Armor Class

A creature's Armor Class, or AC, represents its ability to dodge, block or parry physical blows without getting hurt. It stands opposite to an attacker's THAC0, which describes the accuracy of a creature's physical attacks.
*Should the attacker win a pass through the process of an attack roll, the defender's Armor Class will be "defeated" and a damaging blow will occur. One of the next defences — physical damage resistance — affects how much damage is going to be dealt; this is not the purpose of Armor Class.
*Should the defender successfully dodge, block or parry, the attack will simply be considered a miss.
Like THAC0, the lower the AC is, the better.
==To-hit formula==
An attacker is able to perform up to 10 attacks per round. For each attack, they roll a 20-sided die whose result will support their THAC0 in trying to overcome the defender's Armor Class, so that they can land a blow. However, regardless of AC and THAC0, rolling a 1 always translates to a miss, and rolling a 20 always translates to a critical hit. A roll that is neither a critical hit or miss will result in a miss if AC is good enough — low enough — to validate this inequality:
 Armor Class < THAC0 - Attack roll
In other words, if the value of the die is not big enough to make the THAC0 fall as low as or lower than AC, the attack roll has failed and the defender wins the pass.
==The four types of AC==
As far as game mechanics go, there are four types of physical damage and four types of AC to match: AC vs Slashing, vs Piercing, vs Crushing, and vs Missile damage. When an attacker is attempting to strike with a weapon, its physical damage type will determine which AC will need to be overcome. The other 3 will be ignored.
When it comes to game interfaces, this reality is not advertised as straightforwardly, preferring to talk about an AC value and then about a type modifier. This often leads players to misestimate the value of armors with a large modifier.
==Breakdown of the AC value==
Each AC type is made of three parts later called General AC, Type-exclusive AC and Situational AC.
===General AC===
Advertised prominently in game interfaces and descriptions, this intermediate value contributes to all 4 types of AC. It is the sum of the following:
*Base AC and General AC modifiers, which are combined to form an initial score with a minimum of -20 and a maximum of 20
*Uncapped General AC modifiers, which come on top and only include Dexterity and Single-Weapon style, with the Blindness effect as well in Enhanced Editions.
In Enhanced Editions, General AC is displayed in the shield-shaped icon of the Inventory screen and in the Record screen's Combat stats tab as . In original editions, the shield-shaped icon is present in both the Inventory and the Record screen, which does not have tabs.
===Type-exclusive AC===
Often advertised timidly in-game, this is the second component used in the calculation of each type of AC. As the name suggests, Type-exclusive AC contributes to only one type – so there's actually Slashing-only AC, Piercing-only AC and so on. 
Slashing-only AC, to take it as an example, is the sum of the following:
*Slashing-only AC modifiers, only active against slashing damage. Combined, they provide up to a -20 bonus or a +20 penalty vs slashing weapons.
*Uncapped Slashing-only AC modifiers, which come on top and only include Sword & Shield style. That style only grants a Missile-only AC modifier though.
In Enhanced Editions, one needs to scroll down in the top-right textbox of the Inventory screen to find Type-exclusive AC. It is also present in the Record screen's Combat stats tab as . In the original editions, the values are only listed in the Record screen and found by scrolling two-thirds down.
===Situational AC===
Like General AC modifiers, Situational AC contributes to all 4 types of AC. But it is comprised of modifiers that only affect the to-hit roll when the attacker meets a particular criterion, and stay dormant and ignored the rest of the time. The Inventory and Record screens make no mention of this component of Armor Class.
Situational AC is the sum of the following:
*Improved type of invisibility, a -4 bonus only kicking in against enemies unable to see the invisible.
*Protection versus creature type, a -2 bonus only kicking in against the right type of attackers. It is almost exclusively Evil alignments that are targeted.
*Unarmed or ranged weapon penalty, a +4 penalty only kicking in when the attacker uses a melee weapon.
