IWD uses 3E's Base Attack Bonus instead of the traditional 2nd Ed. THAC0 system. Base Attack Bonus operates in the following manner: each class has a progression rate. Fighters, rangers, paladins, and barbarians have the best rate, followed by monks, clerics, druids, bards, and rogues. Wizards and sorcerers have the slowest progression. As a character gains levels, their BAB increases. Their BAB is applied to their attack roll. When a character's BAB reaches +6, he or she gains an additional attack every round, at -5. Every +5 BAB after that, the character gains another additional attack at -5 beyond that.

E.g., Edward Raith has a +16 BAB. He gets one attack at +16, one attack at +11, one attack at +6, and one attack at +1. Spells that increase a character's number of attacks grant an additional attack at the highest BAB. Spells that increase attack bonus should not be confused with the derivation of BAB, which is done solely through class/level.

Multiclassed characters use the best BAB between their classes and levels.

Monks gain additional unarmed attacks at an increased rate. For monks, they gain an additional attack at +4 BAB, -3. This increases every +3 BAB after that. E.g., Kendel has a +10 BAB. He has four attacks at +10/+7/+4/+1.