d20 Alternate Mechanics – Armor & Damage Reduction

As Bear and Loki pointed out in response to my last entry on the subject of D&D combat: the regular system may in some ways be strange or unrealistic, but it’s simple. There’s one die roll for attack and one set of dice rolled for damage.

There’s plenty of validity to this view. But D&D actually tries to cram alot of detail into the system with spells, combat maneuvers (at least in the 3.0 and 3.5 editions), and class abilities. My feeling is that the standard D&D/d20 system has come a long way but is stuck at a crossroads. On one hand it’s straddling a fence between the old amalgam of “sub-systems” (one set of rules for damage, one for spellcasting, one for skills, many for other abilities) and a unified system (saving throws, skills, and attacks all work similarly). On the other hand it’s straddling a fence between a sort of simulationist attention to detail and a quicker, simpler system.

While there’s alot of room for debate I feel there exist game systems which are better at speed and simplicity than D&D/d20. So d20 may as well try to succeed at the more detailed, simulationist approach. It also seems that a few of the existing sub-systems could be unified or more elegantly implimented.

To this end I’d favor a combat system (complicated though it might be) with the following steps:

  1. Hit/Miss: Roll one die to determine if the attack hits (also determines if attack hits flesh or armor).
  2. Dealing Damage: Roll one or more dice to determine damage (determined by weapon and maybe strength).
  3. Modifying Damage: If the attack hit armor damage is reduced. If creature is large damage is reduced (if creature is small damage is increased).
  4. Consequences: If any damage gets through the character records this and rolls to see if they pay a consequence.

The existing d20 system covers Dealing Damage adequately, and I covered the Consequences angle in my last entry here. So this time round both Hit/Miss and Modify Damage will be covered.

Since it’s early inception (that I know of) the various incarnations of D&D have treated the damage resistance of armor as a hit-probability modifier under the heading of Armor Class (AC). Under this system the character’s armor either turns away an attack completely or allows the full damage of the blow to harm the character. This kind of “binary result armor” may produce a sort of virtual damage resistance as a certain percentage of attacks on average don’t strike the character.

The problem though is that this is only on average. Averages produce a better approximation when there’s a large pool of data (in this chase attack results) to average together. Large totals of character hit points (as standard D&D typically has at higher levels) mean that it takes more attacks to kill a character, thus producing a better average for attacks and supporting the existing AC system.

But what about characters with few hit points at low levels? Or games which use a non-hitpoint based system (as under Injury & Consequences)?

For example: A character with only a few hit points dons a full suit of plate armor and gets attacked by another character with a dagger. Rolling extremely well on their attack the character with the dagger does full damage against the armored character, killing them quickly… this in spite of the fact that they could not physically have bypassed the armor and the dagger wouldn’t do enough damage to overcome the armor’s hardness rating (amount of damage needed to break it).

The following alternate system is intended to avoid or minimize such quirks:

Dodge & Parry


Instead of having Armor Class (AC) characters have a Dodge score. A character’s Dodge is equal to 10+ their Reflex Save. Attack rolls which meet or exceed the defender’s Dodge hit something, although what they hit may be armor.


Instead of Dodging a defending character may make a Parry roll. A Parry roll is basically an attack which does no damage but attempts to deflect an incomming blow. The Parry roll uses the defender’s:

   1d20 + Defender’s Dexterity Bonus + Base Attack Bonus

   (+ additonal modifiers if applicable(1))

If the defender’s roll exceeds that of the attacker then the attack has been successfully turned aside by the defender’s weapon. Otherwise the attack hits some part of the defender’s person, although again it may hit armor.

A Parry roll is usually treated as an attack of opportunity(2) brought on when the defender knows they are being attacked. Thus characters may attempt to Parry as many times in a round as they have attacks of opportunity. A character may choose to forgoe their normal combat action in a round and Parry as many blows as they would normally be able to deal.

Normally Parrying does not work against ranged weapons or when using ranged weapons (alghough if you want to swing your bow to block a sword stroke that might work, not good for the bow though). However, certain Feats(3) may be devised to overcome this deficiency.

(1) – Shields provide a bonus to Parrying.

(2) – “Attacks of Opportunity” are extra chances to attack that characters may recieve if a defender does something to open themselves to attack (such as turning around to flee). Normally characters get one attack of opportunity per round if the opportunity presents it’s self although some Feats allow for additional attacks if multiple opportunities occur. In the case of Parrying the only situation considered an “opportunity” is when the defender is attacked.

