IBU measures the bitterness of beer based on the amount of isomerized alpha acids from hops. The scale typically ranges from 5 (very mild beers) to over 100 (extremely hoppy styles). It is an objective measure, but perception of bitterness also depends on malt sweetness and carbonation. For example, a 60 IBU IPA may taste less bitter than a 40 IBU stout because of malt balance. Brewers use IBUs to fine-tune recipe balance.
