THAC0. “To Hit Armor Class 0”. How does one convert this to a modern equivalent of a hit bonus when looking to use older modules & monsters in a modern game (such as DCC RPG, or D&D 5e)? As a follow up to my investigation into ascending vs descending armor class, I now offer my interpretation on how to simply convert THAC0. Or not… as it turns out, this wasn’t as nearly as easy as I’d hoped..
TL;DR: There is no easy conversion, but as a start, use table 2 below, and assign the THAC0 to the D&D 5e proficiency bonus as a hit bonus and then add your other modifiers (i.e. strength bonus) for either 5e or DCC.
First off, if you want a basic explanation on how to use THAC0, a good blog post is here and some explanation on the origin here. Basically, I refer to one of the comments in the first link to sum up how one uses THAC0 in an original game:
THAC0 – (d20 roll + mods) = You hit anything of this AC (or higher)
But, that doesn’t really help me too much for what I want to do…
Swords & Wizardry uses both an ascending and descending AC, and uses THAC0. But I’m finding there are some discrepancies in the math, which I’ll skip over for now. But it may be a start if you want to look at it, and look at Tenkar’s rules of thumb for converting to pluses.
Maybe if I look at the “base” THAC0 and start there? It is 20. Meaning that at level 1, with no modifiers, to hit an unarmored target (AC 10), you need to roll a 10 or higher. So, essentially, this is like having “no” hit bonus. Now, what happens when a player levels up? This THAC0 sequentially goes down (or is it “Increase” since you’re getting “better”?… so confusing…), and this is linear for the fighter. Based on this, I made a table with a simple inverse, linear, relationship. Turns out this doesn’t really work out; but it’s easier to show than tell:
Table 1. A Simple Inverse Linear Relationship
If you now compare the fighter in AD&D, 2e, and 5e, and the Warrior in DCC RPG, you see that this Table 1 doesn’t really match up. 5e is “flatter”.
Table 2. Comparing Across Editions
|Level*||AD&D 1e**||AD&D 2e||D&D 5e||DCC*|
|THAC0||THAC0||Proficiency Bonus||Deed Die|
* I am using a convention that one level in DCC is equal to 2 levels in any edition of D&D.
** I think... I found this on a thread; I can't make heads or tails of any of this by reading the AD&D PHB or DMG; and the table in OSRIC is a little different.
Looking at table 2, I come to a few observations based on a level 9 D&D fighter (or level 4 warrior) as a test:
- Whereas table 1 would convert to a +8 hit bonus, the level 9 D&D 5e fighter has 1/2 this proficiency bonus of +4; so table 1 isn’t close to the same; i.e. this is not a simple inverse relationship (I can’t find any links, but I believe Mearls referred to a “flat math” when designing 5e).
- However, I find it interesting that the level 9 DCC Warrior in ‘some’ ways is similar. That is, if you looked at the maximum possible deed die result of +7 compared to the predicted +8 on table 1. But, of course, with the random nature of DCC RPG, you’re equally as likely to roll a +1. So, that doesn’t really match 1e and 2e either. HOWEVER, it is equivalent to D&D 5e as the average result of a d7 would be +4.
- Keep in mind that in D&D 5e, proficiency bonuses are the same per level, regardless of class.
To make this even more confusing, the THAC0 tables for all the other PC classes are different, and monsters use a table based on Hit Dice.. and the table in the 2e DMG is confusing on this… God forbid I can figure any of this out in the 1e AD&D PHB or DMG…. and then 1e is different than 2e….. and this THAC0 stat isn’t used for anything else in the game but combat…… I’m sorry but all those comments that respond “oh, but THAC0 is easy, you just subtract this one number from another number….”, you’re missing the fucking point; as a whole, this is confusing as shit!
One more Table
As just mentioned, Monsters use Hit Dice instead of level (and class) to determine THAC0. You might need this table since my whole point of this is to convert weird monsters found in old modules and references.
Table 3. Creature THAC0 in AD&D 2e
|less than 1||20|
|16 or higher||5|
Based on these observations, I conclude that there is no simple conversion. How’s that for a conclusion! Ha!
However, you can probably use table 2, and just take the THAC0 listed, and then cross reference to the appropriate D&D 5e level and assign that proficiency bonus as the “to hit” bonus. Obviously, you then add any other hit modifiers as usual (i.e. strength bonuses) and I think that will be helpful to start.
This is merely meant to be a simple conversion to create a starting point. I assume that common sense should guide any further modifications. And, when in doubt, I recommend that you should round up (in favor of the monster!).