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 â€” 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 80, others have tried to surpass this number but there is no real gauge after 80 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.