One of the bigger changes to the game in the new edition of Dungeons & Dragons concerns Armour Class.
It’s function hasn’t changed: the better (higher) your Armour Class, the harder you are to hit. When an attack is made, the attacker rolls a 1d20 and adds their attack bonus; if the result equals or exceeds the target’s Armour Class, the target is hit.
Although its function in the game hasn’t changed, it doesn’t just keep going up and up and up like in 3E and 4E. A major factor in the new edition’s design is Bounded Accuracy, which means that target numbers can’t change too much. Within the game, this translates to most Armour Classes being in the range of 10 to 20.
When a monster or character goes outside those ranges, you generally can assume there are magical items or spells involved, or the monster is special in some way. In the Basic Rules, the best Armour Class is 19, held by an adult dragon. The Hoard of the Dragon Queen Online Supplement has two monsters with an Armour Class of 20 – a helmed horror and a roper. Nothing in those documents gets better. Almost nothing in the Monster Manual exceeds 20.
This is, in many ways, similar to the original Dungeons & Dragons design, where monster Armour Classes were all in a very limited range: basically from 2 to 9. In those days, instead of providing a target number, they provided a chart reference; you’d cross-reference the Armour Class with the level or Hit Dice of the attacker to see what the target number was. Making Armour Class into the target number was one of the things the 3E designers got absolutely right. Historically, using a look-up chart for Armour Classes allowed non-linear progression of the target numbers, but that concept was largely abandoned by the time original D&D was printed.
With Bounded Accuracy in place, this has several implications to how Armour Class is calculated. Drawing on the terminology of previous editions, you have a Base Armour Class which then has several Armour Class Modifiers applied to it before you get the final result.
In 3E, your Base Armour Class was 10, and then everything else modified it: Armour, Shields, Dexterity, Spells, Amulets, Rings, and so on. In 4E, you also gained a bonus to it equal to half your level.
This is not how it works in 5E. Instead, Armour, Spells or Special Abilities provide you with a Base Armour Class, which is then modified by a very limited number of sources.
If your character has several ways of calculating their base armour class, you only use one method.
Here are a few examples of Base AC calculations:
- No Armour: Base AC = 10 + Dexterity modifier
- Leather Armour: Base AC = 11 + Dexterity modifier
- Chain Shirt: Base AC = 13 + Dexterity modifier (max +2)
- Plate Mail: Base AC = 18
- Mage Armour spell: Base AC = 13 + Dexterity modifier
- Barbarian Unarmoured Defense
ability: Base AC = 10 + Dexterity modifier + Constitution modifier
- Monk Unarmoured Defense ability: Base AC = 10 + Dexterity modifier + Wisdom modifier
- Sorcerer Draconic Resilience ability: Base AC = 13 + Dexterity modifier
Meanwhile, there are several ways of further modifying your Armour Class. A few examples of these
- Shield: +2 bonus to AC*
- Shield of Faith spell: +2 bonus to AC
- Shield spell: +5 bonus to AC
- Half Cover: +2 bonus to AC
- Three-Quarters Cover: +5 bonus to AC
- +1 Armour: +1 bonus to AC
- Ring of Protection: +1 bonus to AC; requires attunement
- Bracers of Defense: +3 bonus to AC when not wearing armour or using a shield; requires attunement
- Arrow Catching Shield: +1 bonus to AC against ranged attacks; requires attunement
There is at least one unusual exception to how AC is calculated:
- Barkskin spell: Your minimum AC is 16.
*: The description of shields is unusual as it says it modifiers your base AC, but for most intents and purposes you can just treat it as a regular modifier.
As you can see, a multiclass Barbarian/Monk does not get to add all of their Dexterity, Constitution and Wisdom modifiers to the Armour Class! They have three ways of calculating their base AC, but can only choose one. However, they can benefit from as many other bonuses to AC as they like.
All the other modifiers stack; they aren’t split into several types of modifier like in 3E. (The one exception is that you can’t benefit from the same effect twice; if two clerics cast Shield of Faith on you, you only get a +2 AC. Multiple copies of the same spell do not stack!)
So, why can’t you just apply modifier after modifier and end up with a fantastic Armour Class that requires a natural 20 to hit? Well, you can in limited circumstances. However, there are three major restrictions on increasing your AC.
The first is that there aren’t all that many ways of modifying AC. Although several magic items in the Dungeon Master’s Guide do so, it’s something that the designers kept an eye one. +1 magic armour is sort of hard to find, but +3 armour is rare and precious, and +5 armour? It doesn’t exist!
The second is attunement. Magic items that provide powerful effects (and most permanent items that affect major numbers on your character fit into that category) have the limitation of “requires attunement”. The rules state that a character may have a maximum of three items attuned to them, so you are limited in that sense as well.
The third is concentration. Ongoing spells that provide bonuses typically require the caster to concentrate on them. That means that the caster can’t have any other spells requiring concentration at the same time. One character could get the benefit from a few spellcasters all providing them with armour class bonuses from different spells, but that will be the exception and not the rule, and the rest of the party wouldn’t be benefitting. It should be noted that there are very few spells that provide a bonus to AC in any case.
Honestly, the best way of increasing your Armour Class? Get behind an arrow slit! They provide three-quarters cover. Unfortunately, dungeon designers typically don’t design their dungeons with arrow slits the characters can take advantage of…
The flip side of this is to consider the range of attack bonuses. Just taking a quick flip through the Basic DM Rules, I see attack bonuses in the range of +0 to +14 (crab to adult dragon), with most being in the range of +3 to +7. A player character can probably expect at high levels to get a regular attack bonus of +12, assuming a 20 in the appropriate ability score, the full +6 proficiency bonus and a +1 weapon. Spells, items and abilities may push it a few points higher.
So, that’s a brief tour of how Armour Class works in the new edition. If you’d like to compare how AC worked in previous editions, I have articles on Armour Class in Original D&D and AD&D, AD&D 2E and D&D 3E. This is unlikely to be my last word on the topic, as my analyses are anywhere but complete!
|Padded||11 + Dex modifier||light, stealth disadvantage|
|Leather||11 + Dex modifier||light|
|Studded leather||12 + Dex modifier||light|
|Hide||12 + Dex modifier (max 2)||medium|
|Chain shirt||13 + Dex modifier (max 2)||medium|
|Scale mail||14 + Dex modifier (max 2)||medium, stealth disadvantage|
|Breastplate||14 + Dex modifier (max 2)||medium|
|Half plate||15 + Dex modifier (max 2)||medium, stealth disadvantage|
|Ring mail||14||heavy, stealth disadvantage|
|Chain mail||16||heavy, stealth disadvantage, Str 13|
|Splint||17||heavy, stealth disadvantage, Str 15|
|Plate||18||heavy, stealth disadvantage, Str 15|
|Shield||+2||modifies Base AC|