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D20fying Classic D&D

Possibly the single most important revision made in 3rd edition Dungeons & Dragons was the way attack (and other) rolls were simplified. The combat system in classic versions of D&D however works on the same underlying principles, the data is simply presented in a different fashion.

The main difference is that in 3rd, and later, editions the Armor Class is the target number for an attack roll, while in classic D&D it is an index number used to look up the actual target number from a table. Classic D&D also calculates level advancements into this target number, while in 3rd edition it is presented as an individual Attack Bonus.

It is relatively easy to convert classic D&D to use a similar mechanism to the one presented in 3rd edition D&D. All you need to do is calculate an appropriate Attack Bonus for each character and convert AC's into a target number. Both can be done using relatively simple formulae, which I will explain below.

(The examples are for Basic and Expert Set D&D and Labyrinth Lord. Similar principles should apply to Advanced Dungeons & Dragons, but you'll need to calculate the exact values for yourself. NOTE: I have not tested or thoroughly checked these examples, so I won't vouch there are no errors.)

Attack Bonus

A character's Attack Bonus is equal to his/her class's Base Attack Bonus plus STR modifier (or DEX modifier for ranged weapons). The Base Attack Bonus can be calculated as 19 minus the class's 'to hit' value for AC 0.

Below I have calculated each class's Base Attack Bonuses for your convenience. First I have values based on the progression in the classic D&D clone Labyrinth Lord. This differs slightly from the progression in D&D Basic and Expert sets. Namely, the values increase more frequently, but in smaller increments (which makes more sense in my mind). The values for clerics, thieves and magic-users appear to be relatively close to each other in both versions, but those of fighters seem to increase a little more rapidly in Labyrinth Lord.

Lvl Fighters & Demi-Humans Clerics & Thieves Magic-Users
1 +0 +0 +0
2 +0 +0 +0
3 +1 +0 +0
4 +2 +1 +1
5 +3 +1 +1
6 +4 +2 +1
7 +5 +2 +1
8 +5 +2 +2
9 +6 +3 +2
10 +7 +3 +2
11 +7 +4 +3
12 +8 +5 +3
13 +9 +6 +4
14 +10 +6 +5
15 +11 +7 +5
16 +12 +7 +6
17 +13 +8 +6
18 +14 +8 +6
19 +15 +9 +7
20 +15 +9 +7

These are the values based on the original table from the Dungeons & Dragons Expert Rulebook:

Lvl Fighters & Demi-Humans Clerics & Thieves Magic-Users
1 +0 +0 +0
2 +0 +0 +0
3 +0 +0 +0
4 +2 +0 +0
5 +2 +2 +0
6 +2 +2 +2
7 +4 +2 +2
8 +4 +2 +2
9 +4 +4 +2
10 +6 +4 +2
11 +6 +4 +4
12 +6 +4 +4
13 +8 +6 +4
14 +8 +6 +4
15 +8 +6 +4

Up to about 8 Hit Dice, a monster's Attack Bonus is basically equal to its Hit Dice minus one (after this point Attack Bonus grows slighly slower). Any plusses in HD however count as the next full HD value (i.e. a monster with HD 1 or less has an Attack Bonus of zero, while 1+1 or 2 HD monster would have an Attack Bonus of +1).

Labyrinth Lord diverges slightly from the tables in the Basic and Expert D&D sets. Namely, higher level monsters in the former are slightly weaker. The table below lists Attack Bonuses based on both these tables:

Hit Dice LL D&D
Up to 1 +0 +0
1+ to 2 +1 +1
2+ to 3 +2 +2
3+ to 4 +3 +3
4+ to 5 +4 +4
5+ to 6 +5 +5
6+ to 7 +6 +6
7+ to 8 +7 +7
8+ to 9 +7 +8
9+ to 11 +8 +9
11+ to 13 +9 +10
13+ to 15 +10 +11
15+ to 17 +11 +12
17+ to 19 +12 +13
19+ to 21 +13 +14
21+ and above +14 +15

Armor Class

The new AC is calculated with a very simple formula: 19 minus original AC.

You can calculate a character's AC as 10 plus his/her DEX bonus plus any armor's AC bonus, calculated here for your convenience (based on values in Labyrinth Lord):

Armor Type AC Bonus
Banded mail +5
Chain mail +4
Leather +2
Padded +1
Plate mail +6
Scale mail +3
Shield +1
Splint mail +5
Studded leather +3

Character Sheet

I designed a simple character sheet for Labyrinth Lord to better go with the ideas presented in this article. Download it here. Also available is a fillable version, and a matching spell sheet in both regular and fillable versions.