The formula for calculating a creature's AC rating is:
effective AC = min(20, max(-20, base AC + general modifiers)) + dexterity modifier + weapon proficiency bonus
- effecive AC is the AC value used by the THAC0 formula to determine if an attack is successful or not;
- base AC is the character's AC before any bonuses or penalties have been applied;
- general modifiers is the sum of all non-damage-specific AC modifiers currently affecting the character;
- the min/max expression is limiting that portion of the equation to the range of -20 to +20;
- dexterity modifier is the bonus (or penalty) conferred by the character's dexterity;
- weapon proficiency bonus is the bonus conferred by the single-weapon fighting-style weapon proficiency.
The sections which follow will discuss these terms in more detail.
Base AC Edit
The base AC is typically determined by a character's armor. A character wearing standard leather armor, for example, would have a base of 8. The base AC value can be set by various pieces of gear and by some potions and spells. The Altering Base AC section lists all the items capable of establishing a character's base AC value.
If a creature's base armor class isn't being set by any gear or effect, the creature's natural AC rating is used as the base. Natural AC ratings vary widely from creature to creature; a red dragon might have a natural AC of -11 (due to its scales) while goblins typically have a natural AC of 7. Characters (except for monks) have a natural AC of 10.
When a character has multiple items or effects that could potentially establish their base AC, the one with the best value is used. For example, if Edwin is wearing a Robe of Invocation, his base AC will be 10 (his natural AC), because those robes offer no protection from physical damage. If he upgrades to Robe of Vecna his base AC would become 5. If he equips Bracers of Defense AC 3, the base will change from 5 to 3. If he then quaffs a Potion of Defense his base AC will become 0. Once the potion wears off, his base will revert back to 3.
A character's effective AC is often the result of various bonuses and penalties altering the base AC. Continuing with the Edwin example from before, if he equips the Ring of Gaxx (-2) and Staff of the Magi (-2), his effective AC would become:
3 + (-2 + -2) = -1. If an enemy successfully casts Blindness on him, his AC would be:
3 + (-2 + -2 + 4) = 3.
Most modifiers cannot raise the effective AC above 20 or lower it below -20. There are two exceptions: the bonuses from dexterity and the single-weapon fighting-style proficiency. If a character with an effective AC of -20 and a dexterity of 14 (which offers no dex bonus) equips the Staff of the Magi (-2 bonus), their effective AC will be unchanged. If that same character manages to increase their dexterity to 15 (-1 bonus), their effective AC will be reduced to -21. If that same character invests a point into the single-weapon fighting-style (and leaves the offhand slot empty) their effective AC will become -22.
Targets with an AC less than 1 can still be hit for a couple of reasons. An attacker's THAC0 bonuses act like AC penalties, so an attacker with a +5 THAC0 trying to strike a target with an effective AC of -3 is the same as an attacker with no bonus trying to strike a target with an effective AC of 2. Even if the target has an extremely low effective AC they are still subject to Critical hits; a critical hit ignores the target's AC, no matter how low it is. Targets immune to critical hits are just immune to the additional damage; the hit will still succeed at dealing normal damage. That means attacks always have a chance of hitting their targets, depending on the attacker's critical hit chance (which is typically between 5% and 10%).
There are various types of physical damage (e.g., crushing, piercing, etc.). It is possible for a bonus or penalty to only apply to one type of damage. For example, the Destroyer of the Hills girdle offers a -4 AC bonus but only against crushing damage. In fact, it's very common for armor to have bonuses and penalties against different types of damage. Chain mail armor, for example, has a base AC value of 5 with a +2 penalty against crushing damage and a -2 bonus against slashing. If a modifier doesn't specify a damage type, it's understood to apply to all damage types.
User Interface Edit
Within the game, detailed information about a character's armor class can be found on the Inventory screen and on the Conbat Stats tab of the Character Record screen. On the inventory screen, the armor class information is located in the upper-right corner. The value inside the shield icon is the character's effective AC. The panel to the right of the shield provides details on how the effective AC was calculated. The AC details are basically the same between the Inventory screen and the Combat Stats tab. The following example will help to better understand the AC details.
