D&D 5e Armor Class (AC) Calculator

The Complete Guide to Armor Class (AC) in D&D 5e & 2024 Rules

In Dungeons & Dragons 5th Edition, your Armor Class (AC) is the foundational metric of physical combat defense. Whether an arrow glances harmlessly off your full plate or a rogue gracefully sidesteps a dragon's claws, every physical attack roll must meet or exceed your AC to deal damage.

Unlike earlier editions such as 3.5e or AD&D—where high-level characters stacked dozens of exotic armor bonuses to reach ACs exceeding 40 or 50—5th Edition was built around the philosophy of Bounded Accuracy. In 5e, Armor Class generally ranges from 10 to 22 for most characters throughout their careers, with rare magical stacking reaching 25 to 30. Because enemy attack bonuses scale modestly (+3 at level 1 to +14 at CR 20+), every single point of AC remains mathematically vital across all twenty levels of play.

How Armor Categories Work in 5e

Mundane armor in the 5e Player's Handbook is divided into three distinct categories, each designed for different character archetypes and ability score investments:

Unarmored Defense & Non-Stacking AC Rules

One of the most frequent rules questions at tabletop gaming sessions is: "Can I stack Barbarian Unarmored Defense with Monk Unarmored Defense or Mage Armor?"

The answer by official Rules As Written (RAW) is an emphatic NO. In 5th Edition, rules that determine your Armor Class fall into two distinct mechanical categories:

• Base AC Calculations: These provide a complete mathematical formula for calculating your base AC. Examples include 10 + Dex (default unarmored), 10 + Dex + Con (Barbarian), 10 + Dex + Wis (Monk), 13 + Dex (Mage Armor or Draconic Resilience), and 18 (Full Plate). You can only ever use one base AC calculation at a time. If multiple calculations apply to your character, you choose which one to benefit from.
• AC Modifiers (Bonuses): These are flat numeric additions that apply on top of whichever base calculation you choose. Examples include Shields (+2), the Defense Fighting Style (+1 while wearing armor), the Shield of Faith spell (+2), and Cover (+2 or +5). These bonuses stack cleanly with your chosen base calculation, provided you meet their specific prerequisites.

Understanding the Math of Hit Probability & Effective HP

Because attack rolls use a twenty-sided die (\(d20\)), every single +1 increase in Armor Class reduces an enemy's chance to hit by exactly 5 percentage points (unless the roll is restricted by automatic Natural 1 misses or Natural 20 hits).

However, the defensive value of that 5% increases exponentially the higher your AC becomes. Consider an enemy with a +5 attack bonus attacking two different characters:

• Character A (AC 12): The monster needs a roll of 7 or higher on the d20 to hit (\(20 - 7 + 1 = 14\) outcomes, or a 70% hit chance). Increasing AC to 13 drops the hit chance from 70% to 65% (a \(\approx 7.1\%\) reduction in incoming damage).
• Character B (AC 20): The monster needs a roll of 15 or higher to hit (6 outcomes, or a 30% hit chance). Increasing AC to 21 drops the hit chance from 30% to 25% (a 16.7% reduction in incoming damage).
• Character C (AC 24): The monster only hits on a 19 or 20 (10% hit chance). Increasing AC to 25 means the monster only hits on a Natural 20 (5% hit chance)—effectively halving all incoming attack damage!

This compounding value is why defensive reactions—such as the wizard's Shield spell (+5 AC until the start of your next turn)—are considered among the most powerful combat survival tools in the entire game.

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Frequently Asked Questions About 5e Armor & AC