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Armor Class
Your Armor Class (AC) represents how hard it is for opponents to land a damaging blow on you. It's the attack roll result that an opponent needs to achieve to hit you.
AC = 10 + size modifier + Dex modifier + armor bonus + shield bonus + deflection bonus + insight bonus + natural armor bonus + dodge bonuses + other modifiers
Size Modifier
The bigger a creature is, the easier it is to hit. The smaller it is, the harder it is to hit. Since this same modifier applies on attack rolls, a Small creature doesn't have a hard time hitting another Small creature. See Size, page 116.
Dexterity Modifier
If your Dexterity is high, you're adept at dodging blows. If your Dexterity is low, you're inept at this task. That's why you apply your Dexterity modifier to your AC. 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 react to a blow, so you're denied your Dexterity bonus to AC, but any Dexterity penalty still applies. You lose your Dexterity bonus when, for example, an invisible opponent attacks you, you're climbing, or you're stunned. Being caught flat-footed at the beginning of combat is also this sort of situation—see Flat-Footed, below.
Armor and Shield Bonuses
Your armor and shield each provide a bonus to AC. This bonus represents their ability to protect you from blows.
Enhancement Bonuses
An enhancement bonus makes an armor bonus, natural armor bonus, or a shield bonus better. The enhancement bonus stacks as if it were part of the bonus to which it applies—armor, natural armor, or shield—so it's not included in the AC formula above.
Deflection Bonus
Magical deflection effects ward off attacks, improving AC.
Insight Bonus
An uncanny knack to sense danger and react to it improves AC. Some creatures have insight bonuses to AC, and a number of magical effects provides such bonuses.
Natural Armor Bonus
Natural armor represents an inborn or magical resistance to attacks that improves AC.
Dodge Bonuses
Dodge bonuses represent actively avoiding blows. Any situation that denies you your Dexterity bonus to AC also denies you dodge bonuses. Wearing armor doesn't limit these bonuses the way it limits a Dexterity bonus to AC. Unlike most other bonuses, dodge bonuses stack with each other.
Other Modifiers
Circumstances can modify your Armor Class.
TOUCH
Some attacks disregard your armor, shield, and natural armor. In these cases, the attacker makes a touch attack roll against
your touch AC.
Touch AC = AC – armor bonus – shield bonus – natural armor bonus
FLAT-FOOTED
You can't react well to danger at the beginning of combat until you've taken your first turn. In such cases, you use flat-footed AC.
Flat-footed AC = AC – Dex bonus – dodge bonuses