International Bitterness (IBU) Calculator use your target Batch Size and Original Gravity to estimate how bitter your beer will be based on up to 6 hops additions. Calculations have been adjusted for full and partial mash batches, and takes into consideration, the average utilization differences between whole leaf hops and pellet hops. IBU Scale starts at 0 for beers with no bitterness (fruit and sour beers), and goes all the way up to 120 for highly–bitter beers (Imperial IPA and Barley Wines).