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Armour Class (AC) is a measurement of how difficult a creature is to be hit by an attack. In order to successfully hit a creature, the results of an attack roll must have a number equal to or greater than the target's Armour Class. AC can be increased by equipping armour and shields, by selecting certain feats when leveling up, or utilizing certain spells.
Formulas
The default formula that determines AC is:
- 10 + Dexterity modifier + armour bonus + shield bonus + other bonuses and penalties
The AC bonus from Dexterity modifier may be capped when wearing medium or heavy armour.
Medium armour caps the Dexterity Modifier to +2,[note 1][note 2] whrereas heavy armour ignores the modifier entirely.
Shields will grant the listed AC bonus to whomever equips it, regardless of which weapon slot they are currently using.
Other formulas
Unarmoured creatures may use a different formula if they have certain features. Creatures always use whichever formula – that they have access to – would result in a higher AC.
and 
:
- 13 + Dexterity modifier + shield bonus + other bonuses and penalties
:
- 10 + Constitution modifier† + shield bonus + other bonuses and penalties
:
- 10 + Wisdom modifier† + shield bonus + other bonuses and penalties
Math
Armour Class becomes exponentially more useful the greater it is – the difference in effectiveness between 20 and 19 AC is greater than the difference in effectiveness between 15 and 14.
To illustrate this, if a defender has 15 AC and 10 HP, and the attacker has +5 (50% chance to hit) to attack rolls, and deals 2 damage per attack, the defender would on average survive for 10 turns.
If the defender's AC was increased to 16 (45% chance to be hit), they would instead survive for an average of 11.1 rounds (an 11% increase in effectiveness).
Meanwhile, if the defender has 19 AC (30% chance to be hit), they would survive for an average of 16.66 rounds. If their AC was increased to 20 (25% chance to be hit), they would be able to survive for an average of 20 rounds (a 20% increase in effectiveness).
The difference between 25 and 24 is even greater, with a 200% increase in effectiveness (50 vs 100 rounds).
Footnotes
- ↑ The Medium Armour Master feat increases the cap from +2 to +3.
- ↑ A few rare armours have an "Exotic Material" trait that allow the wearer to get the full Dexterity bonus to AC. These include Yuan-Ti Scale Mail, Unwanted Masterwork Scalemail, Sharpened Snare Cuirass, and Armour of Agility.