THAC0 (/ˈθækoʊ/, thack-oh) is an abbreviation for To Hit Armor Class Zero (0). To calculate if a hit succeeds you would take the AC of the target and subtract it from the attacker's THAC0, then roll a 20-sided die; if the die rolls equal to or higher than the calculated number, the attack hits. That is, THAC0 − AC = roll needed to hit.
- Example: If a target has an AC of 5 and the attacker has a THAC0 of 18: 18 − 5 = 13. The attacker must roll 13 or greater to hit.
- Example: If a target has an AC of −3 and the attacker has a THAC0 of 8, 8 − (−3) = 11. The attacker must roll 11 or greater to hit.
Fighter classes have the best THAC0 progression and spell users have the worst. Lower THAC0 is better, since you have to roll equal to or higher than the to-hit number.
For more on THAC0, consult the AD&D 1st/2nd edition DM Guide(s).