THAC0 Progression

THAC0 is the d20 number a character needs to hit AC 0. Lower is better. Subtract the target's AC from the attacker's THAC0 to get the number needed on a d20 — see Dice and Rolls → The to-hit roll.

Level	Warrior / Monk	Priest	Wizard	Rogue
1	20	20	20	20
2	19	20	20	20
3	18	20	20	19
4	17	18	19	19
5	16	18	19	18
6	15	18	19	18
7	14	16	18	17
8	13	16	18	17
9	12	16	18	16
10	11	14	17	16
11	10	14	17	15
12	9	14	17	15
13	8	12	16	14
14	7	12	16	14
15	6	12	16	13
16	5	10	15	13
17	4	10	15	12
18	3	10	15	12
19	2	8	14	11
20	1	8	14	11
21	0	8	14	10
22+	0	6	13	10

Class column groups:

• Warrior / Monk — Fighter, Ranger, Paladin, Barbarian, Monk.
• Priest — Cleric, Druid.
• Wizard — Mage, Sorcerer.
• Rogue — Thief, Bard.

Multi-class characters use the best THAC0 of their classes at their respective levels.

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Source: iwdee/original_manuals/IWDEE - Mastering Melee & Magic.pdf — "Appendix 4: THAC0".