THAC0

Attacker's Level/Hit Dice

Attacker	1st	2nd	3rd	4th	5th	6th	7th	8th	9th	10th	11th	12th	13th	14th	15th	16th+
Assassin	20	19	19	18	17	16	15	15	14	13	12	11	11	10	10	—
Bard	21	20	20	20	19	18	18	17	17	16	16	15	15	14	14	13
Cleric	20	20	19	18	18	17	16	16	15	14	14	13	12	12	11	11
Druid	20	20	19	19	18	18	17	16	16	15	14	14	13	13	12	12
Fighter, Paladin or Ranger	20	19	18	17	16	15	14	13	12	12	11	10	10	9	8	8
Illusionist or Mage	21	21	20	20	20	19	19	18	18	17	17	16	16	15	15	14
Monk	20	19	19	18	17	17	16	15	14	13	12	12	11	10	10	9
Thief	21	21	20	20	19	19	18	18	17	16	16	15	14	14	13	13
Commoner	21	—	—	—	—	—	—	—	—	—	—	—	—	—	—	—
Monster or Combat-trained	20	20	19	18	18	17	17	16	15	15	14	14	13	12	12	11

THAC0 is an acronym for To Hit Armour Class 0 (zero). It represents the number a combatant must roll on a d20 to successfully hit an opponent with an armour class (AC) of 0.

From this base value, the number needed to hit opponents with other AC values is determined by simple subtraction. For example, if a combatant has a THAC0 of 17, they would need to roll a 17 to hit AC 0, a 16 to hit AC 1, a 15 to hit AC 2, a 14 to hit AC 3 and so on. The required roll is calculated as:

To-Hit Roll = THAC0 minus Target's AC

This system simplifies attack resolution in combat by eliminating the need for look-up tables for every attack. Instead, a single THAC0 value allows quick calculation of the roll required to land a successful strike.

THACO and Class Progression

THAC0 varies based on the combatant's experience and class. Martial classes such as fighters and paladins have the most favourable THAC0 progression, as they receive the best training and experience in combat. Conversely, mages and other non-combat classes have a slower improvement, reflecting their lack of proficiency with weapons. Thieves and clerics fall somewhere in between, with moderate combat ability.

The table used for THAC0 progression highlights the disparity in weapons training between different classes. Some classes improve their THAC0 every level, while others advance more slowly, reinforcing their reliance on magic, tactics or support roles rather than direct physical combat.

THACO and Monsters

Monsters generally lack formal combat training, relying on instinct or sheer brute force. Their ability to strike is based on their hit dice rather than level progression, with stronger monsters having lower (better) THAC0 values. However, humanoid creatures that gain experience levels — such as NPC fighters, veteran mercenaries or intelligent undead — may use either their hit dice or class level, whichever results in a more advantageous THAC0.

See also,
Attacking in Combat
Combat