===Maximum AC by type===
In theory, the best AC is -48 for each type, except for AC vs Missiles where it's -50. Situational AC can boost these maxima down to respectively -54 and -56 if the attacker meets the right conditions.
In practical terms though, a defender's AC only needs to be 20 points below their attacker's THAC0 to ensure that only critical hits will ever connect. Keep in mind it's also possible to inflict penalties to an opponent's THAC0 with for example these spells — this is equivalent to a bonus to defender AC.
==AC formula==
Each of the four types of AC is calculated like so:
 Final AC vs type =
 CAP( Base AC + General AC modifiers ) IN [-20, 20]
   + Dexterity modifier
 + Single-Weapon style bonus
 + Blindness penalty in EE
 + CAP( Type-exclusive AC modifiers ) IN [-20, 20]
 + Sword & Shield style bonus
 + Improved Invisibility bonus
 + Protection vs. creature type bonus
 + Unarmed or ranged weapon penalty vs. melee attacks
===Legend===
Each element from the AC formula is described below. The game interface will display most of them in both the Inventory screen's top-right corner and the Record screen's Combat Stats tab. The “Appears as” cells in the table below describe how the game interface names the various elements.
CAP( x ) IN [-20, 20]	x cannot go below -20 or above 20
Final AC vs type 	The final Armor Class value as used in the to-hit formula. The damage type of the attacker's weapon decides which of the 4 types of AC is going to be used in the attack roll.
General AC 	General AC
Base AC 	The defender's AC before any bonuses or penalties have been applied. For example Full Plate Armor and Spirit Armor both set Base AC to 1.By default this value is 10 for non-Monk player characters, but many creatures are nimble, have thick skin or even a shell, granting them a lower "natural" default. This value can't be higher than 10. When several sources set Base AC to different values, the best value wins.
Base AC 	Appears as:
 Item_name
 Armor Class: ...
The first line is only displayed if Base AC was set by an item.
General AC modifiers 	The sum of all damage-agnostic AC modifiers currently affecting the defender — they work against all weapons. For example, Helm of the Noble provides such a modifier, but not Destroyer of the Hills. Dexterity and Single-weapon style modifiers are counted separately.
General AC modifiers 	Appears as, either/or:
Item_name: ...
Ability Bonuses: ...
Dexterity modifier 	The modifier conferred by the defender's Dexterity, such as a -4 AC bonus at DEX 18.
Dexterity modifier 	Appears as Dexterity: ...
Single-Weapon style bonus 	AC bonus from the Single-Weapon fighting-style proficiency, active when the defender is wielding a one-handed melee weapon with an empty off-hand. It can be a -1 or a -2 bonus.
Single-Weapon style bonus 	Appears as:
 Single-Weapon Style
 Armor Class: ...
Blindness penalty in EE 	A defender suffering from the Blindness effect receives a +4 AC penalty in Enhanced Editions only.
Blindness penalty in EE 	Appears as Blind: +4
Type-exclusive AC 	Type-exclusive AC
Type-exclusive AC modifiers 	The sum of all Slashing-only AC modifiers, or Crushing-only, Piercing-only or Missile-only. These modifiers only take part in the to-hit roll if the attacker uses a weapon that deals the relevant damage type.Armors often provide a Base AC and a series of Type-exclusive AC modifiers. For example Full Plate Armor, on its own, provides a total AC vs Crushing of 1 since it doesn't have any Crushing-only AC modifier. But its total AC vs Slashing reaches -3 thanks to its large -4 Slashing-only bonus. This armor is a lot easier to hit with Crushing weapons.
Type-exclusive AC modifiers 	Appears as:
 Armor Class Modifiers
 Crushing: ...
 Slashing: ...
 Piercing: ...
 Missile: ...
Sword & Shield style bonus 	AC bonus from the Sword & Shield fighting-style proficiency, active when the defender is wielding a one-handed melee weapon and a shield. It can be a -2 or a -4 Missile-only AC bonus.
