Monsters, Armour Classes & Challenge Ratings, Oh My!

What is the average monster’s armour class? How does AC relate to CR and what does all this have to do with character level? Let’s take a look.

Paisley of Endor, knight errant — paladin of Mielikki1 and Selûne2 — tightens her grip on the reins of her warhorse. She holds the animal steady as she stares down into the valley beneath her. The purulent lair of Oglegrot the Beholder lies somewhere below.

Many knights have ridden into this valley. The few who have returned describe Oglegrot as a fearsome aberration. They admit, with shame, that they struggled to land even a single blow against this appalling creature. Though she has sworn to defeat him, Paisley can’t help but wonder what her chances are against such a foe.

Possibly, Paisley isn’t alone in her pondering. Her player might also wonder just how hard Oglegrot is going to be to hit.

Average Monster Armour

A D&D monster’s3 armour class (AC) represents its ability to “avoid[…] being wounded in battle.”4 Oglegrot’s AC is the target number for an attack roll. Paisley’s player must roll a D20 to attack the Beholder. If the roll, plus Paisley’s Strength modifier5 and proficiency bonus, equals or exceeds Oglegrot’s AC; the attack hits. Otherwise, she misses.

But, what is Oglegrot’s armour class? We could look it up, of course, but where is the fun in that?

Instead, we will be using two different measures of central tendency, namely the median and the mode, to tease out how likely it is that our knight errant will ride out of the valley triumphant.

Having lit the Torches of Central Tendency, we can draw the following graph, which showcases a few tidbits about the armour classes of all the monsters found in 5th Edition’s Monster Manual (MM). Notice that the median armour class skews a little low. This indicates that there are more monsters with an AC below the median than above. Also, the mode represents about 17% of all monsters — thus, a significant, but not overwhelming proportion of the monsters in the MM have an armour class of 12.

This graph is a nice start, but it doesn’t shed too much light on Paisley of Endor’s predicament. We’d like a little more detail.

Armour Class & Challenge Rating

The 5th Edition of Dungeons & Dragons indicates a monster’s threat-level with something called a challenge rating (CR). Challenge ratings range from 0 to 30. In theory, the higher a monster’s challenge rating, the harder it is to defeat.

As the graph below shows; median armour class rises with challenge rating. In fact, there is a 98.5% correlation between median AC and CR. However, there is quite a bit of variation in AC values for a given CR, particularly at lower challenge ratings. For instance, a CR2 monster’s AC can range from 6 to 19! This variation lessens the median’s usefulness in describing low CR monsters — we must keep this in mind as we press on.

Challenge Rating & Character Level

As as we have seen, comparing armour classes with challenge ratings is certainly helpful. We know a monster’s AC tends to rise along with its CR. Unfortunately, Paisley’s player is no more likely to know Oglegrot’s CR than his AC! Perhaps we can make an educated guess as to the Beholder’s challenge rating? In fact, we can — and the Dungeon Master’s Guide (DMG) provides some helpful hints.

Hold on to your hats, this is going to be a bit of bumpy ride!

Paisely’s level is determined by the number of experience points (XP) she has. She earns XP by defeating monsters. And, the amount of XP a monster is worth is determined by its challenge rating — the more dangerous the monster is, the more XP she gains by defeating it.

Now, a confrontation with a monster (or monsters) is called an encounter. The Dungeon Master’s Guide uses XP to determine how difficult an encounter is — we are going to use Paisely’s level, and her intuition about the difficulty of an encounter with Oglegrot, to infer his XP and CR (and ultimately his AC). But first, we need to lay a little groundwork.

To start, an adventuring party’s XP threshold6 is determined with the XP Thresholds by Character Level table found in the Dungeon Master’s Guide.7 We will use it determine the medium and hard XP thresholds for a party of four characters.

Next, we will use the Experience Points by Challenge Rating table8 and the Encounter Multipliers table9 to find upper and lower boundaries for our monster’s challenge ratings.

We will select a CR rating for a single monster whose XP is just under the hard encounter threshold for our upper boundary. These are our end bosses — the most dangerous monsters a character, like Paisley, will come up against (at least until she levels up).

