Homebrew Bittering Units (HBUs) are a simple means of assessing the amount of hops required to achieve a specific bitterness in a beer. The number of bitterness units in a beer is simply the number of ounces used times the alpha acid value of the hop, with alpha acids being the active ingredient that will provide the bitterness.
If a beer recipe requires a specific HBU level, then dividing this level by the alpha acid of the hop will provide an estimate of the number of ounces of hops required. For example, a recipe for 5 gal of beer with a HBU of 10 would require 2.5 oz of a hop with an alpha acid of 4%. Hop varieties have different levels of alpha acid and different additions are required to obtain the same bitterness level in the beer.
Standard bitterness units (IBUs) have a chemical definition based on milligrams of the actual bitterness chemical, iso-alpha acid, present per liter. HBUs take into account the bitterness in the hops and allow a simple calculation of hop addition to be made.
However, the actual bitterness in beer depends on additional factors such as the efficiency of the boil in converting alpha acids to iso-alpha acids and the time of hop addition. More exact analysis of the brewing system used will allow for these factors and permit advanced and professional brewing calculations to be made based on IBU values.