[RPG] Which Armor Class values should be used as controls when making damage calculations

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It is known that occasionally questions will pop up that are about attempts to optimize a character's damage output (sometimes shorthanded as DPR, or Damage-per-Round), and veterans of this game know that which Armor Class values used to perform these optimizations can have pretty significant impacts on how you optimize for damage.

In the past, I've been using what I thought was a pretty good "Standard Array" for Armor Class values to test against: AC0, AC14, AC16, AC18, AC20, AC26, for the purpose of covering the following cases:

AC0: Absolute "Ideal Scenario" gameplay, where bonuses are stacked high enough or creatures are vulnerable enough that their Armor Class is irrelevant
AC14-20: Commonplace "Regular Play" scenarios, representing average-case damage output
AC26: In officially published books, AC26 is greater than any Armor Class that any creature has; even the mightly Terrasque has an Armor Class of 25 in 5th Edition D&D

I'd also backed this with the context of the Dungeon Master's Guide, which in its table (page 274) describing guideline Armor Class values for custom creatures, it never goes below 13. So I've been operating under the premise that it's not realistic to perform DPR calculations against creatures that have an Armor Class of 13 or lower.

However, doing more research into how Statblocks are allocated in the Monster Manual and other supplemental books, I'm beginning to question this assumption. For example, despite AC13 being listed as a nominal minimum for even creatures as low as CR1/4 in the Dungeon Master's Guide, there are lots of creatures whose AC is well below 13, even for significant Challenge Ratings. In fact, nearly a third (369/1205) of the creatures found in officially published 5th edition materials have Armor Classes below 13, and even if you exclude the CR<1 creatures from that list, more than 10% of all creatures still have Armor Class values of 13 or lower. This list also includes relatively high CR creatures like the Corpse Flower, Giant Ape, Tyrannosaurus Rex, and other high CR creatures that have an Armor Class of 13 or lower.

My sense from seeing these statblocks is that while the 14-20 range might be accurate for some adventures and DMs, it may not be accurate for others, depending on what kinds of creatures are pulled from the manual to use in encounters.

Of course, I could just span the entire range from [5,25] to capture every single Armor Class that shows up in published 5th edition materials, but tables constructed with 21 columns of Armor Class values are unwieldy and difficult to read. So in an ideal world, I don't think I want to represent more than 6 or 7 distinct Armor Class values.

So I'd like to solicit some feedback: what would constitute a good array of Armor Class values to use in the future to try to represent the important use cases, if the use cases are:

Creatures whose Armor Class is trivially low
Creatures whose Armor Class is common and typical for a 5th Edition campaign
Creatures whose Armor Class is uncommonly high for a 5th Edition campaign
Creatures that constitute "Boss Monsters" with uniquely high Armor Classes

Best Answer

It depends on the characters' level

Observe the recommended AC for creatures of various CR ratings in the DMG (p. 274) and you'll notice something interesting. The suggested AC goes up based on two factors:

AC rises as Proficiency Bonus rises¹
AC rises at CR 4 and 8

There's a simple explanation for this. These are the levels at which PCs tend to add to the bonus to their Attack rolls. Many PCs will choose to use their first two ASIs (at level 4 and 8) to increase their primary attack stat to 20. Otherwise, their attack roll bonus will mainly rise when their proficiency bonus does (barring unusual jumps, like multiclassing into a class with the Archery fighting style).

This tells us something very simple: the baseline AC recommended for a creature is one that causes an average character whose level matches its CR to hit it by rolling a natural 8 or better on a d20 (without magical assistance).

A DM who follows the recommendations for encounter difficulty in published adventures will often find themselves defaulting to this (perhaps without realizing it) for a "medium" difficulty encounter. But the precise AC used will vary as characters level up, since their proficiency bonuses at least will make them more effective.

How much should we vary the AC?

This question is more subjective, because various features can raise or lower a creature's AC. But we can use some established rules to give a reasonable approximation of viable ranges.

On page 82 of the DMG, it gives recommended XP values (per character) for Easy, Medium, Hard, and Deadly encounters. Assuming that there are 4 players in the party, the "Medium" xp value would usually give us a XP value where CR = Party level. So our "typical" AC value should probably be one where the creature is hit on an 8 or better.

There is definitely some variety in how the table then progresses. But a common pattern is that each difficulty category (roughly) is separated by a change in CR of 1. (E.G., if a medium encounter a CR 3, then a CR 4 would be a hard encounter, and a CR 5 about a deadly one).

