Which Armor Class values should be used as controls when making damage calculations?

Question

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

Answer 1

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 rises ¹
• 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).}