What Your Armor Class Number Actually Represents
In D&D 5e, Armor Class is the difficulty number an attacker must meet or beat on their d20 attack roll to hit you. An AC of 16 means any attack roll of 16 or higher connects; anything below misses. The higher your AC, the smaller the portion of the d20 range that results in a hit — and since attackers add their attack bonus to the roll, the real effect of a higher AC depends on who’s swinging at you.
Against a veteran fighter with +8 to hit, an AC of 18 means they hit on 10–20 (11 outcomes out of 20, or 55%). Raise it to AC 20 and they hit on 12–20 (9 outcomes, 45%). That 2-point difference reduces incoming damage by about 18% — a meaningful swing over a long adventuring day. But against a goblin with +4 to hit, the math shifts entirely. These relationships are why optimized builds care so much about each AC point.
The Different Ways AC Is Calculated in 5e
There isn’t one universal formula — the base depends on your armor type, and several class features replace the standard calculation entirely. Getting this wrong is one of the most common character sheet errors, especially at early levels.
Unarmored and Light Armor: Dexterity Matters Most
Without armor, your AC is 10 + Dexterity modifier. Light armor (padded, leather, studded leather) adds its listed base to your full Dex mod with no cap. A rogue in studded leather with +5 Dex ends up at 17 — competitive with many heavier armors. This is intentional: light armor classes are designed to reward high Dexterity investment rather than being permanently outclassed by heavy armor users.
Medium armor (hide, chain shirt, scale mail, breastplate, half plate) uses its listed base plus Dex — but Dex is capped at +2. This matters enormously for character builds. A Fighter in half plate with +4 Dex gets the same 17 as one with +2 Dex. The extra Dex investment beyond +2 contributes nothing to AC in medium armor.
Heavy Armor: Strength Gates, Flat AC
Heavy armor — ring mail, chain mail, splint, plate — uses a fixed base with no Dex contribution at all. Plate at 18 is the ceiling for base armor in the game. Strength requirements apply (chain mail needs 13, splint needs 15, plate needs 15), and heavy armor imposes disadvantage on Stealth checks. A Fighter with −1 Dex in plate has exactly the same AC as one with +5 Dex.
Class-Specific Unarmored Defense Options
Barbarians use 10 + Dex + Con when unarmored — the only class that adds two ability modifiers to its base. A Barbarian with +3 Dex and +4 Con starts at AC 17, better than studded leather without spending a single gold piece. Monks use 10 + Dex + Wisdom, which rewards the class’s natural stat spread. Neither ability adds to AC when the character puts on armor, so there’s a genuine choice to be made between wearing armor or relying on unarmored defense.
Mage Armor (the spell) sets a character’s unarmored AC to 13 + Dex, making it better than leather for any spellcaster with a Dexterity modifier of +1 or higher.
Stacking Rules: What Does and Doesn’t Combine
Not all AC bonuses stack. Two effects granting the same type of bonus typically don’t add together. The important practical rules:
- Shield of Faith (+2 AC) and Haste (+2 AC) both stack with each other and with armor — they’re different spells granting a bonus, and there’s no rule preventing this combination.
- A +1 shield and Fighting Style: Defense (+1) both stack fine — they’re different sources.
- You cannot benefit from both Mage Armor and wearing physical armor — Mage Armor explicitly ends if you put on armor.
- Barbarian and Monk unarmored defense don’t combine — only one applies (whichever is higher in the rare case of a multiclass).
- The Dodge action and Blur spell both impose attack disadvantage, but they don’t add to AC — they change the attack roll mechanic instead.
The Player’s Handbook combat rules on D&D Beyond and the Sage Advice Compendium are the primary references for ruling edge cases.
What’s the Highest Possible AC in 5e?
This is one of the more entertaining optimization thought experiments in the game. The practical ceiling without magic items or extreme multiclassing is plate (18) + shield (2) + Fighting Style: Defense (1) = AC 21 for a Fighter. Add a +3 magic shield (+3 more), a Cloak of Protection (+1), and a friendly Paladin’s Aura of Protection isn’t AC but worth noting — and you can reach AC 25 before concentration spells. Theoretical maximums involving Shield of Faith, Haste, Bladesong, and specific magic items push higher but require very specific builds and active spell concentration.
FAQs
What’s a good AC for a 1st-level character in 5e?
At 1st level, AC 13–15 is solid for most classes. A Fighter in chain mail starts at 16, which is high for that tier. A Wizard without armor at AC 10–12 is normal but fragile — Mage Armor is worth the spell slot if you expect to be hit. AC 18+ (plate + shield) is exceptional and only available to a Fighter or Paladin at 1st level.
Does a natural 20 hit regardless of AC?
Yes. A natural 20 on the attack roll is a critical hit and always connects, regardless of your AC. Similarly, a natural 1 always misses. No amount of AC can make you immune to a critical hit.
Can you wear a shield with two-handed weapons?
No. Wielding a two-handed weapon (or a weapon with the two-handed property) requires both hands, which means you cannot also hold a shield. This is an explicit rule in the PHB. Versatile weapons (like a longsword) can be used one-handed, freeing up the other hand for a shield.
Does the Dex modifier apply to AC when wearing heavy armor?
No. Heavy armor in 5e — ring mail, chain mail, splint, and plate — uses a flat base AC with no Dexterity modifier added or subtracted. High Dex contributes nothing to AC in heavy armor, though it still affects initiative, Dexterity saving throws, and Dex-based skill checks.
How does the Dodge action affect AC?
Dodging doesn’t increase your AC number — it forces any attacker who can see you to make their attack roll at disadvantage (rolling two d20s and using the lower result). Combined with a high AC, this dramatically reduces the probability of being hit. However, Dodge costs your full action and doesn’t help against attacks you can’t see.
Can Barbarian Unarmored Defense work with a shield?
Yes. The Barbarian Unarmored Defense feature (10 + Dex + Con) applies while you’re not wearing armor — a shield isn’t armor, so it works. A Barbarian using unarmored defense with a shield adds +2 AC on top of their 10 + Dex + Con total. This is a common optimization for Barbarian builds.
What’s the armor class modifier finder for a Monk with 18 Dex and 16 Wis?
A Monk’s Unarmored Defense is 10 + Dexterity modifier + Wisdom modifier. With 18 Dex (+4 modifier) and 16 Wis (+3 modifier), that’s 10 + 4 + 3 = 17 AC. No armor cost, no Strength requirement, no Stealth disadvantage — and it scales with two of the stats Monks naturally want to invest in anyway.
AC is the most visible defensive number on your sheet, but it’s not the only one that matters. Saving throw proficiencies, hit points, and resistances all contribute to survival. Still, knowing exactly where your AC comes from — and which bonuses you’re leaving on the table — is half the work of building a character who can actually take a hit.