Attack Ranks as Attack Bonuses

Following on my discussion of OD&D AC yesterday, here is how to use OD&D attack ranks with the d20 SRD armor system (which uses armor class as target number, also known as ascending AC).

Attack Ranks
Rank Weak (Magic-user) Average (Cleric, Thief) Strong (Fighter) Attack Bonus
levels 1-5
levels 1-4
levels 1-3
levels 6-10
levels 5-8
levels 4-6
levels 11+
levels 9-12
levels 7-9
levels 13+
levels 10-12
levels 13-15
levels 16+


Read the table like: clerics of levels 9 through 12 have attained attack rank 3 and have a base attack bonus of +5.

Again we see the power of three at work here. Weak, average, and strong fighting capabilities are enough to distinguish the classes from each other. Other than a category for not progressing at all, I can’t see any finer granularity adding much value to gameplay.

I have capped the progression of the weak and average classes. It should be obvious how to extrapolate the progression if you want it to be unlimited for all classes. I prefer that the pinacle of fighter combat achievement be higher than other classes.

Yes, it’s a table lookup, but it’s offline, not during the game, so who cares?

Simplified d20 SRD armor bonuses:

Armor Bonuses (Simplified)
Armor AC Bonus Penalty Exploration Tactical
light (leather)
medium (chain)
-4 (-20%)
heavy (plate)
-6 (-30%)
-1 (-5%)

The really cool thing about this is that there are only 24 different possibilities to remember, and all of them are distinct. So players could potentially use whatever interface they prefer, and it would be all the same to me. So, all are equivalent: I hit plate, or I hit AC 3, or I hit AC 16 (my notes generally use the “AC as plate” form).

The penalty is for ability checks and thief abilities. This is taken directly from the d20 SRD, but the numbers work with the original game, so why not? The movement rates are the same as in this encumbrance system, just presented in a slightly different form.

Armor Equivalencies
Armor Descending AC Ascending AC
Leather & shield
Chain & shield
Plate & shield

I’m sure you have seen similar tables to this before, but I include this one here to show the minimum knowledge I need to keep in my head. You’ll also notice that the ascending AC column above is identical to the first OD&D combat rank.

7 thoughts on “Attack Ranks as Attack Bonuses

  1. Matthew James Stanham

    The attack ranks should really begin at +1 and not +0. Whilst this would throw off AC 10/9 it gives the correct probabilities for all other armour values. For example, +0 versus AC 15 (Mail Armour) has a 30% probability of a hit, but THAC0 19 versus AC 5 (Mail Armour) requires a 14+, meaning a 35% chance to hit.

    1. Brendan

      Doesn’t this come down to the fact that “chainmail” in the d20 SRD is not actually the medium armor? It counts as a +5 to AC, which in OD&D would require chain armor and a shield.

      A chain shirt or scale mail would be the proper d20 medium armor equivalent to preserve the steps, which is what I assumed above. Also, banded and splint are the equivalents to OD&D plate (being +6), as both half plate and full plate give too large a bonus. I find this aesthetically pleasing, because I usually assume that Roman legionaries would be wearing heavy armor.

      As Robert Fisher mentioned on G+, AC values of 1 and 0 are still available in the OD&D system were one to desire a super-heavy armor category in addition to the heavy (plate) category. I could see this potentially being useful to distinguish heavy infantry (AC 2) from heavy cavalry in full plate (AC 0).

      I may still have gotten the probabilities wrong, but I don’t think so. Let me know if this still doesn’t make sense.

  2. Matthew James Stanham

    No, I do not think so. What happened in the shift between D&D and AD&D is this:

    No Armour: AC 9 = AC 10
    Leather Armour: AC 7 = AC 8
    Mail Armour: AC 5 = AC 5
    Plate Mail: AC 3 = AC 3

    Beginning THAC0 was then shifted up one place from 19 to 20. What D20/3E did was this:

    No Armour: AC 10 = AC 10
    Leather Armour: AC 8 = AC 12
    Mail Armour: AC 5 = AC 15
    Plate Mail (renamed Half Plate): AC 3 = AC 17

    …which is a direct conversion of AD&D. However, it also jumped fighters back to THAC0 19. It also does the same for studded leather [AC 7 = AC 13], scale mail [AC 6 = AC 14], banded mail [AC 4 = AC 16], and field plate (renamed full plate) [AC 2 = AC 18]. Armour in AD&D works more or less like this:

    AC 10: No Armour
    AC 9-7: Light Armour
    AC 6-4: Medium Armour
    AC 3-1: Heavy Armour

    Mail Armour is right in the middle of the 10 to 0 scale (worst to best armour classes), and the 9-1 scale (worst to best armour types), much like OD&D where it is in the middle of the 7-3 scale (worst to best armour types). What happened with D20/3E is they removed AC 1 type armour and bumped large shields up to a +2 bonus, meaning full plate and large shield = AC 20, also known as AC 0 in AD&D.

    1. Brendan

      Oh, I see. I prefer to consider “distance from unarmored” to be the primary descriptor, as AD&D AC seems to be negatively unbounded (with all the tables in the DMG going down to -10). Thus, I’m not sure that seeing 5 as the middle is well-founded.

      I understand where you’re coming from now though, and the system you describe would work too.

    2. Matthew James Stanham

      Not sure I am following you there. The distance of AC 5 from AC 9 is 4 pips, whilst its distance from AC 7 is 2 pips and its distance from AC 3 is 2 pips, so for it to be a “medium” armour we have to measure its distance only from the other two armour types [i.e. 2 pips] and ignore its distance from unarmoured [i.e. 4 pips]. I guess I am just not seeing the point in having a medium armour if you are then going to have a super heavy armour opposite unarmoured, that makes no sense to me of the term “medium”, and it seems like a confusing change to the D20 tables.

      For what it is worth, the armour class system of AD&D is not unbounded, armour classes 10 to 0 are the full range of what can be achieved with “armour” (which is why the weapon type versus armour class table only takes those values into account).

      Something else worth thinking about is that although OD&D initially suggests that THAC0 19 is for “normal men” this is changed in S&S to accord with the CM fighting ability (man + 1) so that normal men (and goblins) have THAC0 20, which also argues for starting hit bonus of +1.

      Anyway, not meaning to give you a hard time over this, probably I just prefer the 1:1 mapping of fighting ability to fighting-man ability level in conjunction with the 21 point spread! 😉

    3. Brendan

      I don’t plan on including AC 0 and 1 in my game, but that is basically what AD&D did. I am actually inclined towards giving fighters +1 to attack at first level, basically for the reasons you cite, but for now I’m leaving the matrix from Men & Magic as is, for simplicity’s sake.


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