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
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.
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.
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.
Magical deflection effects ward off attacks, improving AC.
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 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.
Circumstances can modify your Armor Class.
|Defender is …||Melee||Ranged|
|Balancing (4 or fewer ranks in balance)1, 2||+0||+0|
|Behind improved cover4||+8||+8|
|Behind limited cover5||+2||+2|
|Climbing (without a climb speed)1, 6||+0||+0|
|Grappling (but attacker isn't)1||+0||+08|
|Kneeling or sitting||-2||+2|
|Off balance due to failing a Swim check1||+0||+0|
|Running (full round action)1||+0||+0|
|Squeezing (Space less than half normal; DC 30 Escape Artist check)1||-4||-4|
|Successfully feinted by a foe1||+0||+0|
|Using total defense||+4||+4|
- Defender is denied its Dexterity bonus to AC
- If defender takes damage while balancing, it must make another Balance check against the same DC or fall prone.
- Defender also receives a +2 bonus on Reflex saves.
- Defender also receives a +4 bonus on Reflex saves and a +10 bonus on Hide checks, and benefits of evasion.
- Defender also receives a +1 bonus on Reflex saves.
- If defender succeeded on a Climb check after adding 20 to the Climb DC, it isn't denied its Dexterity bonus to AC. If defender takes damage while climbing, it must make another Climb check against the same DC or fall.
- Defender takes a -4 penalty to Dexterity, which results in a modifier 2 lower than normal.
- Roll randomly to see which grappler you strike.
- Treat defender's Dexterity as 0 (-5 modifier), but only for determining AC. Defender subject to attacks, such as sneak attack, that rely on a target being denied its Dexterity bonus to AC.
- AC penalty doesn't apply against the pinning creature.
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 = 10 + dex bonus + dodge bonus + deflection bonus + insight bonus + other modifiers
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 = 10 + armor bonus + shield bonus + natural armor bonus + deflection bonus + insight bonus + other modifiers