And, we will choose a CR rating for four monsters whose XP is just under the medium encounter threshold. These monsters are our rank and file minions — they form the bulk of a level’s encounters.

Our results are shown in the table below.

Encounter Challenge Ratings by Character Level
Character Level1234567891011121314151617181920
Upper CR12347789101112141516161719202021
Lower CR1/81/41/41/21112223344445566

Arguably, choosing a likely range of challenge ratings for each character level is a subjective process. You may disagree with the choices we have made. For instance, you will notice that our table does not include monsters with CR 0 or CR 22-30. This is a byproduct of our method, by I think it is a serendipitous one. The Monster Manual describes CR 0 monsters as “insignificant except in large numbers.”10 And, CR 21 and higher monsters are designed to be more than a “typical 20th-level party can handle.”11 Therefore, excluding these monsters from our calculations seems quite reasonable.

Monster Armour & Character Level

Hopefully, you still have your hat on, because we are nearly finished.

We will now calculate the maximum, median, mode and minimum monster armour classes for each character level. For example, 1st level characters are likely face CR 1/8, CR 1/4, CR 1/2 and CR 1 monsters. On the other hand, 5th level characters will probably come up against monsters ranging from CR 1 to CR 7. The following graph shows our results.

Now, we could stop here and be quite happy. But, I’d like to tweak our results just a little more.

Average Effective Monster Armour

In order to refine our results, we are going to make an assumption about Paisley, Paladin of Forest and Moon, and character’s of her ilk. We are going to take for granted that she is good at her job. Or, to put it another way, we will assume that Paisley started the game with a +3 Strength modifier and that her player has used her ability score increases to maximize her hitting ability.

Essentially, we will subtract Paisley’s Strength modifier and her proficiency bonus, for a given level, from our results to find the monster’s effective armour class. If our premise holds, then the graph below shows exactly what her player needs to roll in order to hit.12

Importantly, this graph assumes Paisley is a paladin; it would look a little different if she were, for example, a fighter.

As an aside, I find it interesting that high CR monsters actually get a little easier to hit as characters advance from level to level — despite the fact that their AC tends to increase!

In Conclusion

The last bit of information we need is Paisely’s level. Luckily, Paisley’s player always knows her level! As it happens, she is a 12th level paladin. Now, we are ready to make an educated guess about Oglerot’s armour class. From the above graph; the median effective armour class is 6. However, we know that Oglegrot has bested every knight who has tried to defeat him. He feels more like a boss monster to me! Therefore, Oglegrot’s AC is likely to be closer to the maximum effective AC of 11.

To clarify, Paisely’s player likely needs to roll somewhere between 6 and 11 (and probably closer to 11) or higher to hit the nefarious Oglegrot. This means that Paisley has between a 50% to 75% percent chance of hitting Oglegrot — but probably closer to 50%.

Checking the Monster Manual, we learn that a beholder has an effective AC of 9 when facing a 12th level paladin. The paladin has a 60% chance to hit. Happily, this is well within our results.

Finally, I would be remise if I didn’t point out that this method for guessing a monster’s AC works best for mid to high CR monsters. Because there is a significant AC variation in low CR monsters, we would be less certain of Oglegrot’s AC if he were a relatively low CR monster.

Additional Reading

Check out these pages for more musings on monsters and/or armour class in Dungeons & Dragons.

Notes

  1. Mielikki is described as the goddess of forests on page 294 of the Player’s Handbook (PHB).
  2. Selûne is described as the goddess of the moon on page 294 of the PHB.
  3. In the parlance of D&D, non-player characters are referred to as monsters.
  4. See page 14 of the PHB.
  5. Some weapons use the Dexterity modifier instead of the Strength modifier.
  6. The appropriate amount of XP for easy, medium, hard and deadly encounters is called an XP threshold.
  7. See page 82 of the DMG.
  8. See page 275 of the DMG.
  9. See page 82 of the DMG.
  10. See page 9 of the MM.
  11. See page 9 of the MM.
  12. Other modifiers, such as a +1 weapon, will also lower a monster’s effective AC.
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