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 defender's AC is so low that the value of the attacker's die cannot make their THAC0 fall far enough to reach 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.

Note:	When an AC bonus or penalty is mentioned, it affects all 4 types of AC unless explicitly stated otherwise.

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 AC: [General value] (modifiers: ...). 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 AC: ... (modifiers: [Type-exclusive ACs listed one by one]). 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
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.
	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.
	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.
	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.
	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.
	Appears as Blind: +4
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.
	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.
	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
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.
	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.
	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.
	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
	Cloak of the Dark Moon	-2
	Wondrous Gloves	-1
	Helm of Balduran	-1
	Ring of Gaxx	-2
	Ring of Earth Control	-1
	Axe of the Unyielding	-1
	Scroll of Blur	-3
	Greater Evasion	-6
Total Base + General		-21
Total Base + General once capped		-20
Uncapped general AC	DEX 24 with Potions	-6
	Single-Weapon style ++	-2
Total AC displayed in the Inventory screen's shield icon		-28
Slashing-only AC modifiers	Gorgon Plate	-6
	Golden Girdle of Urnst	-3
Total Slashing-only		-9
Total Slashing-only once capped		-9
Uncapped Slashing-only AC	Sword & Shield style ++	0
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 Barkskin (AC 3)
• 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
  • 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)
• Earth Elemental Transformation and Fire Elemental Transformation (AC -5)
• Slayer Form (AC -4)

Item charges

• Shield Amulet (AC 4, casts Shield)

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.
• Jester's Bracers (AC 5)
• 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 Agility (dexterity set to 18)
• Potion of Mind Focusing (+3 dexterity)
• Violet Potion (dexterity set to 3)

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:

• Shield (-2 vs. missile attacks)
• 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.

Item charges

• Cloak of the Shield (-1 vs. crushing, piercing, and slashing attacks, and -5 vs. missile attacks)
• Shield Amulet (-2 vs. missile attacks, casts Shield)
• The Horn of Kazgaroth (-5 vs. missile attacks)

Weapon hits

• Biter (+1 to target for 1 turn, 15% chance per hit, save vs. Breath, cumulative, bypasses magic resistance)
• Sundermaul (+1 to target for 1 turn, 15% chance per hit, save vs. Spell, cumulative, bypasses magic resistance)
• The Answerer (+2 to target for 4 rounds, on each hit, no save, cumulative, bypasses magic resistance)

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.

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

Main article: 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