Raeth is a level 26 elven Swashbuckler. He had a natural dexterity of 19 which he increased to 20 by reading the Manual of Quickness of Action. While exploring Watcher's Keep, he used the Machine of Lum the Mad to increase it once more to 21.
He is currently wearing Aslyferund Elven Chain which is providing him with a base AC of 0. Other gear affecting his AC: Helm of Balduran (-1), Ring of Gaxx (-2), Ring of Earth Control (-1), and Cloak of Balduran (-1). His other AC bonuses are: Dex bonus (-5), Weapon Proficiency (-1) and Swashbuckler's AC Bonus (-6) and Tear of Bhaal (-2). Raeth also has the Staff of the Magi equipped as an alternate weapon but isn't currently wielding it. After applying all these bonuses, his effective AC is -19.
Screenshots of his inventory and combat stats screens are shown below.
As with other stats, there can be confusion around whether a bonus is positive or negative. The in-game descriptions refer to bonuses as positive numbers (e.g., Cloak of Protection +1) even though they are shown as negatives in the AC details. Just remember that bonuses (regardless of sign) improve AC by lowering it and penalties worsen AC by raising it.
The following table provides a line-by-line description of the AC details.
|AC: effective AC (modifiers: damage-specific modifiers)||This row only appears in the Combat Stats tab. On the inventory screen, the effective AC is shown in the shield icon.||Here we see Raeth's effective AC of -17 along with the damage-type specific modifiers from his Aslyferund Elven Chain|
|Base AC Item Name||The item (if any) responsible for the character's base AC. This row will not be shown if the character's base AC is not being set by an item (e.g., the character's natural AC is being used or the base AC is the result of an effect).||Since Raeth's armor is setting his base AC, we see it listed here.|
|Armor Class : Base AC||The base AC used to calculate the effective AC.||His armor's base AC is 0.|
|Dexterity: modifier value||The bonus (or penalty) conferred by the character's dexterity. As with all modifiers, this row is omitted when the modifier is 0.||Raeth's Dex of 21 is giving him a -5 bonus.|
|Item Name: modifier value||An alphabetized list of every item with a general AC modifier that's contributing to the effective AC. Note that the game will include bonuses from weapons in quick slots, even when they're not being wielded! Further down the list, the game will correct its math.||Note the presence of the Staff of the Magi, even though Raeth is not currently wielding it.|
|Single-Weapon Style Armor Class: bonus||If the character has at least one proficiency point invested in this fighting style, this row will be included within the alphabetized list of items.||Raeth has one point invested in this fighting style.|
|Ability Bonuses: modifier sum||This row shows the sum of any modifiers that weren't accounted for in the preceding rows (i.e., spells, potions, innate abilities and class bonuses). Penalties will be included to balance out any bonuses from weapons not being wielded.||Raeth has a -6 bonus as a swashbuckler and another -2 from a Tear of Bhaal, but the game is including a +2 penalty to back-out the bonus from Staff of the Magi (since it isn't being wielded).|
|Armor Class Modifiers||This is a section heading that separates general modifiers from damage-type specific ones.|
|Damage Type: modifier sum||This row is repeated for each damage type that has a modifier. Each damage type is only shown once, followed by the sum of any modifiers that affected it.||The Aslyferund Elven Chain has a +2 penalty vs crushing and a -2 bonus vs. slashing|
Look at what happens if Raeth performs the following actions:
- quaffs a Potion of Mind Focusing, which raises his Dex to 24, giving him an addition -1 bonus
- uses a scroll of Blur (-3 bonus)
- uses his Greater Evasion HLA (-6 bonus)
The additional -10 bonus points bring his general modifier total to -22. Since the combination of his baseAC and general modifiers have exceed -20, the value will be capped. This still gives him an impressive AC of -27. The only way it could be higher is if he could invest one additional point into the single-weapon fighting style (which, as a pure swashbuckler, he can't). Here's a screenshot of the results:
If Raeth had been a Mage / Thief, he could have achieved these same results, some of them temporarily, by:
- Weapon Proficiency (-1)
- Ring of Gaxx (-2)
- Potion of Mind Focusing (-6 from 24 Dex)
- Blur (-3)
- Tenser's Transformation (-4)
- Shakti Figurine (-5 base AC)
- Greater Evasion (-6)
Altering Base AC Edit
- A monk's natural AC improves with level
- Arcane: Armor (AC 6), Shield (AC 4), Ghost Armor (AC 2), Spirit Armor (AC 1)
- Divine: Barkskin (level dependent)
- Items worn in the armor slot.