Sword & Shield style bonus 	Appears as:
 Sword And Shield Style
 Missile Adjustment: ...
Even though it is already listed independently, the game interface also includes it as a Missile bonus under Armor Class Modifiers, so don't count it twice.
Situational AC 	Situational AC
Improved invisibility bonus 	-4 AC bonus against creatures unable to see the invisible. The Improved type of Invisibility effect is granted by e.g. Mass Invisibility or Mislead, but not Stealth or Potion of Invisibility.
Improved invisibility bonus 	Hidden from game interfaces.
Protection from creature type bonus 	-2 AC bonus against the right type of attackers — thanks to e.g. Protection From Evil or Spirit Ward, but not Protection From Undead. Only one such bonus is counted per attack roll, even if the attacker matches more than one criterion.
Protection from creature type bonus 	Hidden from game interfaces.
Unarmed or ranged penalty in melee 	+4 AC penalty against attackers that use a melee weapon. Unarmed Monks are exempted from this penalty.
Unarmed or ranged penalty in melee 	Hidden from game interfaces.
==Rundown of a large Armor Class==
Fighter / Thief with Use Any Item vs Slashing attacks
Base AC 	Gorgon Plate 	-1
General AC modifiers 	Amulet of the Master Harper 	-3
General AC modifiers 	Cloak of the Dark Moon 	-2
General AC modifiers 	Wondrous Gloves 	-1
General AC modifiers 	Helm of Balduran 	-1
General AC modifiers 	Ring of Gaxx 	-2
General AC modifiers 	Ring of Earth Control 	-1
General AC modifiers 	Axe of the Unyielding 	-1
General AC modifiers 	Scroll of Blur 	-3
General AC modifiers 	Greater Evasion 	-6
Total Base + General 	Total Base + General 	-21
Total Base + General once capped 	Total Base + General once capped 	-20
Uncapped general AC 	DEX 24 with Potions 	-6
Uncapped general AC 	Single-Weapon style ++ 	-2
Total AC displayed in the Inventory screen's shield icon	Total AC displayed in the Inventory screen's shield icon	-28
Slashing-only AC modifiers 	Gorgon Plate 	-6
Slashing-only AC modifiers 	Golden Girdle of Urnst 	-3
Total Slashing-only 	Total Slashing-only 	-9
Total Slashing-only once capped 	Total Slashing-only once capped 	-9
Uncapped Slashing-only AC 	Sword & Shield style ++ 	 0
Final AC vs Slashing damage 	Final AC vs Slashing damage 	-37
Final AC vs Slashing could be further improved against all Slashing attacks coming from Evil-aligned creatures that cannot see the invisible: Should the Fighter / Thief use Improved Invisibility from the Improved Cloak of Protection and double down with Protection From Evil, they would have the equivalent of an AC of -43.
== Altering Base AC ==
=== Permanent ===
* A Monk's natural AC improves with level (up to -6 at level 40)
=== Potions ===
* Potion of Defense (AC 0)
* Potion of Invulnerability (AC 0)
* Potion of Stone Form (AC 0)
=== Spells ===
* Arcane: Armor (AC 6), Shield (AC 4), Ghost Armor (AC 2), Spirit Armor (AC 1), Polymorph Self and Shapechange (AC depends of the chosen form)
* Divine: Barkskin (level dependent)
=== Innate abilities ===
* Shapeshift (AC 7 or 6 for normal Druids, AC 2 or 3 for Avenger) and Shapeshift: Werewolf (AC 1 for Werewolf, -6 for Greater Werewolf)
* Elemental Transformation (Earth) and Elemental Transformation (Fire) (AC -5)
* Slayer Form (AC -4)
=== Equipment ===
*Items worn in the armor slot.
