Damage Reduction (5e Variant Rule)
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In the abstract combat system of the game, a character's armor defends them 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.
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. This variant rule makes combat not necessarily more realistic, but more believable. It also makes combat far more dangerous - even baseline, creatures in armor will generally take more damage than before, and above and beyond that, non-damaging effects that resolve on hit will occur much more often.
Armor Damage Reduction Values
In this system, armor offers two benefits against attacks: an Armor Class, which functions just like it does in the normal rules but is usually lower in value, and damage reduction, which reduces the damage the wearer takes from an attack. All other statstics, such as cost, weight, et cetera, remain unchanged.
|Armor||Armor Class (AC)||Damage Reduction|
|Padded||11 + Dexterity modifier||0|
|Leather||11 + Dexterity modifier||0|
|Studded Leather||11 + Dexterity modifier||1|
|Hide||11 + Dexterity modifier (max 2)||1|
|Chain Shirt||12 + Dexterity modifier (max 2)||1|
|Scale Mail||12 + Dexterity modifier (max 2)||2|
|Breastplate||12 + Dexterity modifier (max 2)||2|
|Half Plate||13 + Dexterity modifier (max 2)||2|
For armor not covered on this table, use the following calculations to determine the armor's new statistics: take the armor's original armor class, subtract 10, and divide the result by 2. If this value is not a whole number, round it down. Disregard any Dexterity modifier you add to the armor class. This value is the armor's damage reduction value. Subtract the armor's damage reduction value from its original armor class to get its new armor class.
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.
Some class features operate differently under this variant rule. The following subheadings detail the changes that should be made to core class features if you are using this variant rule. If a class feature that deals with AC isn't listed here, the DM should adjudicate using the class features detailed here as precedent.
The Unarmored Defense feature from the barbarian class, and that of other classes that use physical ability scores such as Strength is replaced by the Unarmored Damage Reduction feature, detailed below, since the effectiveness of that class feature represents the user's ability to resist physical damage. The Unarmored Defense feature from the monk class, and that of other home-made classes that use nonphysical ability scores such as Charisma, functions normally, since the effectiveness of that class feature represents the user's ability to dodge blows or otherwise prevent attacks from connecting entirely.
Unarmored Damage Reduction
While you are not wearing armor, your Armor Class equals 10 + your Dexterity modifier. While you are using this AC, you have a damage reduction value equal to your Constitution modifier. You can use a shield and still gain this benefit.
This class feature replaces the Unarmored Defense feature of the base barbarian class.
This is a difficult class feature to rule on, because depending on how players imagine their character, the defensive fighting style might represent a character being skilled at parrying and deflecting blows, or it might represent a character "knowing how to get hit" and being sturdy enough to take some blows with minimal injury. Consider allowing a player who chooses this fighting style to choose whether they want to boost their Armor Class or damage reduction value, but do not allow them to "double-dip" if they later get to choose a second fighting style.
The second paragraph of this sorcerer class feature should instead read as follows:
Additionally, parts of your skin are covered by a thin sheen of dragon-like scales. While you are not wearing armor, your Armor Class equals 10 + your Dexterity modifier. While you are using this AC, you have a damage reduction value of 3.
Magic armor such as +1 Armor continues to provide a boost to AC as it normally does, and does not provide any additional damage reduction, unless the armor description specifies otherwise.
Monsters' Damage Reduction
Monsters also gain benefit from this variant rule. If a monster wears armor, consult the table above to find its new armor class and its damage reduction value. If a monster has natural armor, use the following calculations to determine its new armor class and damage reduction value: take the monster's original armor class, subtract (10 + the monster's Dexterity modifier), and divide the result by 2. This number is the monster's damage reduction value. Subtract the damage reduction value from the monster's original armor class to get its new armor class.
For example, a copper dragon wyrmling has an AC of 16. It also has a Dexterity modifier of +1, so you subtract that from its armor class, and you also subtract 10. Then you divide the result, 5, by 2, which gives the result 2.5. Since this is not a whole number, you round down, giving the copper dragon wyrmling a damage reduction value of 2, meaning that any damage it takes from an attack is reduced by 2. Subtract this number from the monsters original armor class, giving the copper dragon wyrmling a new AC of 14 using this variant rule. Add a monster's damage reduction to its AC for the purposes of calculating the monster's challenge rating.