Armor class

Armor Class (AC) is a number representing a creature's ability to avoid being hit in combat. An opponent's attack roll must equal or exceed the target creature's AC to hit it.

AC = 10 + armor bonus + shield bonus + dodge bonus + natural armor bonus + deflection bonus + other bonuses

The average, unarmoured peasant has an AC of 10.

AC types

 * Armor bonus - provided by armor slot and bracers slot (bracers only; gloves provide deflection)
 * Shield bonus - provided by shield slot
 * Dodge bonus - provided by boots slot
 * Natural armor bonus - provided by amulet slot
 * Deflection bonus - provided by all other inventory slots

Only dodge bonuses stack, up to a maximum of +20. For all other types of bonus, the single highest modifier is applied.

Armor class guide
As above, there are several types of Armor Class in Neverwinter Nights; some will work together (i.e., stack) and some will not. Knowing what stacks and what doesn't can often mean the difference between useful equipment and useless equipment.

Below are 6 AC categories; the 5 that NWN uses (natural, armor, shield, deflection, dodge), and one extra "other" category to hold the bonuses that do not fit into the 5 base categories. All these categories will stack together, and ultimately sum together to form a character's total AC.

Sources of natural AC
These will not stack with each other; the highest one is used.
 * Item: Amulet AC Bonus (+1-20)
 * Feat: Epic spell: epic mage armor (+5)
 * Spell: Mage armor (+1)
 * Spell: Barkskin (+3-5)
 * Spell: Shadow shield (+5)
 * Spell: Tenser's transformation (+4)

Sources of armor AC
These will not stack with each other; the highest one is used.
 * Item: Armor AC Bonus (+1-20)
 * Item: Bracer AC Bonus (+1-20) (same item slot as Gloves)
 * Feat: Epic spell: epic mage armor (+5)
 * Spell: Mage armor (+1)
 * Spell: Magic vestment (+1-5)

Sources of shield AC
These will not stack with each other; the highest one is used.
 * Item: Shield AC Bonus (+1-20)
 * Spell: Magic vestment (+1-5)

Sources of deflection AC
These will not stack with each other; the highest one is used.
 * Item: Glove AC Bonus (+1-20) (same item slot as Bracers)
 * Item: Ring AC Bonus (+1-20)
 * Item: Belt AC Bonus (+1-20)
 * Item: Helm AC Bonus (+1-20)
 * Item: Cloak AC Bonus (+1-20)
 * Item: Weapon AC Bonus (+1-20)
 * Item: Dusty Rose Ioun stone (+1)
 * Feat: Epic spell: epic mage armor (+5)
 * Spell: Shield (+4)
 * Spell: Mage armor (+1)
 * Spell: Shield of faith (+2-5)
 * Spell: Protection from evil (+2 versus selected alignment)
 * Spell: Aura versus alignment (+4 versus selected alignment)
 * Spell: Magic circle against alignment (+2 versus selected alignment)

Sources of dodge AC
These will stack with each other, but is limited to a +20 maximum Dodge bonus.
 * Item: Boots AC Bonus (+1-20)
 * Feat: Bard Song (+2-7)
 * Feat: Battle Training vs. Giants (+4 against giant-type enemies)
 * Feat: Mobility (+4 against attacks of opportunity)
 * Feat: Shadow Evade (+1-4)
 * Feat: Divine Shield (+1-20)
 * Feat: Defensive Stance (+4)
 * Spell: Haste (+4)
 * Spell: Mage armor (+1)
 * Spell: Epic spell: epic mage armor (+5)
 * Spell: Undeath's eternal foe (+4)

All dodge AC is lost when caught flat-footed (even if the character has the Uncanny Dodge feat).

Other sources of AC
These are all individual sources, and stack together with no restrictions.
 * Ability: Dexterity (+1-122, but limited by the armor's Maximum dexterity bonus)
 * Item: Armor Base (+1-8)
 * Item: Shield Base (+1-3)
 * Feat: Armor Skin (+2)
 * Feat: Dodge (+1 against current target or last attacker)
 * Feat: Bone Skin (+2-16)
 * Feat: Small Stature (+1)
 * Feat: Monk AC Bonus from Wisdom (+1-122)
 * Feat: Monk AC Bonus from level progression (+1-8)
 * Feat: Draconic Armor (+1-8)
 * Feat: Expertise/Improved Expertise (+5/+10)
 * Skill: Tumble (+1-8)

The dexterity bonus to AC, Dodge, and Tumble AC benefits are lost when caught flat-footed, but the Uncanny Dodge and Defensive Awareness feats allow the dexterity bonus to be retained.