Attack Rolls and Armor Class Rules

Every combat encounter in Dungeons & Dragons pivots on a single mechanical exchange: an attacker rolls a d20, adds modifiers, and either meets or beats a target's Armor Class. That number — AC — is the game's central gatekeeper between "a miss" and "roll damage." Understanding how the two interact explains why a fighter in plate mail feels impervious to goblins and why that same fighter still sweats against a veteran assassin.

Definition and scope

Armor Class is a single integer representing how difficult a creature is to hit with a standard attack. It accounts for physical armor, shield use, dexterity, and sometimes magical protection. The Player's Handbook (5th Edition) sets unarmored AC as 10 + Dexterity modifier — a baseline that a character in no gear whatsoever still possesses.

An attack roll is the d20 roll a creature makes to determine whether an attack connects. The roll uses this formula:

d20 + ability modifier + proficiency bonus (if proficient) ≥ target AC = hit

The scope is broader than it first appears. Attack rolls govern melee weapon strikes, ranged weapon attacks, and spell attacks — the last being a category that trips up newer players who sometimes assume spells always bypass this mechanic. Many spells do (they call for saving throws instead), but spell attacks like Fire Bolt or Eldritch Blast use the same d20 roll structure. For a full breakdown of how these mechanics fit into the broader system, see How It Works.

How it works

The sequence, in practice, follows five steps:

1. Declare the attack and target. The attacker names the creature being attacked and the weapon or ability used.
2. Roll the d20. Natural results of 1 and 20 have special status — a natural 1 always misses regardless of modifiers; a natural 20 is a critical hit and automatically hits, also triggering double damage dice.
3. Apply the attack modifier. For melee attacks with a weapon, this is Strength modifier plus proficiency bonus (if proficient with that weapon type). Finesse weapons — rapiers, shortswords — allow Dexterity instead. Ranged weapons default to Dexterity.
4. Compare the total to the target's AC. The attack hits if the total equals or exceeds AC. A result of exactly AC is a hit, not a miss — a detail that generates genuine table debate roughly once per campaign.
5. Resolve damage on a hit. Damage rolls are entirely separate from the attack roll and use their own dice.

Proficiency bonus scales with character level: +2 at levels 1–4, reaching +6 at levels 17–20 (Player's Handbook, 5e, Chapter 1). This means a 17th-level fighter has a meaningfully higher floor on every attack roll than a 1st-level character using the same weapon.

AC construction varies significantly by character build. A Paladin in plate mail (AC 18) with a shield (+2) sits at AC 20 — meaning an attacker needs a total of 20 or higher to land a hit. Against a starting fighter with a +4 attack modifier and +2 proficiency, the average roll (10 on the d20) produces a 16: a clean miss. That math is intentional. Key dimensions and scopes of D&D covers how these numbers scale across tiers of play.

Common scenarios

The lightly-armored rogue: Rogues frequently rely on Dexterity-based AC — leather armor (AC 11) plus Dexterity modifier. A rogue with +4 Dexterity sits at AC 15. Respectable, but not impervious. Enemies with modest attack bonuses still connect roughly 30% of the time on a flat d20 probability.

The heavily-armored tanker: Full plate (AC 18) is the highest non-magical armor value in the standard equipment list. A fighter with a shield reaches AC 20 without any magical enhancement. Against low-level monsters with +3 or +4 attack bonuses, hits become statistically rare.

Spellcasters and the Mage Armor spell: Wizards proficient only in daggers and wearing no armor default to AC 10 + Dexterity modifier. Mage Armor raises unarmored AC to 13 + Dexterity modifier — a meaningful defensive bump that runs until dispelled or the caster takes a long rest.

Cover mechanics: The Dungeon Master's Guide defines two cover tiers. Half cover grants +2 to AC and Dexterity saving throws; three-quarters cover grants +5. Full cover makes a creature untargetable entirely. These bonuses stack directly onto the AC calculation and can turn an otherwise-certain hit into a miss.

Decision boundaries

The critical distinction players encounter repeatedly is the difference between an attack roll and a saving throw. Spells like Fireball call for a Dexterity saving throw — the defender rolls to reduce damage, and AC is irrelevant. Spells like Scorching Ray use an attack roll — AC matters completely, and the target rolls nothing. Mixing these up is one of the most common mechanical errors at tables, particularly when learning the game. The D&D frequently asked questions page addresses this distinction directly.

A second boundary: the Help action. One creature can use its action to Help an ally attacking a target it can reach, granting advantage — two d20 rolls with the higher result taken. Advantage doesn't raise the attack bonus; it improves the probability of a higher natural roll. Against AC 20, a character with a +6 attack modifier hits on a 14 or higher — roughly 35% of the time on a single roll, rising to approximately 58% with advantage.

Finally, the Dodge action sets an important defensive ceiling. A creature using Dodge forces all attack rolls against it to be made at disadvantage (lower of two d20 rolls) until its next turn — regardless of how high the attacker's bonus is. High attack modifiers still help, but no modifier eliminates the statistical drag of rolling twice and taking the worse result.