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THAC0 and Attack Rolls

THAC0 stands for "To Hit Armor Class 0" — the number a character, NPC, or monster needs to attack an AC 0 target successfully. THAC0 depends on a character's class and level, with magical items, spells, and racial bonuses modifying it.

The attack roll

At the heart of combat is the attack roll — the d20 roll that determines whether an attack succeeds or fails.

To resolve an attack:

  1. Take the attacker's THAC0.
  2. Subtract the target's AC (Armor Class).
  3. The result is the number the attacker must equal or exceed on a d20.

Example: Orthos, a 3rd-level Fighter, has THAC0 18. He attacks a hobgoblin with AC 5. Subtracting (18 – 5 = 13) gives a "to-hit" number of 13. If Orthos rolls a 13 or higher on the d20, he hits and damage is rolled.

Critical hits and misses

  • A natural 20 (the d20 result before modifiers) is a critical hit — damage is doubled. It always hits regardless of AC.
  • A natural 1 is a critical miss — it always misses, and the attacker takes a time penalty before they can act again.

Wear a helmet

Characters wearing headgear are immune to critical hits scored against them. Helmets are cheap and ubiquitous — every front-liner should be wearing one.

Reference tables

See THAC0 Progression for the per-class THAC0 by level.

Source: iwdee/original_manuals/IWDEE - Mastering Melee & Magic.pdf — "THAC0", "The Attack Roll", "Critical Hits and Misses".