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The default formula that determines AC is:
The default formula that determines AC is:
: {{InfoBlob|10 + [[Dexterity|Dexterity modifier]] + armour bonus + shield bonus + other bonuses and penalties}}
: {{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]] or [[Armour#Heavy armour|heavy]] armour.
[[Armour#Medium armour|Medium]] armour caps the Dexterity Modifier to +2.{{ref|The [[Medium Armour Master]] feat increases the cap from +2 to +3.}}{{ref|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]].}} [[Armour#Heavy armour|Heavy]] armour ignores the modifier entirely.


Medium armour 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]].}} whrereas heavy armour ignores the modifier entirely.
Shields will grant the listed AC bonus to whoever equips them, regardless of which weapon slot they are currently using.
 
Shields will grant the listed AC bonus to whomever equips it, regardless of which weapon slot they are currently using.


=== Other formulas ===
=== 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.
Unarmoured creatures may use one of the following different formulas if they have certain features. Creatures always use whichever formula would result in a higher AC among those they have access to.{{ref|Alternative formulas are only used if {{em|no}} items are worn in the chest, hands, helm or boots slots which are labelled as armour. The Monk version also stops working if a shield is carried.|name=slots}}


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


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


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


== Math ==
== Mathematics==
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.
Armour Class becomes more useful the greater it is; for instance, 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.
To illustrate this, if a defender has 15 AC and 10 HP, and the attacker has +4 (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).
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).
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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).
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 (50 vs 100 rounds).
The difference between 25 and 24 is even greater, with a {{em|100%}} increase in effectiveness (50 vs 100 rounds).
 
== Items increasing Amour Class ==
{{Table alignment}}
{| class="wikitable defaultleft col3center col5center sortable"
!Equipment
!Slot
!AC Increase
!Effect
!Found in Act
|-
|{{LgRarityItem|Cloak of Protection}}
|{{Equipment slot|Cloak}}
| +1
|[[Armour Class]] +1
[[Saving throw]] +1
|2
|-
|{{LgRarityItem|Defender Flail}}
|{{Equipment slot|Main Hand Melee}}
| +1
|{{Passive|Steel Physiology}}
[[Armour Class]] +1
|2
|-
|{{LgRarityItem|Ring of Protection}}
|{{Equipment slot|Ring}}
| +1
|[[Armour Class]] +1
[[Saving throw]] +1
|1
|-
|{{LgRarityItem|Wondrous Gloves}}
|{{Equipment slot|Gloves}}
| +1
|{{Passive|Troubadour's Wonder}}
|1
|-
| [[Evasive Shoes]]
|[[Footwear]]
| +1
|
* [[Acrobatics]] +1
* [[Armour Class]] +1
|2
|-
|[[Armour of Moonbasking]]
|[[Armour]]
| +2
|
; [[Lunar Bestial Vitality]]
: You gain  22 [[Temporary Hit Points|temporary hit points]] after casting  [[Wild Shape]]. While those temporary hit points are active reduce all incoming damage by 1.
; [[Lunar Bestial Fortitude]]
: You have a +2 bonus to  [[Armour Class]]. You also have  [[Advantage]] on  [[Saving throw|Saving throws]] against spells. This effect persists while using your druidic  [[Wild Shape]] ability.
|3
|-
|[[Barkskin Armour]]
|[[Armour]]
|Flat 16 ([[Barkskin (Condition)|Barkskin]])
|
* [[Advantage]] on [[Constitution]]  [[Saving throw|Saving throws]].
 
; [[Forest Aegis]]
: You are invested with the power of the meadows and woods of the land, and have the effect of [[Barkskin (Condition)|Barkskin]], increasing your  [[Armour Class]] to 16.
|2
|-
|[[Unwanted Masterwork Scalemail|Unwanted Masterwork Scalemai]]
|[[Armour]]
|Full Dexterity [[Ability Modifier|Modifier]]
|
* [[Resistance]] to [[Fire]] damage.
 
; [[Exotic Material]]
: Add your full Dexterity [[Ability Modifier|Modifier]] to your  [[Armour Class]]. Additionally, this armour does not impose [[Disadvantage]] on Stealth [[Ability Check|Ability Checks]].
|3
|-
|[[Yuan-Ti Scale Mail]]
|[[Armour]]
|Full Dexterity [[Ability Modifier|Modifier]]
|
; [[Exotic Material]]
: Add your full Dexterity [[Ability Modifier|Modifier]] to your  [[Armour Class]]. Additionally, this armour does not impose  [[Disadvantage]] on Stealth [[Ability Check|Ability Checks]].
; [[Ambusher (+1)|Ambusher]]
: Gain a +1 bonus to  [[Initiative|Initiative Rolls]].
|2
|-
|[[Helm of Balduran]]
|[[Headwear]]
| +1
|
* Attackers can't land [[Critical Hit|Critical Hits]] on the wearer.
 
