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.

For a character of level 1 through level 20, consult Table 38 . 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 37 . 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. A priest, for example, improves by two for every three levels he
advances. A 5th level cleric would have a THAC0 of 18. A rogue (a thief or bard)
improves one point every two levels. A 17th-level rogue would have a THAC0 of 12. Table 38 lists the THAC0 number of each group at each level.

Creatures do not have character classes and levels, so they calculate THAC0s
differently, basing it on the Hit Dice of the creature. Table 39 lists the THAC0 number for monsters having various numbers of Hit Dice. When
a creature has three or more points added to its Hit Dice, count another die
when consulting the table.

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