Armor Class

Armor Class (AC) is a number that determines how hard you are to hit with an attack (your opponent must roll a number at least equal to your AC, after applying potential to-hit modifiers, in order to hit you).

Your AC is equal to the following:

10 + armor bonus + shield bonus + dexterity bonus + deflection bonus + dodge bonus + natural armor bonus + size modifier + blocking bonus + misc. modifiers

Each of the bonuses mentioned above stack with each other, although multiple bonuses of the same type will usually not stack (exceptions are listed below). Examples are also given below.

Bonus Types
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.
 * The Armor bonus repesents 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 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 Dodge bonus represents your ability to dodge blows before they land. Dodge bonuses are one of the exceptions in that all dodge bonuses DO stack with other dodge bonuses.  Note that Dodge bonuses are not the same as your dexterity bonus.  Dodge bonuses come from things like the Dodge feat or the Haste spell.
 * The Size modifier is +1 for Halflings (because of their small size), 0 for other playable races
 * Blocking Bonus - This is a +2 bonus you recieve only while actively blocking with a shield
 * There are a few miscellaneous bonuses that do not fall into one of the above four categories.

Dexterity and Armor
The amount of AC bonus you get from the armor you are wearing is dependent on two things:
 * The armor bonus of the armor
 * The dexterity bonus you can get while wearing that armor.

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.

If you have a very high dexterity, you can get the same AC bonus from as a low dexterity character could from wearing.

Example -
 * Rogue has a dexterity of 20, normally giving a +5.
 * Fighter has a dexterity of 12, normally giving a +1.


 * Rogue puts on Studded Leather Armor which has an armor bonus of 3 and a max dex bonus of 5.  Rogue gets an AC bonus of 3+5 = 8.
 * Fighter puts on Half Plate Armor which has an armor bonus of 7 and a max dex bonus of 0.  Fighter only gets a bonus of 7+ 0 = 7.

The rogue actually gets a better bonus to armor here because of their dexterity. If fighter picks up a suit of Full Plate Armor then their bonus would go up to 9 (armor 8, max dex 1).

Mithril is an often sought-after enhancement for armor because of its ability to allow a higher dex bonus in armor that is normally to heavy for a high dexterity character, and also because it makes armor one class lighter (ie - 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 (e.g. when you are immobilzed by a Hold person spell). Note also that some shields, in particular tower shields, have a maximum dexterity bonus.

Typical values
Some typical PnP armor class numbers to give an example of the ranges expected. In DDO, fighters can usually reach an AC around 40 at level 10.
 * 8 - A clumsy person
 * 10 - Average human
 * 12 - Average human in leather armor or average human with a large shield
 * 17 - Average human with chainmail and a shield
 * 18 - Average elf with chainmail and a large shield
 * 20 - Average human with Full Plate and a Large Shield