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 Dexterity modifier bonus may be limited or ignored if the character wears medium or heavy armour (see [[Armour Class#Armour and shields|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.

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

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

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

=== Armour and shields ===
The AC bonus from Dexterity modifier may be capped when wearing [[Armour#Medium armour|medium]] or [[Armour#Heavy armour|heavy]] armour.

Medium armour typically caps the Dexterity Modifier to +2.{{note|The [[Medium Armour Master]] feat increases the cap from +2 to +3.}}{{note|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]].}} 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 {{em|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 {{em|200%}} increase in effectiveness.

== Footnotes ==
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{{NavGameplay}}

[[Category:Gameplay mechanics]]