DnD Armor Rules and Armor Class

Armor Class (AC) is the central defensive statistic in Dungeons & Dragons 5th Edition, determining how difficult a creature or character is to hit with an attack. The rules governing armor and AC intersect with attack roll mechanics, class features, ability scores, and spell effects, making this one of the most frequently referenced rule categories in play. This page covers the armor categories defined in the core rules, how AC is calculated across different scenarios, and the decision points that arise when multiple AC formulas could apply simultaneously.

Definition and scope

Armor Class represents the difficulty of landing a damaging hit on a target. When an attacker makes an attack roll, the result must meet or exceed the target's AC for the attack to connect. The baseline AC calculation for an unarmored creature is 10 plus the character's Dexterity modifier — a rule codified in the Player's Handbook (Wizards of the Coast, 2014) and reproduced in the free-access D&D Basic Rules.

The Player's Handbook organizes armor into three categories:

  1. Light Armor — Padded, Leather, Studded Leather. AC equals the armor's base value plus the wearer's full Dexterity modifier.
  2. Medium Armor — Hide, Chain Shirt, Scale Mail, Breastplate, Half Plate. AC equals the armor's base value plus the wearer's Dexterity modifier, capped at +2.
  3. Heavy Armor — Ring Mail, Chain Mail, Splint, Plate. AC equals a fixed value; Dexterity modifier is not added at all. Plate armor sets AC at 18 by default.

Shields add a flat +2 to AC and are not classified as armor — they function as a separate equipment category. Wearing a shield requires one hand to be occupied, which interacts with rules for two-weapon fighting and spellcasting components covered under spell components.

Proficiency is a prerequisite for using armor effectively. A character wearing armor without proficiency cannot cast spells, has disadvantage on Strength and Dexterity ability checks and saving throws, and cannot make attack rolls — as specified in the Player's Handbook, Chapter 5.

How it works

AC is calculated at the moment of an attack; it is not rolled. The attacker rolls a d20, adds relevant modifiers (Strength or Dexterity, proficiency bonus if applicable, magical bonuses), and compares the total to the defender's AC. A result equal to the AC is a hit; results below are misses.

The foundational armor calculation is straightforward, but the rules accommodate multiple formulas that can replace the standard armor-based AC. These alternative formulas are not cumulative — only one applies at a time:

A character with Mage Armor and Unarmored Defense cannot apply both simultaneously. The player selects one formula; typically the higher result is preferred, though some class features have restrictions on simultaneous use. This interaction is addressed in depth under the multiclassing rules, where combining Barbarian and Monk levels raises this exact conflict.

The broader combat sequence in which AC operates is documented on the DnD combat rules page, and the mechanical foundation of how dice resolve against static numbers is explained in the how D&D works conceptual overview.

Common scenarios

Scenario 1 — New character using medium armor: A Fighter with a +3 Dexterity modifier equips a Breastplate (base AC 14). The Dexterity modifier is capped at +2 for medium armor, so the character's AC is 16, not 17. Adding a shield brings the total to 18.

Scenario 2 — Monk in combat: A Monk with a +3 Dexterity modifier and +2 Wisdom modifier uses Unarmored Defense, yielding AC 15. If the Monk picks up a shield, Unarmored Defense no longer applies (the feature explicitly requires no shield), and AC drops to 13 + Dexterity modifier (10 + 3), making the shield actively detrimental to this character's defense.

Scenario 3 — Stealth and heavy armor: Splint and Plate armor impose disadvantage on Dexterity (Stealth) checks by rule. Characters attempting stealth and hiding must account for this trade-off between AC and operational concealment. The Player's Handbook marks this with a "Disadvantage" property in the armor table.

Scenario 4 — Cover and AC: The cover rules in D&D grant additional AC bonuses — +2 AC for half cover, +5 AC for three-quarters cover — that stack onto a character's existing AC calculation. These are situational modifiers, not changes to the base formula.

Decision boundaries

The central decision boundary involves choosing between armor types and alternative AC formulas. A Barbarian with a +1 Constitution modifier and +4 Dexterity modifier has Unarmored Defense AC of 15. Wearing medium armor (Breastplate) gives AC 16. The armor wins numerically, but the Barbarian's Unarmored Defense scales with Constitution investment, creating a long-term optimization question tied to the ability scores and modifiers system.

A second boundary governs shield use. The +2 AC bonus from a shield is consistent and does not depend on Dexterity, Constitution, or subclass. For classes lacking Unarmored Defense, a shield almost always increases effective durability. For Monks, the shield breaks the Unarmored Defense prerequisite and reduces AC in most builds.

Heavy armor access is gated by class selection — not all classes are proficient — and by Strength minimums. Chain Mail requires Strength 13; Plate requires Strength 15. Failing the Strength requirement imposes a movement speed reduction of 10 feet, connecting armor choice to movement and positioning rules.

Feats such as Lightly Armored, Moderately Armored, and Heavily Armored can extend armor proficiency to classes that lack it, effectively expanding the AC options available to any character build. These feats are documented in the Player's Handbook, Chapter 6, and the full rules index at dndrules.com provides cross-references to related mechanical categories.

References

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