Armor Class, also called AC, represents your chance to be missed by melee attacks - the higher your AC, the less you get hit. This chance is also influenced by the attackers attack bonus.
If the attack misses, it can still result in a grazing hit.
Your AC is equal to the following:
Chance to be hit
The detailed formulas are as follows (; link goes to the beta test version of the formula; where is the final formula listed?):
- Monster's chance to hit: (Monster's Attack Bonus + 10.5) / (Target's Armor Class * 2)
- Note: from testing, monster's Attack Bonus is typically 2.5*CR. Details .
- Player's chance to hit: (Player's Attack Bonus + 10.5) / (Target's Armor Class * 2) + 20% proficiency bonus, rounded to nearest 5%
The following table shows the chance of a player to be hit by an NPC enemy (rounded to the nearest percent for display reasons).
Following a brief explanation of the bonus types:
- The base 10 AC is common to everyone
- The Natural armor bonus represents a creatures hide or magical effects that make your skin thicker (such as Barkskin. Natural armor bonuses do not stack with each other, so if you are wearing an amulet of natural armor +4 and have a natural modifier of +2, only the +4 is applied not both. (Exception: the bonus from Rough Hide stacks))
- The Armor bonus represents armor you are wearing (physically or magically). This includes armor, bracers giving armor bonuses, or the Mage armor spell. Armor bonuses do not stack with each other. So Armor that gives you a +5 armor bonus does not stack with bracers that give you a +4, you only get the +5.
- The Shield bonus is a bonus for using a shield. It is different from, and does stack with, the armor bonus. Only one shield can be used at a time. The Shield spell provides a Shield Bonus, and so does not stack with an actual shield. Certain named armors provide a Shield bonus; this stacks with the armor's normal armor bonus, but not with an actual shield.
- The Dexterity bonus is a bonus equal to your Dexterity modifier, potentially capped by the armor or shield you are wearing (if they impose a maximum dexterity bonus), or your encumbrance
- The Deflection bonus represents an item's or enchantment's ability to deflect blows attempting to hit you. Again, it sounds like armor, but it is different from and stacks with armor and shields. Deflection bonuses do not stack with each other. With a cloak giving +2 deflection bonus and bracers giving +4 deflection bonus, you only get the +4.
- The Size modifier is +1 for Halflings and Gnomes (because of their small size), 0 for other playable races
- The Insight bonus can typically be found on items (e.g. weapons with the Parrying enhancement, or items with Heightened Awareness).
- There are a few Miscellaneous bonuses that do not fall into one of the above four categories. Miscellaneous bonuses are one of the exceptions in that all Miscellaneous bonuses DO stack with other Miscellaneous bonuses.
Dexterity and Armor
The amount of AC bonus you get from the armor you are wearing is dependent on two things:
The heavier an armor is, the more it limits the amount of a bonus you can get from your dexterity. Padded Armor gives you an armor bonus of only 1 but lets you get up to 8 more bonus points from Dexterity. Half Plate Armor gives you an armor bonus of 7, but does not let you get any dexterity bonus.
The dexterity bonus and the AC bonus of the armor are affected by the type of armor. IE: "Mountain" armor will always give better bonuses than "Battle" armor. For more details on this click on the "Armor by Proficiency" link at the bottom of the page.
Mithral is an often sought-after material for armor because of its ability to allow a higher dexterity bonus in armor that is normally too heavy for a high dexterity character, and also because it makes armor one class lighter (i.e. - heavy is medium, medium is light) so that you do not have to be proficient in heavier armor to wear them.
Note that sometimes, you can't use your Dexterity bonus (if you have one). If you can't react to a blow, you can't use your Dexterity bonus to AC. So, when you are immobilized by a Hold person spell you lose your dexterity bonus to armor class.
- Note: Even if your armor/shield limits your dexterity bonus to Armor Class, it will not affect your attack bonus if you use dexterity (if you use weapon finesse or a bow)
As of Update 14, some benefits of defensive stances were changed to grant a percentage bonus to Armor Class. Additional sources of this bonus were added later. Multiple percentage bonuses to AC stack. The percentage bonus is applied after all other Armor Class bonuses.
- Defensive Fighting grants a 5% bonus, while applying a 5% penalty to your Attack Bonus
- Combat Expertise grants a 10% bonus, and a -5 penalty to your Attack Bonus (mutually exclusive with Defensive Fighting)
- Monk Mountain Stances grant 5%/10%/15%/20% and other benefits, and a -2 penalty to Dexterity, and -10% movement speed penalty
- Grandfather's Shield filigree set bonus: +5%
- Bravery Throughout filigree set bonus: +5%
- Unbreakable filigree set bonus: +5%
- Technomage filigree set bonus: +5%
- Wild Shape Bear / Dire Bear: +10% / 15%
- Wizard / Sorcerer Eldritch Knight enhancements: Mage Armor SLA +10% (Exceptional bonus)
- Sacred Defender enhancements 10%
- Stalwart Defender enhancements 15%
- Epic Sirens Ward item set: +10% Artifact Bonus
- Legendary Sirens Ward item set: +15% Artifact Bonus
- Knight of the Shadows item set: +10% Artifact Bonus
- Legendary Knight of the Shadows item set: +15% Artifact Bonus
- Guardian of the Gates item set: +10% Artifact Bonus
- Legendary Guardian of the Gates item set: +15% Artifact Bonus
- Legacy of Levikk item set: +15% Artifact Bonus
- Eminence of Winter item set: +15% Artifact Bonus
- Epic Wards of House Kundarak item set: +10% Artifact Bonus
- Legendary Wards of House Kundarak item set: +15% Artifact Bonus
- Reaper_bonus: 5%
Bonus to shield AC
- Dwarven Shield Mastery: 5%/10%/15%
- (PDK) Shield Mastery: 5%/10%/15%
- Sacred Shield Mastery: 5%/10%/15%
- Stalwart Shield Mastery: 5%/10%/15%
- Perfect Shield Mastery: 20%
- Unyielding Sentinel Shield Prowess: 15%/30%/50%
As of Update 14, nearly every source of dodge bonus to Armor Class was converted to a percentage. This is a rating of your chance to avoid blows entirely.
- It is capped at 25%, or your Maximum Dexterity Bonus to Armor Class, whichever is lower.
- It is calculated separately to the roll to hit, and as such, you can dodge grazing hits
- You cannot dodge spells or spell-like abilities. Evasion and spell resistance has a chance to mitigate these, depending on the type of attack.