; [[Balduran's Vitality]]
: The helmet heals you 2hit points at the beginning of every turn.
; [[Balduran's Favour]]
: You have a +1 bonus to  [[Armour Class]] and  [[Saving throw|Saving throws]].
; [[Stun Immunity]]
: You can't be  [[Stunned (Condition)|Stunned]].
|3
|}


== Footnotes ==
== Notes ==
{{notebegin}}
{{reflist}}
{{notelist}}
{{notelist}}
{{noteend}}
{{NavGameplay}}
{{NavGameplay}}


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

Latest revision as of 13:20, 28 November 2024

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[edit | edit source]

The default formula that determines AC is:

10 + Dexterity modifier + armour bonus + shield bonus + other bonuses and penalties

Medium armour caps the Dexterity Modifier to +2.[1][2] Heavy armour ignores the modifier entirely.

Shields will grant the listed AC bonus to whoever equips them, regardless of which weapon slot they are currently using.

Other formulas[edit | edit source]

Unarmoured creatures may use one of the following different formulas if they have certain features. Creatures always use whichever formula would result in a higher AC among those they have access to.[3]

Mage Armour Mage Armour or Draconic Resilience Draconic Resilience:

13 + Dexterity modifier + shield bonus + other bonuses and penalties

Unarmoured Defence (Barbarian) Unarmoured Defence (Barbarian):

10 + Constitution modifier + Dexterity modifier + shield bonus + other bonuses and penalties

Unarmoured Defence (Monk) Unarmoured Defence (Monk):

10 + Wisdom modifier + Dexterity modifier + other bonuses and penalties

Mathematics[edit | edit source]

Armour Class becomes more useful the greater it is; for instance, the difference in effectiveness between 20 and 19 AC is 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 +4 (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 100% increase in effectiveness (50 vs 100 rounds).

Items increasing Amour Class[edit | edit source]

Equipment Slot AC Increase Effect Found in Act
Cloak of Protection Cloak Cloak +1 Armour Class +1

Saving throw +1

2
Defender Flail Main Hand Melee Main Hand Melee +1
Steel Physiology
Reduce incoming Damage TypesBludgeoning, Damage TypesPiercing, and Damage TypesSlashing damage by 1.

Armour Class +1

2
Ring of Protection Ring Ring +1 Armour Class +1

Saving throw +1

1
Wondrous Gloves Gloves Gloves +1
Troubadour's Wonder
Your Armour Class Armour Class increases by 1. In addition, if you have Bardic Inspiration Bardic Inspiration, you gain 1 more use of it.
 1
Evasive Shoes Footwear +1 2
Armour of Moonbasking Armour +2
Lunar Bestial Vitality
You gain 22 temporary hit points after casting  Wild Shape. While those temporary hit points are active reduce all incoming damage by 1.
Lunar Bestial Fortitude
You have a +2 bonus to  Armour Class. You also have Advantage on Saving throws against spells. This effect persists while using your druidic  Wild Shape ability.
 3
Barkskin Armour Armour Flat 16 (Barkskin)
Forest Aegis
You are invested with the power of the meadows and woods of the land, and have the effect of Barkskin, increasing your  Armour Class to 16.
 2
Unwanted Masterwork Scalemai Armour Full Dexterity Modifier
Exotic Material
Add your full Dexterity Modifier to your  Armour Class. Additionally, this armour does not impose Disadvantage on Stealth Ability Checks.
 3
Yuan-Ti Scale Mail Armour Full Dexterity Modifier
Exotic Material
Add your full Dexterity Modifier to your  Armour Class. Additionally, this armour does not impose Disadvantage on Stealth Ability Checks.
Ambusher
Gain a +1 bonus to  Initiative Rolls.
 2
Helm of Balduran Headwear +1
Balduran's Vitality
The helmet heals you 2hit points at the beginning of every turn.
Balduran's Favour
You have a +1 bonus to  Armour Class and Saving throws.
Stun Immunity
You can't be  Stunned.
 3

Notes[edit | edit source]

  1. ↑ The Medium Armour Master feat increases the cap from +2 to +3.
  2. ↑ 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.
  3. ↑ Alternative formulas are only used if no items are worn in the chest, hands, helm or boots slots which are labelled as armour. The Monk version also stops working if a shield is carried.
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