Gnomish Workshop: I See AC

Gnomish Workshop: I See AC

This article was first broadcast in Episode Two Hundred and Forty Nine on 10th October, 2023.

Note: This article was adapted from an episode script, and so there may be parts that don’t flow well when read, because they were initially designed for broadcast.

Armor class is one of the most consulted stats in D&D 5th edition. It could arguably be *the* most consulted stat, assuming your game has combat as a major part of the play sessions. Even if it’s a war of spellcasters, a number of spells use attack rolls that target AC. In any average combat, though, almost everyone including the DM is going to be looking at that Armor Class statistic multiple times.

Now before we gets into the details, we’re going to talk about AC as a concept. Despite the name and the common ways to increase it, “Armor Class” is not only a representation of the amount of armor your character wears. If you think about it for a minute, you’ll realize that makes no sense. Rogues, Bards, and Monks can easily run around the battlefield with ACs of 16 or 17, and they’re dressed in ninja wraps, a dinner jacket, and a fancy towel, respectively. So Armor Class doesn’t directly translate to the amount of armor someone has on.

In the early days of D&D, AC probably did just mean “how much armor you have on” and Wizards hasn’t wanted to mess with the basics of D&D that much, so they kept it. But if they wanted to be more precise, they should have renamed it something like “DC to hit”.

Part of the Armor class represents the durability of whatever armor you’re wearing. However it also takes into account how nimble the character is. That’s why, absent any other armor, a character’s AC includes their Dexterity modifier. While a fully kitted out Paladin in plate mail with a shield probably is just taking the sword and mace blows on his body and letting the metal absorb them, a rogue or a ranger are more likely dodging out of the way so the attacks miss them completely.

This doesn’t matter much mechanics-wise, but if you add a lot of color commentary to your battles and you constantly struggle to figure out how an owlbear claw gets deflected by the wizard’s cloth robes, remember that what actually allowed the wizard to shrug off the hit was them getting out of the way.

Some people take this farther and change the description depending on how much the attack failed by. For example, if an attack missed by more than 5 the attempt just flat-out missed the character. But if the attack only missed by 1 or 2, they describe the blow being deflected off a shield, or just barely dodged by a character’s sidestep.

The mechanics of armor class calculation support the idea that it is the most important statistic in the game. The number of modifiers and methods to improve or alter armor class outstrip all other statistical manipulation.

To begin with the simple, unaltered approach, a character’s armor class is equal to 10 added to the dexterity modifier of the character. Note: if a character’s modifiers include a negative figure for Dexterity, that will reduce the character’s AC below 10. This is unadvisable.

The most common and easiest method to improve armor class is apropos employing armor. The effects will now be reviewed based on armor category.

Light armor, in general, simply bumps up the base number from 10 to 11 or 12 and then still lets you add in your Dexterity modifier, whatever it is. It gives most characters a little boost but doesn’t help a lot unless you’re like me and you focus on improving your dexterity so you can have a 17 AC while wearing the finest supple rabbit-hide clothing gold can buy.

Medium armor is the halfway point. As we mentioned earlier, AC is a mix of how much of a hit your armor can take and how good you are at getting out of the way. But of course, the more armor you put on, the harder it is to move, either because of the weight or because the armor isn’t as flexible. You *can* do a cartwheel in plate armor, but it’s not easy.

Most medium armor has a slightly better increase to the base value, usually something in the 14 to 16 range. Then you can add in some of your Dexterity modifier, but not all of it. Most medium armor only allows you a maximum of 2 or 3 out of your dexterity modifier.

Heavy armor is where you no longer have to worry about moving around because you’re wearing so much metal it’s just easier on everyone if you stand there and take the hit. Almost all heavy armor just gives you a flat AC, and the Dexterity modifier doesn’t enter into it. The vaunted plate mail, for example, sets your AC at 18, and it doesn’t care if you can dance with cirque du solei; that’s as good as it gets.

Until you pick up a shield that is. For the low, low price of 10 gold and one hand you can’t use for a weapon anymore, you get +2 to your AC.

In general, without special exceptions, most characters designed to fight in the front lines, like Fighters and Paladins, can get up to 20 AC just with base equipment. Mid-range skirmishers and off-tanks like clerics, barbarians, and Monks usually end up around 16 or 17, and the so-called “squishies” are often trying to make do with something between 11 and 13.

All of those are base values, however, and represent unmodified mechanics. The number of exceptions to the basic calculations are so numerous that situations which should be edge cases manifest as commonplace. To begin with, any and all armor can be of a magical variety, which provides a flat bonus from +1 to +3 applied to the Armor Class Calculation. Given that shields and armor are two separate pieces of equipment and can be enchanted separately, it is theoretically possible to acquire an extra 6 points to the armor class value, assuming your DM lacks fortitude and the ability to say “no”.

There are also a number of separate magic items, such as cloaks and rings of protection, which provide unrestricted bonuses to Armor Class.

At this point we will briefly review the premise behind bounded accuracy. Attack rolls are calculated based on the attack attribute modifier, the proficiency bonus, and the d20 roll. At highest values for most characters, this means the maximum possible bonus without magical assistance receives 11 from static values and 19 from the die roll. 20s are discounted as they bypass Armor class.

That means any armor class value above 30 represents mundane invulnerability; it would be impossible to hit the creature with a regular attack unless it was enhanced by magical spells or items.

Now monsters aren’t quite as constrained as characters in their statistics; it’s not too hard to find creatures with Strength and Dexterity numbers well above 20. But ROSTRO apparently put the research beholders to work and they figured out that if you discount the gods and demigods from the mix, the highest bonuses the monsters can manage is +15, and most of them are well below that.

There are a lot of different ways to get your character’s AC up above 20. The most popular methods usually involve creating a Monk, Barbarian, or Bladesinger wizard. Those classes all get to add another modifier to their AC calculation along with Dexterity, which means they can pull +6 or +7 from their stats without even skewing their build into min/maxing too much. Add the armor and shields back in and those people are sitting comfortably on ACs of 22 or 23.

If the DM isn’t stingy with protective magical items then the average top number goes even higher. As we already said, if a character somehow convinces the rest of the party that both +3 protective items should go to them, they can put another six points on top of where they were before, so now we’re up to 27 or 28 and we’re rapidly approaching that “you can’t hit me” mark that was mentioned earlier.

Realistically, in the low teen levels with average magic item accrual, a character who’s focused on getting their AC maxed out can probably manage a safe 23 to 25 without having to worry about special circumstances or extra bonuses.

However, there exists the reality of the so-called “Burst AC”. In common parlance, burst AC refers to the ability of various spells and abilities to confer a temporary bonus to the AC value that dissipates after a predetermined trigger. The most common example of this is the “shield” spell, which provides a static +5 bonus to AC for the duration of the turn it was cast. Most if not all of these effects stack, and many of them do not require predetermined triggers.

Employing advantageous racial, class, and equipment builds and assuming one’s DM has determined they no longer care about realism or order in their game, theoretical calculations posit it is possible for a character to achieve a maximum burst AC value between 79 and 83. The variability is the result of some uncertainty regarding interactions between stacking bonus mechanics.


We would reiterate that the maximum bonus to attack that currently exists within 5e belongs to, alternatively, the Tarrasque or Tiamat, either of which is capable of making an attack with a +19 to hit. Again discounting criticals, that means the maximum possible attack value a character could face is 38. Increasing one’s armor class beyond that value represents a practical waste of resources.