Armor Class = 10 + all modifiers that apply
Modifiers typically include:
- Armor Bonus
- Armor AC Bonus (normal armor and armor enchantments)
- Shield Bonus
- Shield AC Bonus (normal shield and shield enchantments)
- Natural Armor Bonus
Modifiers below are applied against Touch attacks; the ones above do NOT.
- Deflection bonus
- Dodge bonus
- Dexterity modifier (note: Dexterity modifiers behave exactly the same as Dodge bonuses but do not count towards the dodge bonus limit of +20)
- Luck bonus (note: the luck bonus from Luck of Heroes is an unarmored bonus)
- Unarmored AC bonus
Your Armor Class (AC) represents how hard it is for opponents to land a solid, damaging blow on you. It’s the attack roll result that an opponent needs to achieve to hit you. Your AC is equal to ten (10) plus the applicable sum of the following six AC subtypes:
Each subtype, other than unarmored, can be a base bonus or an enhancement bonus. Base bonuses stack and tend to be granted by persistent feats, while enhancement bonuses do not stack (with the exception of Dodge which stacks to +20) and are granted by spells, equipment and spell-like feats.
Against regular attacks all the types of AC are used for defense. Against touch attacks Deflection, Dodge and Unarmored are used for defense. When caught flat footed, Armor, Deflection, Natural and Unarmored are used for defense.
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.
Dodge bonuses: Some other AC bonuses represent actively avoiding blows. These bonuses are called dodge bonuses. Any situation that denies you your Dexterity bonus also denies you dodge bonuses. (Wearing armor, however, does not limit these bonuses the way it limits a Dexterity bonus to AC.) Unlike most sorts of enhancement bonuses, dodge bonuses stack with each other.
Loss of Dodge and Dexterity bonuses
Whenever a character is flat-footed, she loses any applicable dexterity and dodge bonuses. Examples include:
- Those who have been feinted (for that current combat round only)
- Opponents who are stunned, asleep or paralysed, or else not actively engaged in combat
Some feats, such as Uncanny Dodge, may allow a character to retain Dex bonuses to armor class if the character is caught flat-footed.
NOTE: Although being flanked will render a character vulnerable to sneak attacks just as being caught flat-footed will, a character who is the target of a sneak attack is not automatically flat-footed.
Touch attacks: Some attacks disregard armor, including shield bonuses and natural armor. In these cases, the attacker makes a touch attack roll (either ranged or melee). When you are the target of a touch attack, your AC doesn’t include any armor bonus, shield bonus, or natural armor bonus. All other modifiers, such as your size modifier, Dexterity modifier, luck, deflection and dodge bonus (if any) apply normally.
The reason you disregard your armor, shield, and natural armor bonuses is while these modifiers help stave off weapon blows, it is still assumed that you are sometimes "hit" by a weapon, or block with a shield for no damage, and since the goal of a touch attack is to merely touch you, these bonuses are not taken into account.
AC would be calculated at:
10 + deflection bonus + ( Dexterity bonus + Dodge AC bonus ) + Unarmored AC Bonus