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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.

The bittering effect is less noticeable in beers with a high quantity of malt, so a higher IBU is needed in heavier beers to balance the flavor. For example, an Imperial Stout may have an IBU of 50, but will taste less bitter than an English Bitter with an IBU of 30, because the first beer uses much more malt than the second. The technical limit for IBU's is around 100, others have tried to surpass this number but there is no real gauge after 100 IBUs when it comes to taste threshold.

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, sometimes it can range up to 100 (e.g. german Bitterpils)
• India Pale Ale: 40 or higher
• An Irish stout like Guinness: 45–60

A formula craft brewers use to calculate IBU is:

W_h × AA% × U_aa ⁄ ( V_w × 1.34 ), where

• W_h 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
• U_aa is the percentage of alpha acid that is actually used during the boiling process
• V_w 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

There are several different methods for finding U_aa, which can yield very different results. Generally, U_aa increases with longer boiling times and decreases with higher boil density.

Further Reading: Brew Your Own's Explaination