IPA is largely responsible for entering the beer term IBU into the modern lexicon. In theory, a higher number represents more bitterness and many brewers offer the data in one form or other since IPA is a style known for its bitter quality. The following displays how the figure is calculated and the shortcomings of the number as an end all be all measurement.
In the beer world bitterness is analytically measured as "bittering units" or "international bitterness units." The numerical value is a measure of a specific hop compound and will not consistently coincide with individual's perception of bitterness intensity.
a. Due to genetics and other differences, individuals will have varying sensitivity to bitterness. Some will sense high intensity bitterness, while others perceive no bitterness in the same beer. The descriptions of bitterness in these guidelines are inclined towards representing average sensitivity to bitterness.
b. Other beer ingredients can contribute bitterness to beer.
c. The intensity and quality of hop flavor and aroma derived from oils, pellets, whole hops or other hop formats can greatly alter the perception of bitterness intensity.