Editing Armour Class

{{guidenote}}
'''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|attack roll]] must have a number equal to or greater than the target's Armour Class. AC can be increased by equipping [[Armour|armour]] and [[Shields|shields]], by selecting certain [[Feats|feats]] when leveling up, or utilizing certain [[Spells|spells]].

== Formulas ==
The default formula that determines AC is:
: {{InfoBlob|10 + [[Dexterity|Dexterity modifier]] + armour bonus + shield bonus + other bonuses and penalties}}
The AC bonus from Dexterity modifier may be capped when wearing [[Armour#Medium armour|medium]] and is ignored entirely when wearing [[Armour#Heavy armour|heavy]] armour.

Medium armour caps the Dexterity Modifier to +2,{{note|The [[Medium Armour Master]] feat increases the cap from +2 to +3.}}{{note|Four rare armours have an "Exotic Material" trait that allow the wearer to get the full Dexterity bonus to AC, namely [[Yuan-Ti Scale Mail]], [[Unwanted Masterwork Scalemail]], [[Sharpened Snare Cuirass]], and [[Armour of Agility]].}} whereas 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 one of the following different formulas if they have certain features. Creatures always use whichever formula – which they have access to – would result in a higher AC.{{note|Alternative formulas are only used if {{em|no}} items are worn in the chest, hands, helm or boots slots which are labelled as armour.|name=slots}}

{{SAI|Mage Armour}} or {{SAI|Draconic Resilience}}:
: {{InfoBlob|13 + Dexterity modifier + shield bonus + other bonuses and penalties}}

{{SAI|Unarmoured Defence (Barbarian)}}:
: {{InfoBlob|10 + Constitution modifier + Dexterity modifier + shield bonus + other bonuses and penalties}}

{{SAI|Unarmoured Defence (Monk)}}:
: {{InfoBlob|10 + Wisdom modifier + Dexterity modifier + other bonuses and penalties}}

== Mathematics==
Armour Class becomes more useful the greater it is – the difference in effectiveness between 20 and 19 AC is {{em|greater}} than the difference in effectiveness between 16 and 15.

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 {{em|100%}} increase in effectiveness (50 vs 100 rounds).

== Ways to increase AC ==
* Certain clothing and armours grant bonus AC outside of their Base AC. [[Potent Robe]] is an example of this.
* [[Fighter|Fighters]], [[Paladin|Paladins]], and [[Ranger|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 (Resource)|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 [[Proficiency|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|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 ==
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[[Category:Gameplay mechanics]]