Armour Class

Armour Class (AC) is a measurement of how difficult a creature is to be hit by an attack. In order to successfully hit a creature, the results of an attack roll must have a number equal to or greater than the target's Armour Class. AC can be increased by equipping armour and shields, by selecting certain feats when leveling up, or utilizing certain spells.

Formulas

The default formula that determines AC is:

10 + Dexterity modifier + armour bonus + shield bonus + other bonuses and penalties

Medium armour caps the Dexterity Modifier to +2. Heavy armour ignores the modifier entirely.

Shields will grant the listed AC bonus to whoever equips them, regardless of which weapon slot they are currently using.

Other formulas

Unarmoured creatures may use one of the following different formulas if they have certain features. Creatures always use whichever formula would result in a higher AC among those they have access to.

 Mage Armour or  Draconic Resilience:

13 + Dexterity modifier + shield bonus + other bonuses and penalties

 Unarmoured Defence (Barbarian):

10 + Constitution modifier + Dexterity modifier + shield bonus + other bonuses and penalties

 Unarmoured Defence (Monk):

10 + Wisdom modifier + Dexterity modifier + other bonuses and penalties

Mathematics

Armour Class becomes more useful the greater it is; for instance, the difference in effectiveness between 20 and 19 AC is greater than the difference in effectiveness between 16 and 15.

To illustrate this, if a defender has 15 AC and 10 HP, and the attacker has +4 (50% chance to hit) to attack rolls, and deals 2 damage per attack, the defender would on average survive for 10 turns.

If the defender's AC was increased to 16 (45% chance to be hit), they would instead survive for an average of 11.1 rounds (an 11% increase in effectiveness).

Meanwhile, if the defender has 19 AC (30% chance to be hit), they would survive for an average of 16.66 rounds. If their AC was increased to 20 (25% chance to be hit), they would be able to survive for an average of 20 rounds (a 20% increase in effectiveness).

The difference between 25 and 24 is even greater, with a 100% increase in effectiveness (50 vs 100 rounds).

Items increasing Amour Class

Equipment	Slot	AC Increase	Effect	Found in Act
Bracers of Defence	Gloves	+2	Become the Bulwark You gain a +2 bonus to  Armour Class as long as you are not wearing armour or holding a shield.	1
Bracing Band	Ring	+1	Personal Space After shoving an enemy, the wearer gains a +1 to their  Armour Class until their next turn.	1
Ring of Protection	Ring	+1	Armour Class +1 Saving throw +1	1
Cloak of Protection	Cloak	+1	Armour Class +1 Saving throw +1	2
Evasive Shoes	Footwear	+1	Acrobatics +1 Armour Class +1	2
Defender Flail	Main Hand Melee	+1	Steel Physiology Reduce incoming Bludgeoning, Piercing, and Slashing damage by 1. Armour Class +1	2
Ring of Twilight	Ring	+1	Treader by Twilight You gain a +1 bonus to  Armour Class while obscured.	2
Sharpened Snare Cuirass	Armour	14 + Full Dexterity Modifier	Exotic Material Add your full Dexterity Modifier to your  Armour Class. Additionally, this armour does not impose Disadvantage on Stealth Ability Checks. Sharpened Snare Creatures have Disadvantage on Saving throws when resisting your attacks and spells that inflict  Restrained.	2
Yuan-Ti Scale Mail	Armour	15 + Full Dexterity Modifier	Exotic Material Add your full Dexterity Modifier to your  Armour Class. Additionally, this armour does not impose Disadvantage on Stealth Ability Checks. Ambusher Gain a +1 bonus to  Initiative Rolls.	2
Bonespike Boots	Footwear	+1	Brutal Leap Leap at a target and possibly knock it  Prone. ( Recharge: per turn.) Evasive Instinct You have a +1 bonus to  Armour Class and Saving throws as long as you are not wearing armour or holding a shield. Refined Vaulting Jump distance is increased by 1.5 m / 5 ft.	3
Helm of Balduran	Headwear	+1	Attackers can't land Critical Hits on the wearer. Balduran's Vitality The helmet heals you 2hit points at the beginning of every turn. Balduran's Favour You have a +1 bonus to  Armour Class and Saving throws. Stun Immunity You can't be  Stunned.	3
Unwanted Masterwork Scalemai	Armour	16 + Full Dexterity Modifier	Resistance to Fire damage. Exotic Material Add your full Dexterity Modifier to your  Armour Class. Additionally, this armour does not impose Disadvantage on Stealth Ability Checks.	3
Armour of Agility	Armour	17 + Full Dexterity Modifier	Saving throws +2. Exotic Material Add your full Dexterity Modifier to your  Armour Class. Additionally, this armour does not impose Disadvantage on Stealth Ability Checks.	3
Wondrous Gloves	Gloves	+1	Troubadour's Wonder Your  Armour Class increases by 1. In addition, if you have  Bardic Inspiration, you gain 1 more use of it.	1