DnD Armor Rules and Armor Class

Armor Class is the single number that decides whether a sword connects, a fireball singes, or an arrow finds its mark — and the rules behind it are more layered than a single stat suggests. This page breaks down how AC is calculated, what types of armor exist in the fifth edition ruleset, and the moments where players and Dungeon Masters most often reach for the rulebook mid-session.

Definition and scope

Armor Class represents how difficult a creature or character is to hit with an attack roll. When an attacker rolls a d20 and adds their relevant modifier, the result must meet or exceed the target's AC to deal damage. Simple enough in principle — but the calculation of that AC number involves equipment choices, ability scores, class features, and spell effects that can interact in ways that aren't always obvious.

The fifth edition Player's Handbook (Wizards of the Coast) defines three broad categories of armor: light, medium, and heavy. Each category handles the Dexterity modifier differently, which is where a lot of the strategic texture lives. For anyone still finding their footing with the broader system, the DnD overview is a useful orientation before diving into the specifics here.

How it works

The baseline AC calculation depends entirely on what a character is wearing — or not wearing.

1. No armor (unarmored): AC equals 10 + the character's Dexterity modifier. This is the default for any creature without special traits.
2. Light armor (leather, padded, studded leather): AC equals the armor's base value + full Dexterity modifier. Studded leather, for instance, has a base AC of 12, so a character with a +3 Dex modifier sits at AC 15.
3. Medium armor (hide, chain shirt, scale mail, breastplate, half plate): AC equals the armor's base value + Dexterity modifier, capped at +2. A breastplate (base AC 14) with a Dex modifier of +5 still yields only AC 16.
4. Heavy armor (ring mail, chain mail, splint, plate): AC equals a fixed value — no Dexterity modifier applies at all. Plate armor sets AC at 18, regardless of whether the wearer has a Dex of 8 or 20.
5. Shields: Add 2 to AC. They stack with any armor type and require only proficiency to use without penalty.

Two class features complicate this neatly ordered picture. The Monk's Unarmored Defense sets AC to 10 + Dexterity + Wisdom when wearing no armor or shield. The Barbarian's version replaces Wisdom with Constitution. Both are explicitly verified as alternative calculations — a character uses whichever single formula produces the highest AC, but does not add the formulas together. The Player's Handbook is unambiguous on this point: AC calculation methods don't stack with each other, though a shield bonus always applies on top of whatever base formula is in use.

The how it works section of this site covers the broader mechanical framework for those who want context on how attack rolls and saving throws fit into the same resolution system.

Common scenarios

The Dexterity-based fighter: A character proficient in medium armor who wants to leverage a high Dexterity score hits a wall at +2. A player with a Dex modifier of +4 wearing a breastplate gets AC 16 — the same as a character with only +2 Dex in the same armor. This often pushes Dexterity-primary characters toward light armor or unarmored builds rather than medium.

The Wizard in a robe: Without armor proficiency, a Wizard who dons chain mail still suffers disadvantage on attack rolls, saving throws using Strength or Dexterity, and ability checks using those same scores. The Player's Handbook calls this "donning armor you lack proficiency with" — the armor still technically sets the AC, but the mechanical penalties make it largely impractical.

Mage Armor: The spell sets a character's base AC to 13 + Dexterity modifier for 8 hours, targeting only unarmored creatures. A Sorcerer with a +3 Dex modifier reaches AC 16 with no equipment required. This frequently outperforms what light or medium armor could offer a low-Strength caster.

Dual-class edge cases: A character who multiclasses into Monk retains their Monk Unarmored Defense only while wearing no armor. The moment they equip a breastplate, the Monk formula stops applying entirely — the armor's base AC takes over. The key dimensions and scopes of DnD page addresses how class features interact across multiclass builds in more detail.

Decision boundaries

The practical threshold for heavy armor becomes relevant at a Constitution or Dexterity modifier below +2. A character with AC 18 from plate beats anything medium armor can produce without exceptional Dexterity — but plate imposes a Strength requirement of 15 and a Stealth disadvantage in every edition that lists it. For a Paladin or Fighter built around frontline combat with no need for stealth, plate is straightforwardly optimal. For a Ranger or Rogue who needs to move quietly, it isn't.

The shield decision is almost always mathematically favorable. Adding 2 to AC for the cost of one hand is a significant defensive gain — the tradeoff is losing two-handed weapon damage (typically 1d12 or 2d6 versus 1d8 or 1d10 for one-handed weapons). Whether that trade is worth it depends on the character's role and the campaign's encounter design.

Magic armor modifies the base AC value before other calculations apply. A +2 breastplate has a base AC of 16, so a character wearing it with a +2 Dex modifier reaches AC 18 — matching plate without the Strength requirement or stealth penalty. At higher tiers of play, this matters considerably. The DnD frequently asked questions page covers common points of confusion around magic item stacking and AC calculation in those later stages.