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Armour Class: Difference between revisions

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(an attack roll that equals the AC hits, so a +5 attack has 55% chance of hitting a 15 AC, not 50%. +4 is what works with the rest of the math here)
 
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{{ up to date | 2023-08-18 }} <!-- Date format is YEAR-MONTH-DAY, like 2023-08-07. -->[[Category:Gameplay Mechanics]]
{{guidenote}}
'''{{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]].
'''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]].


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}}
[[Armour#Medium 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]].}} [[Armour#Heavy armour|Heavy]] armour ignores the modifier entirely.


<code>Armour Class = Base Armour Class + [[Dexterity#Dexterity Modifier Chart|Dexterity Modifier]] + Other Bonuses and Penalties</code>
Shields will grant the listed AC bonus to whoever equips them, regardless of which weapon slot they are currently using.
*Base Armour Class is the number seen in shields in the descriptions of armour. When naked, a creature naturally has a Base AC of 10.
**[[Mage Armour]] sets the base armour class to 13 if the target isn't wearing any armour
*The AC bonus from Dexterity Modifier may be capped when wearing heavier types of [[Armour]].
** Clothing and Light Armour doesn't cap the Dexterity AC.
** Medium Armour typically allows adding up to +2 Dexterity Modifier to AC, although there are some armours with an "Exotic Material" trait that allow the wearer to get the full Dexterity bonus to AC. For example, [[Yuan-Ti Scale Mail]], [[Sharpened Snare Cuirass]], or [[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 are also relatively rare
=== Other formulas ===
** [[Barbarian]] and [[Monk]] have access to the [[Unarmoured Defence (Disambiguation)|Unarmoured Defence]] feature. Allowing them to add their Constitution Modifier and Wisdom Modifier to AC respectively.
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.{{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. The Monk version also stops working if a shield is carried.|name=slots}}
** Certain Clothing and Armours grants bonus AC outside of their Base AC. [[Potent Robe]] is an example of this.
 
** A few spells grants bonus AC: [[Shield of Faith]], [[Shield|Shield,]] [[Mirror Image]], and [[Haste]]. But all of these either don't last long, or requires concentration.
{{SAI|Mage Armour}} or {{SAI|Draconic Resilience}}:
** Spells that decreases AC is 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.
: {{InfoBlob|13 + Dexterity modifier + shield bonus + other bonuses and penalties}}
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 each point of AC makes a creature normally 5% harder to hit.{{GameplayNavbox}}
 
{{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; 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 +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 {{em|100%}} increase in effectiveness (50 vs 100 rounds).
 
== Footnotes ==
{{notelist}}
 
{{NavGameplay}}
 
[[Category:Gameplay mechanics]]

Latest revision as of 18:41, 7 October 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.[note 1][note 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.[note 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).

Footnotes[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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