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Chapter 9: Playing the Game / General Rules / Specific Checks / Attack Rolls

Armor Class

Source Core Rulebook pg. 447 2.0
Attack rolls are compared to a special difficulty class called an Armor Class (AC), which measures how hard it is for your foes to hit you with Strikes and other attack actions. Just like for any other check and DC, the result of an attack roll must meet or exceed your AC to be successful, which allows your foe to deal damage to you.

Armor Class is calculated using the following formula.

Armor Class = 10 + Dexterity modifier (up to your armor’s Dex Cap) + proficiency bonus + armor’s item bonus to AC + other bonuses + penalties

Use the proficiency bonus for the category (light, medium, or heavy) or the specific type of armor you’re wearing. If you’re not wearing armor, use your proficiency in unarmored defense.

Armor Class can benefit from bonuses with a variety of sources, much like attack rolls. Armor itself grants an item bonus, so other item bonuses usually won’t apply to your AC, but magic armor can increase the item bonus granted by your armor.

Penalties to AC come from situations and effects in much the same way bonuses do. Circumstance penalties come from unfavorable situations, and status penalties come from effects that impede your abilities or from broken armor. You take an item penalty when you wear shoddy armor.