Player-Facing Defense Rolls in Basic D&D - Part I: Ascending Armor Class & Attack Bonuses

The first stop on our journey to add player-facing defense rolls to Basic D&D!

If you're playing Old-School Essentials or another retroclone that handles this conversion for you, you're all set. Skip to Part II: DEF Bonus & Attack Ratings unless you got some old Basic supplements to convert or you're particularly interested in seeing how the sausage is made.

Descending to Ascending Armor Class

My playgroup has been playing Basic D&D lately (BECMI/Rules Cyclopedia, specifically), which uses Descending Armor Class (DAC). For those of you uninitiated, that's just the method from these older editions where the lower the AC was, the harder it was to hit.

However, my players and I have a preference for using Ascending Armor Class (AAC), where higher AC is harder to hit. For Basic D&D, its easy to convert DAC to AAC with a simple formula:

19 - DAC = AAC

Why 19? Because in Basic D&D, unarmored AC is 9 (not 10, as in AD&D).
If you don't want to do more math later, just use 19.

Going by the armor table above: leather armor is now 12 AC; scale mail is 13 AC; chain mail is 14 AC; banded mail is 15 AC; plate mail is 16 AC; suit armor is 19 AC; and shields give a +1 bonus to AC.

It's not listed here, but using the formula, an unarmored DAC of 9 becomes an AAC of 10.

For PCs, any bonuses or penalties from Ability scores still apply as normal. A PC with 13 Dexterity has a bonus of  +1 AAC.

For NPCs, it's exactly the same as the formula above: 19 - DAC. A goblin has 6 DAC, therefore they have 13 AAC. 

THAC0 to Attack Bonus - Part I: NPCs

Now that we have an easy method of determining AAC for both PCs and NPCs, we have to take a look at the way attacks work and adapt them to the new scale (while preserving the math!).

And thankfully, we have another handy formula that makes it a little easier for some NPCs:

19 - THAC0 = Attack Bonus

Wow! Same as above!

For NPCs with a THAC0 value listed in its statblock: That's it! You're done. Just use 19 - THAC0 to find the NPC's attack bonus. A zombie has a THAC0 of 18, therefore it has an attack bonus of +1.

However, THAC0 wasn't always the rule, and there are modules and other books that (unfortunately for us) don't list the THAC0 for each NPC right in the statblock. For example, this is the goblin statblock straight from the Rules Cyclopedia:

Even though the book mentions THAC0 is a convenience you'll come across in other published material, there are no THAC0 values listed in the statblocks in the Rules Cyclopedia itself...

Pre-THAC0, when presented with a statblock like this, you would need to look up the NPC's attack values in a table, finding the row for the NPC based on their Hit Dice (HD), and the column for the target's DAC. Cross-referencing these would show the GM the target number they would need to roll to successfully hit the target or not. To demonstrate, here is a bit of the NPC attack rolls table from the Rules Cyclopedia:

But wait, look at that last column: AC 0.

Aka, the number that needs to be rolled to successfully to hit AC 0.

Aka, the NPC's "to hit AC 0" value, or THAC0.

The last column - the AC 0 column - is the NPCs THAC0!

Delete the other columns! We don't need them! We only need two: the Hit Dice column and the AC 0 column.

Keeping this in mind while looking at the goblin statblock and attack rolls table above, the AC 0 column for creatures with "Up to 1" HD lists a value of 19. So goblins have a THAC0 of 19, and we do the math just like before: 19 - THAC0. A goblin has a THAC0 of 19, therefore it has a +0 Attack Bonus.

This is definitely more complicated than if the THAC0 is just listed for the creature, but we can use what we just learned to make a handy chart for reference while prepping or at the table:

Will you need to go as high in your own game?

Probably not, but that's BECMI/Rules Cyclopedia for ya!

THAC0 to Attack Bonus - Part II: PCs

I bet you can guess where this is going.

Now that we've taken care of NPCs, we need to convert the PC's to have an Attack Bonus, too. And we're going to do it the same way we've been doing it:

19 - THAC0 = Attack Bonus

So then, simple enough. We just have to find the THAC0 values for each PC, and we do that with another table!

This time, the table has rows for the PC's class and level, and the same columns for the target's DAC. Similar to before with NPCs and their Hit Dice, we're only looking for the AC 0 column.

Getting rid of all the unneeded columns and applying the same 19 - THAC0 formula as before, we get another table we can use to determine a PCs base Attack Bonus based on their class and level:

Ignore the DH column and all the letters under it. Those aren't relevant here.

As always, ability score and magic weapon bonuses/penalties still apply the attack roll as normal. For example a Strength of 13 and a +1 magic sword together still equal a +2 bonus to hit on top of any class/level bonus, exactly as before (actually, same goes for NPCs with exceptional ability scores or magic items too!).

And that's it!

You're now playing Basic D&D with Ascending Armor Class (AAC) and Attack Bonuses, and all the math is preserved! Everyone still has the same chance to hit, and to be hit, as before.

Now that you've done that, head over to Part II and learn how to implement player-facing defense rolls to keep the players feeling like it's on them to guard against or dodge attacks in combat!