* The various Bracers of Defense:
** Bracers of Defense AC 8
** Bracers of Defense AC 7
** Bracers to the Death (AC 6)
** Bracers of Defense AC 5
** Bracers of Defense AC 4
** Bracers of Defense AC 3
*Cloak of the Sewers, used to polymorph, AC depends of the creature type.
*Staff of the Woodlands (AC 3)
* Shakti Figurine (AC -5)
== AC Bonuses ==
=== Permanent ===
* Kensai, Swashbuckler, and Dragon Disciple kits boost AC by modifiers
* Dexterity adjustment
* Test of Selfishness evil route boosts AC by -2
===From Weapon Proficiency===
* Single Weapon Style proficiency, when wielding a single one-handed melee weapon
* Sword and Shield Style proficiency, bonus AC vs. missile when wielding a melee weapon with a shield
=== Potions ===
* Potion of Absorption (-10 vs. crushing attacks)
* Potion of Mind Focusing (+3 dexterity)
=== Abilities ===
* Class abilities: Skald song (-2 to -4), Defensive Spin (-1 to -10, depending on level), Enrage (-2), Chaos of Battle (25% chance of -1 to -5)
* HLAs: Enhanced Bard Song (-10 for Bard, -4 for companions), Evasion (-4), Greater Evasion (-6)
=== Spells ===
Arcane Magic:
*Blur (-3), 
*Tenser's Transformation (-4),
*Protection From Evil (-2, or 2 thaco malus for enemies, which is identical, except when AC is capped)
*Improved Invisibility and Mass Invisibility (-4 THAC0 penalty for enemies that don't see through invisibility)
Divine:
*Defensive Harmony (-2)
*Aura of Flaming Death* (-4)
*Protection From Evil 
*Protection From Evil, 10' Radius
*Draw Upon Holy Might (raises dexterity)
*Physical Mirror protects against missiles
There are approaches other than Armor Class to be protected against physical attacks: Reflected Image and Mirror Image will absorb one/some hits, Stoneskin and Iron Skins will absorb only the physical damage of some hits, mantles will make you immune to non magical weapons (Protection From Normal Weapons; Aura of Flaming Death) magical weapons (Protection From Magical Weapons); weapons with an enchantment level inferior to 3 (Mantle); 4 (Improved Mantle); 6 (Absolute Immunity).
There is also damage resistance approach (Armor of Faith, Damage Resistance).
Some effects affecting the defender will essentially disable their AC, allowing the attacker to hit without an attack roll: Time Stop, Hold, Sleep, Stun, ... See the Helpless article for more information.
=== Equipment ===
Shields offer a +1 or greater bonus. They are numerous and haven't been listed.
The following equipment offers AC bonuses that can improve a creature's base AC. Items marked as restricted cannot be worn together or with most magical armors. See the section Item restrictions for more details.
Slot 	Item 	AC Bonus	Damage Specific Bonus 	Restricted
Amulet 	Amulet of Protection +2 	-2 	 	Y
Amulet 	Amulet of the Master Harper 	-3 	 	
Amulet 	Heart of the Mountain 	-2 	 	
Amulet 	The Protector 	-1 	 	Y
Amulet 	Wooden Horse Necklace 	-2 	 	Y
Belt 	Destroyer of the Hills 	 	-4 vs Crushing 	
Belt 	Elves' Bane 	 	-3 vs missile and piercing	
Belt 	Golden Girdle of Urnst 	 	-3 vs slashing 	
Belt 	Ox-Tail Belt 	 	-4 vs missile attacks 	
Boots 	Boots of Phasing 	-2 	 	
Boots 	Moonlight Walkers 	-2 	 	
Boots 	Senses of the Cat 	 	-5 vs missile attacks 	
