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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 Dexterity modifier bonus may be limited or ignored if the character wears medium or heavy armour (see below).
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
Armour and shields
The AC bonus from Dexterity modifier may be capped when wearing medium or heavy armour.
Medium armour typically caps the Dexterity Modifier to +2.[note 1][note 2] Heavy armour ignores any bonus (or penalty) from the wearer's Dexterity.
Shields will grant the listed AC bonus to whomever equips it.
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 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.
Ways to increase AC
- Certain clothing and armours grant bonus AC outside of their Base AC. Potent Robe is an example of this.
- Fighters, Paladins, and Rangers have access to the Defense Fighting Style feature, which grants +1 AC if they are wearing armour.
- The Defensive Duellist feat allows a character to spend their Reaction to functionally add their Proficiency Bonus to their AC for a single incoming attack. The character must be wielding a Finesse weapon they are Proficient in. (This is actually implemented by imposing a penalty to the incoming attack roll.)
- A few spells grants bonus AC: Barkskin, Shield of Faith, Shield(Spell), Mirror Image, and Haste. But all of these either don't last long, require concentration, or are otherwise limited in its bonus.
- Spells that decreases AC are even rarer: Slow can do this directly, but is subject to a saving throw; Chromatic Orb: Acid and Melf's Acid Arrow can achieve this via an Acid Surface, but can be swiftly negated when the victim leaves the surface.
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.