Revisiting Armour Class

Converting D&D’s AC system to “higher equals better”

Author’s Note: This article was originally published in 2002, when Basic D&D had no official mechanic for high-value AC. Since then, thanks to OSR publications (and especially Swords & Wizardry), the concept has entered the mainstream. It’s posted here for posterity and reference. -EDS

As a component of OD&D’s combat rules, the armour class (AC) system is generally good: it provides a single number indicating the degree to which a combatant is protected from physical blows. However, for reasons not entirely known to this writer (and I am open to any enlightenment on this point), the original D&D authors chose to present armour class in the rather unintuitive mode wherein lower values were more advantageous than higher values.By itself, this is perhaps acceptable. However, nearly every other aspect of the game promotes high values as better; indeed, even enchanted armour is described with magical “plusses” such that, bafflingly, a suit of leather +2 provides more protection than a suit of leather –1. This article means to set AC values back along the path of intuition. With little effort, players and DMs will recognise numerically high AC values as good, and converting existing characters’ AC is a snap.

Base Armour Class

A being’s base armour class is his unprotected AC. For most OD&D beings, base AC is 9. Creatures with natural armour or DEX adjustments possess better base AC values. For example, a character with a Dexterity of 16 (+2 bonus) has a base AC of 7; a black widow spider (RC/206) has a base AC of 6.

To set armour class back on the intuitive track, where high numbers are better than low ones, we need first to convert the existing AC data to higher values. The official AC values occupy the low side (i.e., 1 to 9) of the d20 used to make “to-hit” rolls. Since we’re converting to high numbers, we shift AC values to the d20’s high side, or, precisely, from 11 to 20. Universally, this is done by subtracting the original base AC value from 20. Thus, the new base AC value for most beings is 11 [20 – 9 = 11]. For creatures with improved armour classes, the same formula applies; a black widow’s base AC under the new system is 14 [20 – 6 = 14].

Converting Existing AC Values

As mentioned, the quick-and-dirty method for calculating armour class under the new system is to subtract the existing AC value from 20. For example, An official OD&D character with plate mail and a shield +1 (AC 1) becomes AC 19 [20 – 1 = 19].

However, it is important to determine the base AC of any combatant instead of merely converting his existing armour class to the new system. This is because, for simplicity, the AC contribution of any protective device, bonus, or penalty is reflected as a modifier to the base AC. As a result, armour assumes a more modular role, and it becomes a simple matter to determine the effective armour class of creatures who, possessing a natural or otherwise improved AC, don additional armour or enchanted protections.

Determining Armour Class

To determine a being’s AC under the new system, calculate the total modifier of all protective devices worn and all bonuses enjoyed. This modifier is then added to the being’s base AC; the sum is the final AC. Within reason, all such modifiers are cumulative (e.g., a character clearly could not wear two suits of armour simultaneously, though he could wear leather armour and a helm).

New AC Values in Combat

Effective use of our new, intuitive AC values requires one significant change to how OD&D combat is conducted: the existing combat matrices (RC/106-7) no longer need be consulted. Instead, attack success is instantly revealed if a “to-hit” roll result equals or exceeds the defender’s total AC value. As before, “to-hit” rolls are made with 1d20, and all existing combat modifiers are applied directly–as is logical, high “to-hit” roll results are desired.