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 dice; if the dice rolls equal to or higher than the calculated number, the attack hits. That is, THAC0 − AC = roll needed to hit.

If you have a low THAC0, it means that you're more likely to hit your opponent.

If you have a low AC, it means that you're less likely to be hit by your opponent.

This means that a good character will try to get both their AC and THAC0 values as low as possible!

Example: If the player has an AC of 5 and THAC0 of 10, and the enemy has an AC of 10 and THAC0 of 18:

Chance of player hitting the enemy: 10 - 10 = 0. The player will have a very easy time to hit the enemy. Although it may seem that with these numbers it is impossible to miss, a roll is still required as a roll of a natural 1 is ALWAYS considered a miss.

Chance of enemy hitting the player: 18 - 5 = 13. The enemy will have a hard time hitting the player, since a roll of 13 or above is required.

Another 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).

A rough guide to convert THAC0 and AC to 3.0 and higher is to subtract the number from 20. so 20-THAC0= Attack bonus, 20-AC= (New)AC.

using the Above examples the Player now has AC 15 and attack bonus of +10, and the enemy has AC 10 and attack bonus of +2