5E Ac Calculation

Quickly calculate Armor Class for Dungeons & Dragons 5e using armor type, Dexterity modifier, shield, defense style, cover, magical bonuses, and class-based alternatives like Unarmored Defense or Draconic Resilience. Then review hit probability charts and a full expert guide on how AC really works in play.

AC Calculator Inputs

This calculator follows common 5e rules logic: choose one AC formula, then apply shield, valid fighting style bonus, magic bonuses, misc bonuses, and cover if relevant for the current attack.

Tip: Cover is situational. If you are behind an obstacle, apply it only for attacks affected by that cover. The same AC formula should not stack with another full AC formula unless a specific rule says so.

Expert Guide to 5e AC Calculation

Armor Class, usually shortened to AC, is one of the most important defensive numbers in Dungeons & Dragons 5th Edition. If an attack roll equals or exceeds your AC, the attack hits. If the roll is lower, the attack misses. That sounds simple, but players regularly get confused because 5e includes many different ways to determine AC: light armor, medium armor, heavy armor, shields, natural armor, class features like Unarmored Defense, spells like Mage Armor, temporary cover, and magical bonuses. The trick is understanding that most of the time you begin with a single base AC formula, then layer on only the extra bonuses the rules specifically allow.

In practical terms, 5e AC calculation asks one main question: what is the correct base formula for your character right now? After that, you add approved modifiers such as a shield, a magic item bonus, the Defense fighting style if you are wearing armor, or a cover bonus from terrain. You do not generally add multiple full formulas together. For example, a Monk cannot combine Unarmored Defense with Mage Armor to become 10 + Dex + Wis + 3. Instead, you compare the available legal formulas and use the one that applies.

Core rule of thumb: pick one legal AC formula, then add only the separate bonuses that explicitly stack with it, such as a shield, a magic bonus, or cover. Most calculation errors happen when players stack two full formulas that should be mutually exclusive.

The basic AC formulas in 5e

The standard unarmored formula is 10 + Dexterity modifier. Light armor usually adds your full Dexterity modifier. Medium armor adds Dexterity but usually caps it at +2. Heavy armor ignores Dexterity for AC entirely and instead uses a fixed armor value. Here are the core categories:

• Unarmored: 10 + Dex modifier
• Mage Armor: 13 + Dex modifier
• Light armor: armor value + full Dex modifier
• Medium armor: armor value + Dex modifier, maximum +2
• Heavy armor: fixed armor value, no Dex modifier added
• Shield: +2 AC if wielding one and the rules allow it with your setup

Several races and classes introduce alternatives. Barbarian Unarmored Defense is 10 + Dex + Con. Monk Unarmored Defense is 10 + Dex + Wis. Draconic Resilience is 13 + Dex when not wearing armor. A Tortle has a fixed natural armor value of 17. These are alternatives, not add-ons to armor unless a rule specifically says otherwise.

How to calculate AC step by step

1. Choose the active AC method. Are you wearing armor, using Mage Armor, relying on a racial natural armor trait, or using a class feature like Unarmored Defense?
2. Apply the appropriate ability modifier limits. Light armor takes full Dex, medium armor usually caps Dex at +2, and heavy armor takes no Dex bonus.
3. Add a shield if applicable. A normal shield adds +2 AC.
4. Add legal fixed bonuses. These can include a magic armor bonus, a ring or cloak bonus, the Defense fighting style while armored, or other explicit AC bonuses.
5. Add cover if the situation calls for it. Half cover gives +2 AC and three-quarters cover gives +5 AC against the relevant attack.
6. Compare, do not stack, competing formulas. If two full formulas are available, use the one that applies or the better one when the rules allow that choice.

Common examples

Suppose a Rogue wears studded leather and has a Dexterity modifier of +4. Studded leather is 12 + Dex, so that Rogue has AC 16. If the Rogue uses a shield because of multiclassing or a special build, AC becomes 18. If the Rogue is behind half cover from a low wall, the effective AC against the incoming attack becomes 20.

Now consider a Fighter in chain mail with a shield and the Defense fighting style. Chain mail is fixed at 16. The shield increases that to 18. Defense adds +1 while armored, making AC 19. Dexterity does not matter to chain mail AC because heavy armor ignores Dex.

For a Monk with Dex +3 and Wis +4, Unarmored Defense gives 10 + 3 + 4 = 17. If that Monk also receives three-quarters cover from a stone column, the effective AC against that attack becomes 22. But the Monk still is not combining Unarmored Defense with Mage Armor or leather armor. They are using one formula, then adding situational cover.

