International Bitterness Units (IBU) is the standard measurement of perceived bitterness in beer, a quality imparted by the addition of hops during its brewing process. This standard measurement of bitterness is a function of the alpha acid concentration in a strain of hops (see the list of hops and alpha acid content here) and the total boil time of a beer where the greater the boil time, the greater a beer's IBUs.
This is an important factor in determining how true to a particular style a beer is, whether it fits within brand expectations, or as a measure of whether or not a beer falls within the brewer's specifications. The measurement of IBUs does not represent parts-per-million of iso-alpha acids in a beer solution though the measurement was developed to illustrate the correlation between the perceived bitterness in a beer and the concentration of bittering compounds in it.
The formula for calculating IBUs is:
Wh × AA% × Uaa ⁄ Vw
- Wh represents the weight of the hops
- AA%: the percent of alpha acids present in the hops
- Uaa: the hop utilization, or percent of alpha acids used during brewing
- Vw: volume of wort, or beer before fermentation
The amount of alpha acids in the hops is multiplied by the utilization (a measure of how much of the alpha acids will actually be used during brewing) divided by how much wort there is.
A simpler way of saying this is it depends on three things: first, how bitter the hops are, second, how much hops are and can be used, and lastly, how much beer the hops will be going into.
The Four Ingredients
All beer consists of, at a minimum, four ingredients: malted grains, hops, yeast, and water, each of which contributes differently to the character of a beer.
While hoppier beers like India pale ales and English bitter are generally much more bitter than other fruitier and maltier styles like German Kölsch, all beers benefit from the addition of hops in varying capacities.
The measurement of International Bitterness Units was developed in the 1950s and 60s during which time most brewers used unrefrigerated hops. By the time hops were used in brewing, a great deal of the bittering potential would have been lost. In addition to this, as hops age, they begin to oxidize, and it is this oxidation along with other transformation products that begin to contribute more to the bitterness of the resulting beer. Initially, a factor of 5/7 was applied to the calculation of IBUs as a method of closer estimating the product content derived from hop resin in beer, though this estimate did not hold true.
Beer Styles and IBUs
- American Lager: 5-15 IBUs
- American Light Lager: 8-17 IBUs
- Barley Wine: 50-100 IBUs
- Belgian Dubbel: 10-25 IBUs
- Belgian Tripel: 14-25 IBUs
- Bitter, Extra Special: 30-40 IBUs
- Bitter, Ordinary: 20-35 IBUs
- Bock: 20-30 IBUs
- Brown Ale: 15-60 IBUs
- Dunkel Lager: 16-25 IBUs
- Imperial Stout: 50-80 IBUs
- India Pale Ale: 40-80 IBUs
- Irish Stout: 25-60 IBUs
- Lambic: 11-23 IBUs
- Pale Ale (American): 20-55 IBUs
- Pale Ale, English: 20-45 IBUs
- Pilsner (German or Czech): 30-45 IBUs
- Porter: 20-60 IBUs
- Rauchbier: 20-25 IBUs
- Saison: 20-30 IBUs
- Scotch Strong Ale: 25-35 IBUs
- Weizen: 13-17 IBUs