An International Bitterness Unit (IBU) is a measure of the bitterness in beer. One IBU is equal to one milligram of isomerized alpha acid per liter of beer. Isomerized alpha acids are the main bittering acids derived from hops. IBU's can be estimated using one of several equations if one knows factors such as the alpha acid of the hops used, boil time and specific gravity of the wort. See the technical reference for more details on estimating IBUs.