Cloak 	Cloak of Balduran 	-1 	 	
Cloak 	Cloak of Displacement 	 	-4 vs missile attacks 	
Cloak 	Cloak of Protection +1 	-1 	 	Y
Cloak 	Cloak of the Dark Moon 	-2 	 	
Cloak 	Cloak of the High Forest 	-1 	 	
Cloak 	Cloak of the Sewers 	-1 	 	
Cloak 	Cloak of the Shield 	 	-1 vs melee, -5 vs missile	
Cloak 	Improved Cloak of Protection	-2 	 	Y
Cloak 	Montolio's Cloak 	-1 	 	
Cloak 	The Spirit's Shield 	-2 	 	Y
Gauntlets	Gloves of Missile Snaring 	 	-2 vs missile attacks 	
Gauntlets	The Winged's Gauntlet 	 	-5 vs all melee attacks 	
Gauntlets	Tzu-Zan's Bracers 	-1 	 	
Gauntlets	Wondrous Gloves 	-1 	 	
Helmet 	Dusty Rose Ioun Stone 	-1 	 	
Helmet 	Helm of Balduran 	-1 	 	
Helmet 	Helm of the Noble 	-1 	 	
Helmet 	Lavender Ioun Stone 	-1 	 	
Helmet 	Wong Fei's Ioun Stone 	-1 	 	
Helmet 	Vhailor's Helm 	-1 	 	
Ring 	Koveras's Ring of Protection	-1 	 	Y
Ring 	Ring of Earth Control 	-1 	 	
Ring 	Ring of Gaxx 	-2 	 	
Ring 	Ring of the Princes 	-1 	 	Y
Ring 	The Claw of Kazgaroth 	-1 	-4 vs missile 	
Ring 	The Guard's Ring 	-2 	 	Y
Ring 	The Warder's Signet 	-3 	 	Y
Weapons
Type 	Item 	AC Bonus	Damage Specific Bonus
Axe 	Axe of the Unyielding	-1 	
Dagger 	Moonblade +3 	-1 	
Flail 	Defender of Easthaven	-1 	
Katana 	Corthala Family Blade	 	-2 vs slashing attacks
Katana 	Dak'kon's Zerth Blade	-1 	
Katana 	Malakar 	 	-2 vs slashing attacks
Quarterstaff	Staff of Curing 	-2 	
Quarterstaff	Staff of Power 	-2 	
Quarterstaff	Staff of the Magi 	-2 	
Scimitar 	Twinkle 	-2 	
Wakizashi 	Bartleby's Wakizashi 	 	Kensai only: -2 vs slashing attacks
Wakizashi 	Yamato 	-1 	
==Item restrictions==
Ideally, we want to stack AC bonuses from as many items as possible, but in reality many of the items may not be worn together.  Based on ITEMEXCL.2da, you cannot combine any of the items from the following sections.
==== Armor ====
==== Amulets ====
==== Rings ====
==== Cloaks ====
== BGWiki writing formats ==
Due to inconsistency in how AC changes are described in-game, BGW will unify the format by writing -n bonus and +n penalty for modifiers and list the exact subdivided AC values (vs. slashing, crushing, piercing and missile) instead of their modifiers.
== Mod content ==
With the Tweaks Anthology mod, the restrictions on protection items could be relaxed. An option is to allow the use of magical armor with one protection item.
The Spell Revisions mod contains many spells that penalize an opponent's armor class rating. In addition, there are several boosts to armor class or specific damage type armor class bonus inducing spells provided with that mod installed, for arcane and divine casters, above and beyond the standard game's spell system.
Perhaps the greatest armor class boosting spell, from any modification so far, can be realized as the replacement spell for Improved Mantle, known as "Moment of Prescience". See the Improved Mantle page and Mod content section for a description.
== Achievements ==
===Untouchable BG1===
Have an Armor Class of -15 or lower during Baldur's Gate: Enhanced Edition or Baldur's Gate: Siege of Dragonspear.
===Untouchable BG2===
Have an Armor Class of -15 or lower during Baldur's Gate II: Enhanced Edition.
== See also ==
* Armor
* THAC0
* Saving throw
Category:Game mechanics