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| {{guidenote}} | | {{ 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]]. |
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| == 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}}
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| 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.
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| 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]]. |
| | ** 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. |
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| 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 each point of AC makes a creature normally 5% harder to hit.{{GameplayNavbox}} |
| : {{InfoBlob|13 + Dexterity modifier + shield bonus + other bonuses and penalties}}
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| {{SAI|Unarmoured Defence (Barbarian)}}:
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| : {{InfoBlob|10 + Constitution modifier + Dexterity modifier + shield bonus + other bonuses and penalties}}
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| {{SAI|Unarmoured Defence (Monk)}}:
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| : {{InfoBlob|10 + Wisdom modifier + Dexterity modifier + other bonuses and penalties}}
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| == Mathematics==
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| 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.
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| 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.
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| 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).
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| The difference between 25 and 24 is even greater, with a {{em|100%}} increase in effectiveness (50 vs 100 rounds).
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| == Footnotes ==
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| {{notelist}}
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| {{NavGameplay}}
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| [[Category:Gameplay mechanics]]
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