Publication:Unearthed Arcana/Armor as Damage Reduction

Armor as Damage Reduction
In the abstract combat system of the D&D game, a character's armor defends him by reducing the chance that an attack will deal damage. That system simplifies the realities of battle in order to streamline combat resolution. An attack that fails due to a character's armor or natural armor doesn't really fail to connect, but rather fails to connect with enough force to deal any damage. (That's why touch attacks ignore a character's armor and natural armor - the touch attack only needs to connect to deliver its effect, and need not actually breach the target's armor.)

If you're willing to add a layer of complexity to your combats, consider this variant. In this system, armor reduces the amount of damage dealt by an attack instead of merely turning would-be hits into misses. Armor still prevents some hits outright, but also reduces the deadliness of attacks that do connect. In essence, the system "gives up" some of armor's ability to turn hits into misses in exchange for a small reduction in the damage dealt by any given attack.

Armor Damage Reduction Values
In this system, armor offers two benefits against attacks: an armor bonus to Armor Class, which functions just like the armor bonus in the standard D&D rules but is usually lower in value; and damage reduction. See Table: Armor and Damage Reduction for the armor bonus and DR values for common armor types. (All other armor statistics, such as maximum Dexterity bonus, armor check penalty, and arcane spell failure chance, are unchanged.)

For armors not covered on Table: Armor and Damage Reduction, you can determine the new armor values and damage reduction based on the standard D&D armor bonus. To determine the armor's damage reduction, divide the armor's normal armor bonus by 2 (rounding down). To determine the armor's new armor bonus, subtract the damage reduction from the normal armor bonus. For example, studded leather has a normal armor bonus of +3. That gives it a damage reduction of 1/- (half of 3, rounded down) and a new armor bonus of +2 (3 minus 1).

Magic Armor
An armor's enhancement bonus (if any) increases its armor bonus to Armor Class, but has no effect on the armor's damage reduction. A +3 chain shirt, for example, adds +5 to Armor Class and grants damage reduction 2/-.

Stacking Damage Reduction
The damage reduction granted by armor stacks with other damage reduction of the same type (that is, damage reduction that has a dash after the number). A 7th level barbarian wearing a breastplate has DR 3/- (1/- from his class level and 2/- from his armor). A fighter wearing full plate armor who is the target of a stoneskin spell, however, has DR 4/- from the armor and 10/adamantine from the spell (see page 292 of the Dungeon Master's Guide, under Damage Reduction, for rules on characters with multiple types of damage reduction).

Shields
Shields function normally in this variant, granting their full shield bonus to Armor Class. Unlike with armor, a shield's effectiveness is measured wholly by its ability to keep an attack from connecting with your body.

Natural Armor
A creature's natural armor also provides a modicum of damage reduction. Divide the monster's natural armor bonus (not including any enhancement bonus) by 5 to determine the monster's damage reduction. The same value is subtracted from the monster's natural armor bonus to Armor Class to find the monster's new Armor Class. These calculations are summarized in Table: Natural Armor and Damage Reduction

If the creature already has damage reduction, either add the value granted from natural armor (if the existing damage reduction is of the same type) or treat it as a separate DR value (if it is of a different type). See page 292 of the Dungeon Master's Guide for rules on creatures with multiple types of damage reduction.

For example, a mummy normally has a natural armor bonus of +10. This gives it DR 2/-, and its natural armor bonus is reduced by 2 points to +8 (making its Armor Class 18). Since the mummy already has DR 5/- as a special quality, its total damage reduction becomes DR 7/-.

A mature adult red dragon has a natural armor bonus of +24. This gives it DR 4/-, and its natural armor bonus is reduced by 4 points to +20 (making its Armor Class 28). The dragon's existing damage reduction is 10/magic, so the two damage reduction values remain separate.

Finally, a frost giant has a +9 natural armor bonus, so it gains DR 1/- from natural armor. The chain shirt it wears gives it an additional DR 2/-. If that frost giant were a 7th-level barbarian, the barbarian class levels would give it DR 1/-. These three values add up to DR 4/-. The frost giant's Armor Class would be 20 (10, +8 natural armor bonus, +2 chain shirt).