Armor Class

Armor Class, also called AC, represents the number that a monster must have to hit you. That means that his attack roll must be higher or equal to your AC. That means, the higher you AC, the less you get hit. However, that also means that past a certain point, your AC may become worth useless.

In fact, past a certain point, AC can become useless to you. Since a natural 1 is an automatic missed hit, if a monster can hit you on a 2, your AC is equal to none. For example, if you have 8 AC and a monster has +6 to-hit, your AC is worth nothing as this monster will never miss you.

Your AC is equal to the following:

10+armor bonus+shield bonus+dexterity bonus+deflection bonus+dodge bonus+natural armor bonus+miscellaneous bonus+insight bonus+size modifier

Each of the bonuses mentioned above stack with each other, although multiple bonuses of the same type will usually not stack with exception of dodge bonus and miscellaneous bonus.

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
 * 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 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 light armor as a low dexterity character could from wearing heavy armor as  light armor have lower maximum dexterity bonus.

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 (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. So, when you are immobilzed by a Hold person spell you lose your dexterity bonus to armor class.

Note also that some shields, in particular tower shields, have a maximum dexterity bonus.