Alternative Armor
Traditionally, armor class is designed entirely for avoidance. While unarmored or lightly armored, that's dodging the attack entirely. In medium or heavy armor, however, that's absorbing the attack on your armor in such a way that it deals no damage. If you want a different approach to armor, try this variant rule.
Armor Class (AC)
Base armor class (AC), without consideration of any special traits, class features, or armor and shields, is always calculated as 10 + Dexterity modifier, with armor replacing or augmenting that base AC calculation. Using this system, your base armor class is instead calculated as follows:
Base AC = 8 + your proficiency bonus + your Dexterity modifier
Damage Reduction (DR)
Damage reduction (DR) is a static value that you subtract when you take damage if certain conditions are met. Certain features, such as the Defense fighting mastery or the absorptive armor property, grant DR that only applies when you take damage from weapons. DR is expressed as a number. DR provided by armor only affects damage you take as a result of being hit by an attack roll, in the same way that AC only helps you avoid being hit by an attack roll.
If passive DR would reduce the damage you take to 0, you instead take 1 damage.
Armor Class vs. Damage Reduction
Instead of focusing solely on damage avoidance, this variant rule balances armor class and damage reduction, to better emphasize how armor is used. Each type of armor affects your AC and DR differently, as shown below.
Light Armor
As normal, light armor does not prohibit the maximum Dexterity bonus you can receive to your AC. However, if the light armor had a higher base AC, extra AC instead becomes damage reduction (DR).
For instance, fiber armor normally increases your AC to 12 + Dexterity modifier. If you have a Dexterity modifier of +3, while wearing fiber armor you would have an AC of 15. With this variant rule, however, fiber armor instead grants you DR of 2 while allowing you to add your Dexterity modifier to your AC. If you have a proficiency bonus of +2, you would have an AC of 13 (8 + your proficiency bonus of +2 and your Dexterity modifier of +3) and DR of 2.
Medium Armor
Medium armor strikes a balance between AC and DR. As normal, it still allows you to add up to +2 to your AC if your Dexterity modifier is +2 or higher.
For instance, weave armor normally increases your AC to 14 + Dexterity modifier (max 2). If you have a Dexterity modifier of +2 or higher, while wearing weave armor you would have an AC of 16. With this variant
rule, however, weave armor instead grants you DR of 4 while allowing you to add your Dexterity modifier (max 2) to your AC. If you have a proficiency bonus of +2, you would have an AC of 12 (8 + your proficiency bonus of +2 and your Dexterity modifier of +2) and DR of 4.
Heavy Armor
Heavy armor focuses entirely on DR. You gain no benefit to AC from your Dexterity modifier, but you also aren't penalized if your Dexterity modifier is negative.
For instance, assault armor normally increases your AC to 17. With this variant rule, however, assault armor instead grants you DR of 7, but it grants no benefit to your AC. If you have a proficiency bonus of +2, you would have an AC of 10 (8 + your proficiency bonus of +2) and DR of 7.
Unarmored Defense and Natural Armor
Certain species (such as wookie or colicoid), classes (such as monk or berserker), or archetypes (such as Blademaster Specialist fighter) grant unarmored defense or natural armor. When using this variant, the following rules apply.
Unarmored Defense. If your unarmored defense includes Strength, Dexterity, Intelligence, Wisdom, or Charisma, you add that modifier to your AC as normal. If your unarmored defense includes your Constitution modifier, however, you instead treat that modifier as DR. For instance, a monk with a proficiency bonus of +2, a Dex modifier of +4, and a Wisdom modifier of +3 would have an AC of 17. A berserker with a proficiency bonus of +2, a Dex modifier of +4, and a Constitution modifier of +3, however, would have an AC of 14 and DR of 3.
Natural Armor. If your natural armor includes an adjustment to your base AC, that adjustment instead becomes DR. If your natural armor allows you to add your Dexterity modifier to your AC, you still add it as normal. For instance, a wookiee's hide grants them a base AC of 13 + Dexterity modifier. If the wookiee has a proficiency bonus of +2 and a Dexterity modifier of +2, instead of having an AC of 15 they would have an AC of 12 and DR of 3. A colicoid, however, who would have an AC of 17, with a proficiency bonus of +2, would have an AC of 10 and a DR of 7.
Shields
As normal, shields add their bonus to AC.
Enhanced Items
If an enhanced armor would add an additional bonus to AC, it instead increases DR. If an enhanced shield would add an additional bonus to AC, it continues to do so. If that bonus only applies to melee or ranged AC, it still only applies to melee or ranged AC or DR, as appropriate.
Other AC Calculations
Work with your GM to determine how other AC calculations, such as the battle precognition power or Way of Endurance consular's Upheld by the Force feature, should translate using these rules.