Armor Class

Armor Class(AC) is a number that determines how hard you are to hit with an attack. A medium size creature has a base armor class of 10, which is then further modified by their dexterity modifier and also any armor that they are wearing. Your AC is equal to the following:


 * 10 + armor bonus + shield bonus + Dexterity modifier + size modifier + misc modifier

Note that armor limits your Dexterity bonus, so if you’re wearing armor, you might not be able to apply your whole Dexterity bonus to your AC.

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).

Other modifiers
There are specific stacking rules with armor class bonuses. Bonuses with the same name don't stack (only the highest bonus applies). Dodge bonuses are an exception.
 * A Dodge bonus stacks with all the bonuses including other dodge bonuses. (Your dexterity modifier is considered a dodge bonus). Any situation that denies you your Dexterity bonus also denies you dodge bonuses.
 * A Natural armor modifier that represents creatures hide. You can have only one natural armor bonus 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 to give +2.
 * An Armor bonus, which varies dependant on the type of armor worn.
 * A Shield bonus, which varies depending on the shield used. Only one shield can be used at a time.
 * A Cover bonus if you are hiding behind some cover or concealed by smoke.
 * A Size bonus if your race is smaller than medium, races larger than medium have a penalty.
 * A Deflection bonus from certain spells or items ward off attacks and improve your AC.
 * An Armor enhancement bonus from Magical armor.
 * A Shield enhancement bonus from Magical shields.

Typical values
Some typical armor class numbers to give an example of the ranges expected.
 * 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