In the rules for Monster Creation (DMG p. 274-277), we can see that raising (or lowering) a creature's AC by 5 will change its Defensive CR by 2, changing its overall CR by 1. And raising its AC by 9 will raise its Defensive CR by 4, raising its overall CR by 2.

Thus, as a general guideline:

A "trivially low" AC will be (at the lowest) hittable on a natural 3 or better.
A "typical" AC will be hittable on a natural 8 or better.
An "unusually high" AC (at the highest) will be hittable on a natural 13 or better.
A "boss level" creature with a "uniquely high" AC will be (at the highest) hittable on a natural 17 or better.

Naturally, these are just abstractions. They assume that the changes to CR are based on nothing but a change in AC, which is undoubtably an edge case. And some of these AC values are nearly impossible at certain levels (for example, a level 17 character would only encounter this "boss level" AC if an enemy had an AC of 28). But they are useful when defining a reasonable range to use when making damage calculations.

_{1 Except at CR 9, where it waits until one higher (CR 10).}

Related Solutions

[RPG] Tiny Creatures sharing a square

D&D 5e doesn't have a specific rule on this because the fifth edition of the D&D ruleset doesn't make use of 5' squares when determining positioning of creatures -- it relies more heavily on an abstraction of what is happening mechanically in the game to determine this. Note that there are variant rules in the DMG for using a grid where 1" square = 5' square.

That being said, what we do have is a general idea of what space a creature of a given size "controls":

Size       Space
Tiny       2½   by  2½  ft.
Small       5   by  5   ft.
Medium      5   by  5   ft.
Large      10   by  10  ft.
Huge       15   by  15  ft.
Gargantuan 20   by  20  ft. or  larger

Source: 5e SRD pg 92

And just below that, in the SRD:

A creature’s space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions. A typical Medium creature isn’t 5 feet wide, for example, but it does control a space that wide. If a Medium hobgoblin stands in a 5-foot-wide doorway, other creatures can’t get through unless the hobgoblin lets them.

A creature’s space also reflects the area it needs to fight effectively. For that reason, there’s a limit to the number of creatures that can surround another creature in combat. Assuming Medium combatants, eight creatures can fit in a 5-foot radius around another one.

Because larger creatures take up more space, fewer of them can surround a creature. If five Large creatures crowd around a Medium or smaller one, there’s little room for anyone else. In contrast, as many as twenty Medium creatures can surround a Gargantuan one.

Based on the amount of space a tiny creature takes up (2.5 x 2.5, or 1/4 of a 5 foot square) I think it's safe to say that the breakdown is the same as in previous editions -- you could fit four 2.5' squares inside a single 5' square, so you could fit 4 2.5' tiny creatures in the space occupied by a creature who occupies a single 5' area.

However, I think it's important to remember that things are not often so neatly organized into 5' squares, and that's one of the reasons grid-based combat in 5e is a rule variant and not the base assumption. Allowing combat and positioning to occur more organically might be a bit more work than using a grid system but in my experience it has allowed for much greater creative freedom from players.

[RPG] How does the AC evolve with high levels

AC is only one form of defense, and doesn't typically grow with level.

What you're running into is the concept of Bounded Accuracy. One of the design pillars of D&D 5e is the idea that even high level characters don't have passive defenses that make them invulnerable to the attacks of lower-level characters. It is an intentional feature of the system that level 1 characters with an attack bonus around +5 can hit the AC of a level 20 character.

What this means is that, as characters level up, one of the ways that they get more powerful is that they hit more often, and are hit more often in turn. It's totally normal for most attacks to hit in a high-level combat where everyone is swinging with attacks above +10.

Generally speaking, without magic items, the highest AC a character can reach is in the low 20s. A paladin with a shield wearing plate armour isn't going to go much past 20 without magical assistance. A breakdown of how a character hits maximum AC can be found here. In short: Barbarians can get their AC a little higher than 20 without magic because of their higher stat cap, and magic item can push your AC another few points.

What you're missing is the other kinds of defense that make a high level character hard to kill. Higher level characters have many more hit points and access to defensive buffs than low level characters, and generally have more passive and active defenses in general. For example: your Paladin player will eventually get an aura that protects the party from various effects, based on their Oath. Also, Lay on Hands acts as an effective HP buffer that makes the party much more difficult to take down as the Paladin keeps them healed. In addition, your casters likely have a large number of battlefield control spells that make it possible to shut down enemies before they can even attack at all.

Your players probably won't have their AC increase, but they will get many more ways of dealing with threats defensively.

Note: The link to the Wizards article on Bounded Accuracy goes to the Wayback Machine, because the original article is no longer on the WotC site.