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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.

Note: Item and spell descriptions will list AC bonuses as positive values, but the game does count them as negative, as can be seen in the Inventory screen. When it comes to descriptions, it's simpler to focus on the word bonus or penalty rather than the sign.

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.

Note: To-hit roll is another common name for the attack roll.

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.

Note: Type-exclusive AC and Situational AC are both included in the second value of the Combat log's attack report. The lower that value, the better it is for the defender.

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.

Note: The shield-shaped icon in the game's interface will not display anything better than -28; this is only the theoretical maximum for the General AC component.

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
Note: The first block is General AC, the second one is Type-exclusive AC, and last one is Situational AC.

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[]

Screenshots of the Inventory and Report screens for this rundown
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[]

  • Monk's natural AC improves with level (up to -6 at level 40)

Potions[]

Spells[]

Innate abilities[]

Equipment[]

AC Bonuses[]

Permanent[]

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[]

Spells[]

Arcane Magic:

Divine:

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.

Abilities[]

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.

Achievements[]

Main article: Achievements
Untouchable Achievement icon BG1EE

Untouchable BG1[]

Have an Armor Class of -15 or lower during Baldur's Gate: Enhanced Edition or Baldur's Gate: Siege of Dragonspear.

Untouchable Achievement icon BG2EE

Untouchable BG2[]

Have an Armor Class of -15 or lower during Baldur's Gate II: Enhanced Edition.

See also[]

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