Comparison table: official armor baselines and common 5e values

Armor or Method	Base Formula	Dex Included?	Typical Range
Unarmored	10 + Dex	Full	10 to 15+
Mage Armor	13 + Dex	Full	13 to 18+
Leather	11 + Dex	Full	11 to 16+
Studded Leather	12 + Dex	Full	12 to 17+
Chain Shirt	13 + Dex max 2	Capped at +2	13 to 15
Breastplate	14 + Dex max 2	Capped at +2	14 to 16
Half Plate	15 + Dex max 2	Capped at +2	15 to 17
Chain Mail	16	No	16
Splint	17	No	17
Plate	18	No	18

Why AC matters mathematically

5e uses a d20 attack roll system. Ignoring advantage, disadvantage, and automatic outcomes for a moment, each +1 to AC changes the hit threshold by one face on a d20. That means one point of AC usually changes an enemy hit probability by about 5 percentage points. This is why AC upgrades feel so meaningful. Going from AC 16 to AC 18 is not a cosmetic increase. It often cuts incoming hit rates by around 10 percentage points, which can significantly extend survival over many rounds.

To estimate hit chance, a creature with attack bonus B hits AC A on a roll of A – B or higher. Because a natural 1 misses and a natural 20 hits, exact values are bounded by those rules. For most tables, though, the simple practical math is enough: increasing your AC by 1 generally lowers an ordinary attacker’s hit chance by 5%.

Hit probability table by attack bonus

The following table shows exact single-roll hit probabilities under standard 5e assumptions, including the automatic miss on a natural 1 and automatic hit on a natural 20. This gives players a much clearer idea of how much defensive value a higher AC really provides.

Target AC	Attack Bonus +5	Attack Bonus +7	Attack Bonus +9	Attack Bonus +11
14	60%	70%	80%	90%
16	50%	60%	70%	80%
18	40%	50%	60%	70%
20	30%	40%	50%	60%
22	20%	30%	40%	50%

Notice the pattern: every 2 points of AC usually drop hit probability by about 10 percentage points across the center of the curve. That is why shields, cover, and small magical bonuses can dramatically alter combat durability. The effect compounds over a long adventuring day because reducing hit frequency also reduces concentration checks, healing pressure, and the chance of dropping to 0 hit points.

Most common AC mistakes players make

• Stacking two base formulas. Mage Armor, Unarmored Defense, and a racial natural armor formula usually do not stack with one another.
• Adding full Dexterity to medium armor. Medium armor usually caps the Dex contribution at +2.
• Adding Dexterity to heavy armor. Heavy armor uses a fixed AC.
• Applying Defense fighting style when not armored. The style gives +1 AC only while wearing armor.
• Treating cover as permanent AC. Cover is situational and attack-specific.
• Forgetting shields are separate from base armor. A shield is often a clean +2, but only if the character is actually wielding it and can benefit from it.

When to prioritize AC in character building

Not every build should chase maximum AC. Front-line Fighters, Paladins, Clerics, and some Rangers often get excellent value from high AC because they absorb many weapon attacks over the course of a campaign. On the other hand, a Wizard may gain more from positioning, control spells, mobility, and concentration protection than from squeezing out a tiny AC increase. AC is strongest when your character is likely to be targeted repeatedly by attack rolls. It is less useful against effects that force saving throws instead of attack rolls.

Even so, AC remains broadly efficient. A shield can be one of the most impactful defensive items in the game. Half cover and three-quarters cover are also underused by many groups despite their huge mechanical value. If your table uses tactical maps, learning to fight around obstacles can provide “free AC” more often than players realize.

How advantage and disadvantage interact with AC

AC itself does not change when an attacker has advantage or disadvantage, but the effective chance to be hit changes a lot. Advantage increases the likelihood that at least one d20 roll meets or exceeds your AC. Disadvantage lowers it. This means that avoiding enemy advantage can be nearly as important as gaining +1 or +2 AC. Conditions, visibility, cover, positioning, and class features all influence this. In some encounters, denying advantage is more valuable than a static AC boost.

Advanced tactical advice

1. Know your expected opponents. If your campaign features many melee brutes with modest attack bonuses, each point of AC is premium value.
2. Use cover actively. Half cover and three-quarters cover are enormous. A +5 bonus from three-quarters cover can turn a dangerous volley into a low-probability threat.
3. Protect concentration. Better AC means fewer hits, which means fewer concentration saves for spells that define encounters.
4. Balance AC with HP and saves. AC does nothing against many area effects, so a complete defense plan includes hit points, Constitution, and key saving throw support.
5. Avoid overinvesting where returns flatten. Going from AC 14 to 18 is often huge. Going from 23 to 24 may matter less depending on enemy attack bonuses.

Helpful probability and statistics references

If you want to understand the math behind d20 systems more deeply, these authoritative resources are useful for probability and statistical reasoning: NIST, Penn State STAT 414, Carnegie Mellon probability notes.

Final takeaway

5e AC calculation becomes easy once you separate formulas from bonuses. First, identify the one correct base AC method: armor, Mage Armor, natural armor, or Unarmored Defense. Second, add only those bonuses that explicitly stack, such as shield, magical bonuses, Defense fighting style when armored, and situational cover. Third, think about AC as probability control. Every point changes the odds on a d20, and those changes matter over dozens of attack rolls. Use the calculator above to test builds, compare equipment choices, and see how much practical survivability your current AC really provides.