only 0.3 IBU for the example.
• Comparing a Rager figure directly with a Tinseth figure for the same recipe. The formulas differ, so the numbers do not match; judge each recipe within one method.