Preface: I imagine that this will likely fall under the “stuff only Mike cares about” category, but I thought I’d share this little thought experiment anyways. Being a big fan of both 2nd Edition AD&D and Pathfinder, I’ve often looked at the differences between the two games and how to transition content from one to the other. One of the big differences, of course, is how a character’s basic ability to hit enemies is calculated. Let’s take a quick look at both systems.

THAC0 (2nd Edition AD&D)

Definition
THAC0 is an abbreviation for To Hit Armor Class Zero (0). To calculate if a hit succeeds you 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
Pavo the intrepid warrior has a THAC0 of 14 and wants to hit an Orc that possesses an AC of 5. Pavo rolls 10 on a d20 as his attack roll, beating his target roll of 9 (14-5). The Orc is hit with a mighty blow!

Notes
The math works out, but I bet you had to read through the description a couple of times for it to make sense. Although THAC0 was meant as an alternative to the to-hit tables of earlier editions, the math was a bit tough for folks to wrap their heads around. I remember being quite stumped by the concept when I was younger!

BAB (Pathfinder)

Definition
A base attack bonus is an attack roll bonus derived from character class and level or creature type and Hit Dice (or combination’s thereof). Base attack bonuses increase at different rates for different character classes and creature types. A second attack is gained when a base attack bonus reaches +6, a third with a base attack bonus of +11 or higher, and a fourth with a base attack bonus of +16 or higher. A character adds his BAB to a d20 roll (plus any additional modifiers), and if the result is equal or greater to the AC of their target, then the attack hits.

Example
Pavo the intrepid warrior has a BAB of +6 and wants to hit an Orc with an AC of 15 (AC is different in Pathfinder, more on this later). Pavo rolls 10 on a d20 as his attack roll, beating his target’s AC of 15. The Orc is hit with a mighty blow!

Notes
Well, it seems like things have been streamlined a bit! I mean, there is a little bit of extra stuff in there regarding iterative attacks, but we’ve switched to an all-addition, all-the-time system. This is especially useful when you consider that other attack bonuses (strength, magic, etc…) can be simply added to the roll.

How To Convert

In order for the conversion to work properly, you have to change the THACO of the attacking creature as well as the Armour Class of the defending creature. We’ve looked at the attack calculations already, but in regards to Armour Class I’ll just say this: AC starts at 10 for both editions. In 2nd Edition a lower AC is better, and in Pathfinder a higher AC is better.

So what are the magic equations that let you convert from 2nd Edition to Pathfinder? Behold!

20 – THAC0 = Pathfinder BAB

20 – AC = Pathfinder AC

 
Easy, right? You can use the examples I gave above to see how the conversion looks like in action. Although the math is a bit different, the numbers and dice rolls work out the same!

Pavo’s To-Hit Calculation
20 – 14 = BAB of +6

Orc’s Armour Class
20 – 5 = AC of 15

So there you have it! It’s actually a pretty simple system, and I hope that it proves useful to some of you home-brewers out there who tinker around with variant systems. It should be noted that the math works just as well for Dungeons and Dragons 3.5, which happens to share most of the same rules of Pathfinder. Have fun converting anything from Aboleths to Zombies!