Armour Class (AC) is a measurement of how difficult a creature is to be hit by an attack. AC can be increased by equipping Armour and Shields, by selecting certain Feats when leveling up, or utilizing certain Magic Spells.
The formula that determines AC is:
Armour Class = Base Armour Class + Dexterity Modifier + Other Bonuses and Penalties
- Base Armour Class is the number seen in shields in the descriptions of armour. When unarmoured, most creatures naturally have a Base AC of 10.
- Mage Armour sets the base armour class to 13 if the target isn't wearing any armour
- The AC bonus from Dexterity Modifier may be capped when wearing heavier types of Armour.
- Light Armour doesn't cap the Dexterity AC. Clothing doesn't either, but they don't count as armour at all.
- Medium Armour typically caps the Dexterity Modifier to +2
- Heavy Armour ignores any bonus (or penalty) from the wearer's Dexterity.
- The final term, "Other Bonuses and Penalties" is a very valuable source of AC, it is subject to the least amount of restrictions, but also are relatively rare, demands certain trade offs, or otherwise require the expenditure of Resources.
- Shields will grant its listed AC bonus to whomever equips it. This means the user's off hand will be occupied, denying the bonus they would get for two-handing a versatile weapon, and wouldn't be able to equip a two-handed weapon or an off-hand weapon.
- Certain Clothing and Armours grant bonus AC outside of their Base AC. Potent Robe is an example of this.
- Barbarians and Monks have access to the Unarmoured Defence feature. Allowing them to add their Constitution Modifier and Wisdom Modifier to AC respectively, but only when unarmoured.
- Fighters, Paladins, and Rangers have access to the Defense Fighting Style feature, which grants +1 AC if the they are wearing armour.
- The Defensive Duellist feat allows a character to spend their Reaction to functionally add their Proficiency Bonus to their AC for a single incoming attack. The character must be wielding a Finesse weapon they are Proficient in. (This is actually implemented by imposing a penalty to the incoming attack roll.)
- A few spells grants bonus AC: Barkskin, Shield of Faith, Shield(Spell), Mirror Image, and Haste. But all of these either don't last long, requires concentration, or are otherwise limited in its bonus.
- Spells that decreases AC are even rarer: Slow can do this directly, but is subject to a saving throw; Chromatic Orb: Acid and Melf's Acid Arrow can achieve this via an Acid Surface, but can be swiftly negated when the victim leaves the surface.
In order to successfully hit a creature, the Attack Roll must have a number equal to or greater than the target's Armour Class. This means, typically, each point of AC makes a creature 5% harder to hit.