- The various Bracers of Defense:
- Staff of the Woodlands (AC 3)
- Shakti Figurine (AC -5)
AC Bonuses Edit
- Kensai, Swashbuckler, and Dragon Disciple Sorcerer kits boost AC by modifiers
- Dexterity adjustment
- Sword and Shield Style proficiency, bonus AC vs. missile when wielding a melee weapon with a shield
- Test of Selfishness evil route boosts AC by -2
- Potion of Absorption (-10 vs. crushing attacks)
- Class abilities: Bard songs, Defensive Spin (-1), Enrage (-2)
- HLAs: Enhanced Bard Song (-10), Evasion (-4), Greater Evasion (-6)
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.
|Type||Item||AC Bonus||Damage Specific Bonus|
|Long Sword||Corthala Family Blade||-2 vs. slashing attacks|
|Katana||Malakar||-2 vs. slashing weapons|
|Katana||Dak'kon's Zerth Blade||-1|
|Flail||Defender of Easthaven||-1|
|Axe||Axe of the Unyielding||-1|
|Quarterstaff||Staff of Curing||-2|
|Quarterstaff||Staff of the Magi||-2|
|Quarterstaff||Staff of Power||-2|
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.
- Aeger's Hide
- Armor of Deep Night
- Armor of Faith
- Armor of Missile Attraction
- Armor of the Hart
- Armor of the Viper
- Ashen Scales
- Aslyferund Elven Chain
- Big Metal Unit
- Bladesinger Chain
- Bloody Bone Plate
- Blue Dragon Plate
- Chain Mail +1
- Chain Mail +2
- Chain Mail +3 (Throne of Bhaal)
- Chain Mail +3
- Changeling's Vest
- Commander's Chain Mail
- Cornugon's Revenge
- Corthala Family Armor
- Corwin's Armor
- Crimson Chain
- Daeros's Full Plate
- Delver's Plate
- Drow Adamantine Chain +5
- Drow Elven Chain
- Drow Full Plate
- Elven Chain +1
- Enkidu's Full Plate
- Fallorain's Plate
- Firecam Full-Plate Armor
- Full Plate Mail +1
- Full Plate Mail +2 (PLAT19)
- Full Plate Mail +2 (PLAT23)
- Goblin Hide Armor
- Goblin Leathers
- Gorgon Plate
- Grandmaster's Armor
- Hide Armor +1 (BDLEAT08)
- Hide Armor +1 (OHLEAT50)
- Hide Armor +2 (OHLEAT51)
- Hide Armor +3 (OHLEAT52)
- Human Flesh +5
- Jegg's Leathers
- Jester's Chain
- Karajah's Life and Death
- Kruntur's Armor
- Leather Armor +1
- Leather Armor +2
- Leather Armor +3
- Magma Bulwark
- Mail of the Dead
- Mail of the Hallowed Hero
- Melodic Chain
- Mithral Chain Mail
- Mithral Field Plate (BRUENPLA)
- Orc Leather
- Plate Mail +1
- Plate Mail +2 (OHPLAT50)
- Plate of Balduran
- Plate of the Dark
- Pride of the Legion
- Protector of the Second
- Raconteur's Regalia
- Red Dragon Scale
- Rugged Leathers
- Shadow Armor
- Shadow Dragon Scale
- Shuruppak's Plate
- Silver Dragon Scale
- Skin of the Ghoul
- Splint Mail +1
- Splint Mail +2 (ohchan50)
- Stalwart Scales
- Studded Leather Armor +1
- Studded Leather Armor +2
- Studded Leather of Thorns
- Sylvan Chain
- T'rachie's Plate
- The Night's Embrace
- The Night's Gift
- The Practical Defense
- White Dragon Scale
BGWiki writing formats Edit
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.