Calculating THAC0

To make an attack roll, the character's THAC0 must be known. This depends on the group and level, if the attacker is a player character or NPC, or the Hit Dice if the attacker is a monster or an animal. All 1st-level characters have THAC0s of 20, regardless of class.

For a character of level 1 through level 20, consult
Table 53. This table lists the THAC0 number of each group through 20th level, so players don't have to perform any calculations.

For a character higher than 20th level, find the Improvement Rate for the character's group in
Table 54. There you'll find the number of levels a character must advance to reduce his THAC0 by 1 (or more) points. Calculate the character's THAC0 according to his level.

DMG contains the information on monster THAC0s.

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