Calculating AC 5e: Armor Class Rules and Examples

In Dungeons & Dragons 5th Edition (5e), Armor Class (AC) is the target an attack roll must meet or beat to hit a creature. To calculate it, start with the character’s armor or unarmored defense formula, then apply a shield and any other applicable bonuses; the result is AC, a number with no unit.

Formula

For the basic unarmored calculation shown here: AC = 10 + Dexterity modifier + shield bonus. In the calculator, “Armor base AC” represents the starting AC supplied by the chosen defense rule. Its default of 10 makes the example an unarmored character. The more general expression is AC = armor base AC + applicable Dexterity modifier + shield bonus, but the Dexterity contribution depends on the armor or feature being used.

• Armor base AC: The starting value from the armor entry or applicable unarmored-defense rule, in AC points. For ordinary unarmored defense, the base is 10.
• Dexterity modifier: The character’s Dexterity ability modifier, in AC points. It can be negative, and armor rules may cap or omit it.
• Shield bonus: The AC bonus from a shield, in AC points. The example uses +2; enter 0 when no shield bonus applies.
• AC: The final Armor Class, a game statistic rather than a physical measurement.

This simple calculator is for an unarmored-style calculation with a separate shield bonus. It is not a universal armor calculator: use the rules for the armor or class feature actually governing the character.

Worked Example

Using exactly the calculator defaults—armor base AC 10, Dexterity modifier +2, and shield bonus +2:

1. Start with the base: 10.
2. Add the Dexterity modifier: 10 + 2 = 12.
3. Add the shield bonus: 12 + 2 = 14.

Result: AC 14. The calculator displays a whole-number AC because these inputs and the standard formula produce an integer.

How Armor Class Works in 5e

When a creature makes an attack roll, it generally rolls a d20 and adds the relevant attack modifiers. The attack hits if the total equals or exceeds the target’s AC. For example, against AC 14, a total of 14 hits; a total of 13 misses, subject to rules that alter the ordinary attack procedure.

AC is a defense target, not damage reduction. A successful hit does not mean the target loses a fixed amount of damage based on its AC: damage is determined by the attack and other applicable rules. Likewise, a higher AC makes attacks less likely to hit, but it does not make a character impossible to hit. Natural 1s and 20s have special rules for attack rolls, so AC is not the only consideration in every case.

Choosing the Correct AC Calculation

First identify what sets the character’s AC. A character wearing armor uses the AC calculation for that armor, including any Dexterity limit or exclusion specified by its rules. A character without armor normally uses an unarmored AC rule. Some class features provide a different formula, and a character cannot freely combine multiple alternative ways of calculating AC.

For example, standard unarmored defense starts at 10 and adds the Dexterity modifier. A character with a feature that specifies a different unarmored formula must use that feature’s stated ability modifiers instead. Do not add the two formulas together. If armor sets a fixed AC, use its listed value and apply only the Dexterity contribution that armor permits.

• Unarmored: Use the relevant unarmored formula. Under the basic formula, AC is 10 + Dexterity modifier.
• Light armor: Its AC calculation generally includes the full Dexterity modifier.
• Medium armor: Its calculation generally adds Dexterity only up to the limit specified by the armor rules.
• Heavy armor: Its listed calculation generally does not add Dexterity.
• Shield: Add its stated bonus when the character can use it under the rules. A shield is not simply another armor formula.

Consult the character’s edition-specific rules and the exact armor entry, since features and exceptions can change the calculation.

Common Mistakes and Limits

The most frequent error is adding a Dexterity modifier when the selected armor does not allow it, or adding the full modifier when medium armor limits it. Another is treating a negative Dexterity modifier as zero when the applicable formula calls for adding the modifier; a negative modifier can lower AC. Check the ability modifier itself rather than adding the full Dexterity score.

Also avoid adding bonuses twice. If a feature replaces the normal AC formula, it does not usually stack with another alternative formula. A shield bonus can be added where permitted, but whether the character can benefit from a shield depends on the game rules and the character’s situation. Temporary effects and magic items may alter AC, so include them only when they actually apply and avoid counting the same effect twice.

The calculator above has three inputs and deliberately does not encode every armor type, class feature, temporary effect, or rule exception. Its arithmetic is reliable only when the entered base and modifiers match the character’s applicable rules. It cannot determine which rule applies from a character sheet, and it does not decide whether an attack hits; that requires comparing an attack roll with the final AC.

Frequently Asked Questions

What is the basic formula for AC in 5e?
For ordinary unarmored defense, AC is 10 + the Dexterity modifier. Add a shield bonus when the rules allow it. Armor and other features can use different calculations.

Do you add Dexterity to AC while wearing armor?
It depends on the armor. Light armor generally adds the full Dexterity modifier, medium armor limits it, and heavy armor generally does not add it. Use the armor’s stated calculation.

Does a shield add 2 to AC?
A standard shield commonly provides a +2 AC bonus when its benefit applies. Check the relevant item and character rules, and do not add the bonus if the character cannot use the shield in that situation.

Can two AC formulas be combined?
Usually not. If two features offer alternative ways to calculate AC, choose the applicable formula rather than adding both results together. A separate shield or other bonus may still apply if its rules permit it.

Can AC be higher than 20?
Yes. Armor, shields, abilities, and other applicable effects can produce AC above 20. An attack can still hit if its roll meets or exceeds that AC, subject to special attack-roll rules.

What happens when an attack roll equals AC?
The attack hits when its total equals the target’s AC. For instance, an attack roll total of 14 hits a creature with AC 14.

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in armor | Armor base AC | - | 10 | 0
in dex | Dexterity modifier | - | 2
in shield | Shield bonus | - | 2 | 0
out ac | Armor Class | - | armor + dex + shield | 0