International Bitterness Units scale
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The International Bitterness Units scale, or simply IBU scale, provides a measure of the bitterness of beer, which is provided by the hops used during brewing. 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 scale, while a barley wine can range up to 100. Measures for other beverages include:
- 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
- Stone Ruination IPA: 100+
One formula craft brewers use to calculate IBU is
- Wh × AA% × Uaa × Vw × 1.34, where
- Wh refers to the weight of the hops used, in ounces
- AA% refers to the alpha acid percentage, which is influenced by many factors, including cultivation method, species, and time of year — hops are often sold labeled with this percentage
- Uaa is the percentage of alpha acid that is actually used during the boiling process
- Vw means the volume of the wort, in gallons
- 1.34 is a constant factor that adjusts the measurement to account for the use of U.S. customary units