How to calculate armor class in DnD 5e

Armor class (AC) is the measure of how hard it is for an attacker to hit you when rolling to attack.

The higher your armor class, the harder it is to hit you. AC is really important in Dungeons and Dragons because if an enemy attack misses, you don’t take damage.

There are a fair few factors that are used to calculate armor class including natural defense, armor worn, dexterity modifier, shields, spells, magic items and more.

Calculating a characters armor class can get a bit confusing so I’ve broken it down into a step by step approach.

Understanding the basics of Armor Class

Armor Class (AC) is how we determine a character or monster’s ability to to avoid being hit in combat.

AC considers both the creature’s ability to dodge out of the way of incoming attacks and it’s defenses such as thick hide, armor or magical protection.

When an attacker is rolling to hit, if the number on the dice + their modifiers is greater or equal to the armor class of the defender, the attack will hit the defender. 

I remember it as "Meets it? Beats it!"

Most of the time the AC will be written on the character sheet or stat block you’re using, but it can change throughout the day or even an encounter so it’s useful to understand how to do the armor class calculation yourself.

Step 1: Calculating Base Armor Class

The base armor class in DnD 5e is the default value before modifiers and bonuses are applied.

For most characters their base armor class is 10.

Certain races and class features can provide a bonus or penalty to the base armor though.

For example, Warforged have a base armer of 13 + Dexterity modifier when not wearing armor.

Barbarians and monks have unarmored defense as a class feature. This means that when they are not wearing armor their armor class equals 10 + their dexterity modifier + their constitution modifier.

Example

For our example we’re going to look at Ethel, the barbarian librarian.

She may look like an unassuming human, but overdue books send her into a rage.

Ethel doesn’t wear armor because she finds her cardigan more comfortable for everyday combat.

Since she is a human, Ethel’s base armor class starts at 10.

Because Ethel is a barbarian with unarmored defense, her base armor class also includes her dexterity modifier and her constitution.

 10 + 3 + 2 means Ethel's base armor class is 15.

Step 2: Incorporating Armor Types

Different armor types provide different AC bonuses. Makes sense right? Full plate adamantine armor is going to do a lot more to protect you than simple leather.

Here’s a guide to the AC bonuses for common armor types:

Side note: Why proficiency with armor is important

Anyone can put armor on, but if you aren’t proficient with it you will find it comes with some hefty drawbacks.

A character wearing armor they don’t know how to wear properly will have disadvantage on any ability checks, attack rolls and saving throws that involve strength and dexterity. Not only that, a character wearing armor they’re not proficient is also unable to cast spells.

This is pretty significant. Wearing armor you’re not trained in may increase your armor class and make it harder to hit you, but it also makes it much, much harder to hit back.

You basically become a heavily armored sitting duck.

Example

Ethel is going on a quest to find the last existing copy of the fable “The Adventurer’s Pack”. Her granddaughter offers to let her borrow her plate armor.

As a barbarian, Ethel only has proficiency in light and medium armor.

She turns down the offer of the full plate which would hinder her and asks instead to borrow just the breastplate.

A breastplate is medium armor.

In the description it says it gives a bonus to armor class of 14 + her dex modifier (max 2).

When Ethel wears the breastplate she has an armor class of 14 + 2 = 16.

In this case, her unarmored defense armor class is 15, but her armor class whilst wearing the breast plate is 16.

Ethel will put the breastplate on over her cardigan.

Step 3: Factoring in Dexterity Modifier / How Dexterity Modifier influences Armor Class

We’ve mentioned dexterity a few times now when calculating armor class. This is because a character’s dexterity modifier is used as a measure of how good they are at quickly dodging attacks.

The rule of thumb is to always add a character’s dexterity to their base armor class.

The exception to the rule is when a character is wearing heavy armor.

For heavy armor, the dexterity modifier does not affect the armor class.

The idea is that wearing heavy armor impedes a character’s nimbleness when dodging.

We’ve already included Ethel’s dexterity into her armor class so we’ll skip straight to the next step.

Step 4: Enhancing Armor Class with Shields

The Dungeons and Dragons rules for using shields are nice and simple. If the character is using a shield, add the shield's AC bonus (usually +2) to the total armor class.

In general, a character can only benefit from one shield at a time.

Wearing a shield can limit you.

A shield has to be carried with one hand, so you only have one other hand free to carry a weapon.

This means you can’t make two-handed weapon attacks whilst wearing a shield.

Shields suffer the same penalties as body armor if worn by a character who don’t know how to use them.

It’s only really worth equipping a shield if your character has proficiency with them.

Example:

As a barbarian, Ethel has proficiency in shields.

She also doesn’t like making two-handed weapon attacks - the extra twisting involved makes her arthritis worse.

With that in mind, Ethel will equip a wooden shield for her quest, adding a +2 to her AC and bringing her armor class up to 18.

(14 + 2 + 2 = 18)

Step 5: Identifying Additional Bonuses and Penalties

We’ve covered the basics but there are a few other things that can give bonuses to armor class.

These are spells, magic items, feats and cover.

We’re getting into the nitty-gritty here so I wouldn’t worry too much, if it feels a bit complicated and confusing. Most of these are “speciality” defenses that only come up because you’ve chosen to use them.

Spells are probably the most common way of getting additional bonuses to armor class.

Good options are the Shield Spell, which allows a character to add +5 to their armor class as a reaction, or Mage Armor.

There are a number of different magic items that can give bonuses to armor class.

These range from a rare Cloak of Protection, to legendary armor such as +3 Chain Mail.

Feats and Class Features are also options.

Defensive Duelist is a feat that allows characters to add +1 to their armor class as a reaction to being hit with a melee attack.

The Defensive Fighting Style however is a Class Feature of Fighters that adds +1 to a character’s AC whilst they’re wearing armor.

Cover is quite literally how much of you is behind cover.

If you are hidden behind a small bush you might have half cover, and have a +2 added to your AC.

Hiding behind a big boulder might give you 3/4 cover, and +5 to your AC.

Example:

Ethel’s late husband used to be part of the same adventuring party as her. Their wedding rings to each other were Rings of Protection. With him gone, she wears both of them and has two +1 bonuses to her armor class.

14 + 2 + 2 + 1 + 1 = 20

With an armor class of 20, Ethel is going to be a tough but to crack!

She also knows the shield spell. When attacked she can create a magical barrier around her as a reaction, bringing her armor class up to 25.

That gives Ethel the same armor class as Tiamat, a God.

Not bad for a 74 year old librarian!

The final takeaway

To sum it up, armor class is used to determine if an incoming attack will hit.

A character’s base armor class is usually 10, but specific dnd races and classes can change that. Armor and shields are commonly worn to increase a character’s AC, though heavier armor has penalties to dexterity related tasks. Other things can be used to increase your armor class such as spells and magic items, as well as environmental effects like cover.

Now you can calculate armor class for your DnD 5e characters yourself and build a bridge club that will make the Gods tremble!

 

Hi! I'm Annabelle! I'm the author of this blog and a huge nerd!

I also make magnetic, double-sided, modular dungeon tiles!

My DnD terrain contains secret spinning magnets so that each piece snaps instantly to every other. They even come in a box disguised as a spellbook to store away on your bookshelf!

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