The International Bitterness Units scale reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

International Bitterness Units scale

The hoppy character of beer is measured by the International Bitterness Units scale, or IBU scale for short. An IBU is one part per million of isohumulone. The higher number, the greater the bitterness.

A light American lager might be as low as 5 on the IBU scale. At the other extreme, a barley wine can range up to 100. Other examples:

• Belgian Lambics: 11-23
• Blonde ale: 15-30
• Kölsch: 18-25
• Märzen/Oktoberfest: 18-25
• Ordinary English bitter: 20-35
• Porter: 20-40
• Brown ale: 15-25, with North American styles higher, 25-45
• Bohemian-style Pilsener: 30-45
• India Pale Ale: 40 or higher
• An Irish stout like Guinness: 45-60

One formula craft brewers use to calculate IBU is

hop weight × AA% × utilization × volume × 1.34

where

• hop weight refers to the weight of the hops used in ounces.
• AA% refers to the alpha acid percentage, which is influenced by many factors, including method of cultivation, species, and time of year. Hops are often sold labeled with this percentage.
• utilization is the percentage of Alpha acid that is actually used during the boiling process.
• volume means the volume of the wort in gallons.
• the constant (1.34) adjusts the measurement to account for the use of U.S. customary units.

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