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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 AC is, the better.

Note: Item and spell descriptions will list AC bonuses as positive values, but they are properly counted 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. 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 inequation:

Armor Class < THAC0 - Attack roll

 

How Armor Class is calculated[]

The various ways to alter AC can be sorted in four categories:

  1. Base AC and General AC modifiers, which are combined to form an initial AC score with a minimum of -20 and a maximum of 20
  2. Uncapped general AC modifiers, which only include Dexterity and Single-Weapon style.
  3. Damage-specific AC modifiers, only active against a single type of Physical damage, for example Slashing. Combined, they provide up to a -20 bonus or a +20 penalty vs that damage type.
  4. Uncapped damage-specific AC modifiers, which only include Sword & Shield style.

The sum of #1 and #2 provides the AC value written in the shield icon of the Inventory screen. (Meaning this value can only ever be as low as -20-6-2 = -28.)

Final AC though, which is the final AC value used in the to-hit formula, also includes #3 and #4. (Meaning it could theoretically go as low as -26-20-4 = -50 vs Missile damage and -28-20 = -48 vs other Physical damage.)

AC formula[]

Final AC =
  CAP( Base AC + General AC modifiers ) IN [-20, 20]
  + Dexterity modifier
  + Single-Weapon style bonus
  + CAP( Damage-specific AC modifiers ) IN [-20, 20]
  + Sword & Shield style bonus

Minor notes:

  • The first part computes the value displayed in the Inventory screen's shield icon. It also appears as AC in the Record screen's Combat Stats tab.
  • The second part is comprised of defender AC bonuses that are included as attacker THAC0 penalties (or AC penalties included as THAC0 bonuses) in the second value of the Combat log's attack report.

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 The final Armor Class value as used in the to-hit formula
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: ...

Damage-specific AC modifiers The sum of all AC modifiers vs Slashing, Crushing, Piercing or Missile damage. The defender's Final AC is altered by such a modifier only against weapons that deal the relevant damage type.

Armors often provide a Base AC and a series of Damage-specific AC modifiers. For example Full Plate Armor, on its own, provides a total AC of 1 against weapons that deal Crushing damage — since it doesn't provide extra AC vs Crushing — but of -3 vs Slashing damage — thanks to its large -4 bonus vs Slashing.
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 -4 bonus vs Missile damage.
Appears as:

 Sword And Shield Style
 Missile Adjustment: ...


Despite being separate from Damage-specific AC modifiers, the game interface also includes it as a Missile bonus under Armor Class Modifiers, so don't count it twice.

 

Altering the THAC0 of incoming attacks[]

In four cases, a characteristic of the defender will affect the THAC0 of incoming attacks. Of those, only the Blindness penalty will be displayed in the Inventory screen's AC section and the Record screen's Combat Stats tab.

Improved invisibility bonus A defender that benefits from the Improved type of Invisibility effect — as granted by e.g. Mass Invisibility or Mislead, but not Stealth or Potion of Invisibility — will inflict a +4 THAC0 penalty to all physical attacks against them. Attacks from creatures able to see the invisible are exempted from this penalty.
Protection from creature type bonus A defender protected from certain types of opponents — thanks to e.g. Protection From Evil or Spirit Ward, but not Protection From Undead — will inflict a +2 THAC0 penalty to all physical attacks against them if they're coming from the right type of attackers.
Unarmed or ranged penalty in melee A defender not wielding a weapon (Monk excepted) or wielding a ranged weapon will grant a -4 THAC0 bonus to all melee attacks against them
Enhanced Editions two letters logo.pngBaldur's Gate: Enhanced Edition Trilogy
This icon indicates content from all Enhanced Editions of the Baldur's Gate games.
 Blindness penalty
A defender suffering from the Blindness effect will grant a -4 THAC0 bonus to all physical attacks against them, in Enhanced Editions only.
Appears as Blind: +4

Minor note: All four modifiers will be included in the second value of the Combat log's attack report.

 

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-specific AC modifiers Gorgon Plate -6
Golden Girdle of Urnst -3
Total Slashing-specific -9
Total Slashing-specific once capped -9
Uncapped damage-specific 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.

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.

 

Altering Base AC[]

Permanent[]

  • Monk's natural AC improves with level

Potions[]

Spells[]

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 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 the High Forest -1
Cloak Cloak of the Shield -1 vs melee, -5 vs missile
Cloak Cloak of Protection +1 -1 Y
Cloak Cloak of the Dark Moon -2
Cloak Cloak of the Sewers -1
Cloak Improved Cloak of Protection -2 Y
Cloak Montolio's Cloak -1
Cloak The Spirit's Shield -1 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
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.

Achievements[]

Main article: Achievements
Untouchable Achievement icon BG1EE.jpg

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.jpg

Untouchable BG2[]

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

See also[]