- How do you calculate IBU?
Here is a quick formula for determining International Bitterness Units (IBUs) from Fred Eckhardt: For a 5 gallon batch of homebrew, you take Ounces x Alpha Acid x Percentage Utilization(boil time) divided by 7.25.

- How is IBU calculated Whirlpool?
My own rule of thumb is to assume you get the IBUs as if you had boiled for half the time of the whirlpool. For example, 20 minutes whirlpool = 10 minutes in the boil; 30 minutes whirlpool = 15 minutes boiled; etc. Might not be 100% accurate, but it's in the right ballpark for certain.

- What is IBU scale?
IBU stands for International Bitterness Units, a scale to gauge the level of a beer's bitterness. More specifically, IBUs measure the parts per million of isohumulone from hops in a beer, which gives beer bitterness.