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{{ up to date | 2023-08-18 }} <!-- Date format is YEAR-MONTH-DAY, like 2023-08-07. -->[[Category:Gameplay Mechanics]]
'''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]].


== Formulas ==
The 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}}
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.
<code>Armour Class = Base Armour Class + [[Dexterity#Dexterity Modifier Chart|Dexterity Modifier]] + Other Bonuses and Penalties</code>
*Base Armour Class is the number seen in shields in the descriptions of armour. When unarmoured, 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]].
** 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 , 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.


Shields will grant the listed AC bonus to whomever equips it, regardless of which weapon slot they are currently using.
* 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
 
** [[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.
=== Other formulas ===
** Certain Clothing and Armours grants bonus AC outside of their Base AC. [[Potent Robe]] is an example of this.
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}}
** 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.
 
** 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.
{{SAI|Mage Armour}} or {{SAI|Draconic Resilience}}:
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.{{GameplayNavbox}}
: {{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).
 
== Footnotes ==
{{notelist}}
 
{{NavGameplay}}
 
[[Category:Gameplay mechanics]]
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