If your bitterness is higher or lower than the Recipe Creator predicted, make sure that you adjusted the alpha acids to match the alpha acids of your batch of hops.

Alpha acid content can vary from crop to crop and having the wrong AA will affect the IBU calculation.

IBU is a measure of the concentration of iso-alpha-acids and can easily be thrown off if your wort volumes don’t match what you have entered in the calculator or if your hops aren’t accurately weighed.