A few of my players had only played 5th Edition AD&D before they came to my table. That’s fine, but I don’t run 5th Edition games, so they often get confused when they have to learn the ancient ways. Of all the old rules, THAC0 seems to be the most intimidating. It shouldn’t be.
Look, it’s just a little math. Just a tiny bit. If you can figure out how much damage you took after successfully saving against that fireball, you can do THAC0.
THAC0 stands for the number a character needs to roll on a d20 “To Hit Armor Class Zero.” (Remember, in the early editions, a low AC was good.)
THAC0 – Number Rolled = AC Hit
If your character is 1st level (any class), your THAC0 is 20. That means you need to roll a 20 to hit an opponent whose AC is 0.
20 – 20 = 0
As the character gains levels, the THAC0 goes down, and you can hit opponents more easily. We like hitting things, right?
Example: How THAC0 Works in Play
Bjorn the Badass is a 5th level fighter. His THAC0 is 16. He has a STR bonus of +2 and a +1 broadsword. He’s fighting a poor defenseless zombie, whose AC is 8.
Bjorn rolls an 11 to hit.
Bjorn’s total hit roll: 11+2+1 = 14
Did he hit? Well, 16 – 14 = 2. Yes. He hit.
Just Like Algebra
If you like, you can change the values on either side and write the equation this way:
THAC0 – AC = Roll Needed
This works well if you know the monster’s AC.
Example: Serena is a 7th level mage with a THAC0 of 18; the monster’s AC is 5. Serena’s out of spells, so she tries to hit it with a rock, and rolls a 12.
18 – 5 = 13
Unless Serena has a good STR bonus or the rock is magical, she missed it by 1. Should have saved some of those Magic Missiles.