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{{ up to date | 2023-08-18 }} <!-- Date format is YEAR-MONTH-DAY, like 2023-08-07. -->
'''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]].
'''{{ArmourClass}}''' ('''AC''') is a measurement of how difficult a creature is to be hit by an attack. AC can be increased by equipping [[Armour]] and [[Shields]], by selecting certain [[Feats]] when leveling up, or utilizing certain Magic [[Spells]].


The formula that determines AC is:
== 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]]).


<center>Armour Class = {{InfoBlob|Base Armour Class + [[Dexterity#Dexterity Modifier Chart|Dexterity Modifier]] + Other Bonuses and Penalties}}</center>
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.


*Base Armour Class is the number seen in shields in the descriptions of armour. When unarmoured, most creatures naturally have a Base AC of 10.
{{SAI|Mage Armour}} and {{SAI|Draconic Resilience}}:
**[[Mage Armour]] and [[Draconic Resilience]] both sets the base armour class to 13 if the creature isn't wearing any armour
: {{InfoBlob|13 + Dexterity modifier + shield bonus + other bonuses and penalties}}
*The AC bonus from Dexterity Modifier may be capped when wearing heavier types of [[Armour]].
** Light Armour doesn't cap the Dexterity AC. Clothing doesn't either, but they don't count as armour at all.
** Medium Armour typically caps the Dexterity Modifier to +2 
*** 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]].
** Heavy Armour ignores any bonus (or penalty) from the wearer's Dexterity.


* The final term, "Other Bonuses and Penalties" is a very valuable source of AC, it is subject to the least amount of restrictions, but also are relatively rare, demands certain trade offs, or otherwise require the expenditure of [[Resource|Resources]].
{{SAI|Unarmoured Defence (Barbarian)}}:
** [[Shields]] will grant its listed AC bonus to whomever equips it. This means the user's off hand will be occupied, denying the bonus they would get for two-handing a versatile weapon, and wouldn't be able to equip a two-handed weapon or an off-hand weapon.
: {{InfoBlob|10 + Constitution modifier† + shield bonus + other bonuses and penalties}}
** Certain Clothing and Armours grant bonus AC outside of their Base AC. [[Potent Robe]] is an example of this.
 
** [[Barbarian|Barbarians]] and [[Monk|Monks]] have access to the [[Unarmoured Defence (Disambiguation)|Unarmoured Defence]] feature. Allowing them to add their Constitution Modifier and Wisdom Modifier to AC respectively, but only when unarmoured.
{{SAI|Unarmoured Defence (Monk)}}:
** [[Fighter|Fighters]], [[Paladin|Paladins]], and [[Ranger|Rangers]] have access to the '''Defense [[Fighting Style]]''' feature, which grants +1 AC if they are wearing armour.
: {{InfoBlob|10 + Wisdom modifier† + shield bonus + other bonuses and penalties}}
** 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.
=== Armour and shields ===
** 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.
The AC bonus from Dexterity modifier may be capped when wearing [[Armour#Medium armour|medium]] or [[Armour#Heavy armour|heavy]] armour.
In order to successfully hit a creature, the [[Attack Roll]] must have a number ''equal to'' or ''greater than'' the target's Armour Class. This means, typically, each point of AC makes a creature 5% harder to hit.{{NavGameplay}}
 
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 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.
 
== 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 ==
{{notelist}}
{{NavGameplay}}


[[Category:Gameplay mechanics]]
[[Category:Gameplay mechanics]]
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