(3)Feats are individually purchased special abilities in the d20 system (rather than skills which increase incrimentally or abilities gained by advancing each level in a character class).

Shield Bonuses

Shields provide benefits to parry attempts in two ways. First of all they increase the chance that a Parry attempt will be successful due to a Parry Bonus (added to each Parry roll made). Secondly, some shields increase the number of Parrys that may be made in a round. The benefits in this regard for each type of shield are noted below:

Shield TypeParry BonusExtra Parriesper round
Light Wooden+4+2
Light Metal+5+2
Heavy Wooden+7+3
Heavy Metal+8+3
Tower+15All (and only) frontal attacks

A Tower shield is very effective but only works against frontal attacks. It is also bulky and requires a move-equivelant action to reposition to face a different direction.

Armor Coverage & Damage Reduction

Damage Reduction

If attacks hit a character’s armor then the armor stops some portion of the damage from getting through (sometimes all the damage). This is referred to as Damage Reduction (DR).

Although only Armor is covered here there are other phenomena which also provide Damage Reduction, such as thick hide, physical makeup (piercing weapons don’t do much damage to creatures which lack bodily fluids), or sheer size. Damage Reduction from these phenomena “stacks” (ie. “is cumulative”) with DR from armor.

The amound of DR an armor provides is determined by the armor’s style of construction and material. A few kinds of armor may be particularly effective against particular sorts of attacks (such as dragon hide armor).

Armor Coverage

A full suit of plate armor protects the entire body against most physical attacks and a hazmat suit protects against most biological or ones. This is simply because, due to their whole-body coverage, nothing can bypass the suit and any attacks must violate the suit’s integrity to get through.

Unfortunately not all armor is so whole-body encompassing. Most suits only cover the vital organs, groin, and maybe the head or arms.

Each type of armor has a Coverage Factor (CF). The following chart relates percentage of the body covered, Coverage Factor, and body parts.

Armor Coverage Factor Chart

% Body
Body Parts covered
+15%Hands or face
+210%Entire head, each arm, each leg
+525%Upper torso or lower torso
Total (+20)100%Whole Body
/2n/aFront or rear surface only (ie. shin guards, chest plate only)

Note: Any combination of armor with a Coverage Factor of +20 or more is considered “full coverage” and automatically applies Damage Reduction without a roll.

Combining It All

In combination this is how Dodge/Parry, Coverage Factor, Damage Reduction work:

  • If an attack is less than the defender’s Parry roll or Dodge score it misses entirely.
  • If an attack is less than the defender’s Parry roll or Dodge score + Coverage Factor then it hits but damage is reduced by the armor’s DR.
  • If an attack equals or exceeds the defender’s Parry roll or Dodge Score + Coverage Factor then the attack hits and bypasses the armor’s DR, thus doing full damage to the character beneath.

The following chart sums up the types of armor common in D&D provides DR and CF for each. Assume that monitary costs, arcan spell failure chance, and other factors stay the same as presented in standard d20 documents.

Common Armors

Armor TypeCoverage
Studded Leather+10+3
Chain Shirt+15+4
Splint Mail+18+5
Banded Mail+18+5
Half Plate+16+6
Full PlateFull (+20)+6

Padded, leather, and studded leather armors covers the torso only.

A shirt of chain mail covers the whole body except the legs and face.

A breastplate covers the torso, but also has a helmet protecting the head (but not the face) and shin guards protecting the front of the legs.

Splint mail and banded mail cover the whole body except the face and hands.

Half plate covers the whole body except the legs.

Full plate covers the whole body.

There are other things about the D&D/d20 combat system I’m not especially happy with, such as the actions per round allocation. But changes along these lines begin to make some core elements of the system (such as standard class abilities) more difficult to reconcile with each other. So I probably won’t be poking at these for the time being.

But D&D magic system watch out: you could be next.

About Peter

This guy lives in Boston MA with his beloved wife and two kids. You can get some idea of his likes and dislikes from posts on this website or elsewhere.

1 thought on “d20 Alternate Mechanics – Armor & Damage Reduction

  1. The notion of feats seems interesting.

    So what would you change on the magic system? How you regained spells – or combat? My biggest beef about spells was spell components. But that was a beef just more from a player standpoint. I wanted to cast spells all the time and not have to worry about foraging for stuff like pine cones and rust monster toe nails.

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