International Bitterness Units, commonly known as IBUs, are the internationally agreed-upon standard for measuring bitterness in beer.

IBUs are calculated values made up of the quantity of material in wort or beer derived from the alpha acids in hops and multiplied by the fraction 5/7. 

Beers can range from about 1 to 100 IBUs. Because IBUs were created as a way for brewers to ensure consistency from batch to batch, it is debatable whether they are useful to the end consumer.