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In D&D 4e, you could tell most monster's AC just from its level and role (Brute, Skirmisher, Soldier).

This is not the case in 5e, but an estimation would be very useful when you try to compare builds.
My group uses 13 + proficiency bonus, this has the benefit that you can completely remove the proficiency bonus from the calculation. However, this is a bit high at the beginning, and low at the end.

  • Does someone have a better formula?
  • Better yet, has anyone found a compilation of existing monsters, that could be used to calculate this for myself?
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DMG 274 suggests these values for AC as part of the table for defensive CRs (HP also factors into defensive CR, but is not listed here):

  • CR 0-3: 13 AC
  • CR 4: 14 AC
  • CR 5-7: 15 AC
  • CR 8-9: 16 AC
  • CR 10-12: 17 AC
  • CR 13-16: 18 AC
  • CR 17+: 19 AC

According to the guidelines, the ultimate CR of a creature is the average of their offensive and defensive CRs. However, the DMG encourages additional tweaking and adjustments to individual monsters beyond the listed guidelines:

Alternatively, you can determine an appropriate AC based on the type of armor the monster wears, its natural armor, or some other Armor Class booster (such as the mage armor spell). Again, don't worry if the monster's AC isn't matching up with the expected challenge rating for the monster.

There are also a number of features that a monster can modify the effective AC for CR calculation purposes. These are tabulated starting on DMG 280. For example, giving a monster magic resistance boosts its effective AC by 2, which might result in a defensive CR that's higher than the one calculated strictly from HP and AC.

Thus, there is going to be significant variation in the AC of monsters at any CR, because there are so many factors that can change the final AC relative to the CR, including the judgment of the person designing the monster.

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    \$\begingroup\$ Like I said in the other answer's comment, you forget to mention that AC is just a part of a monster's defensive CR, and that monsters vary greatly from the values you presented here. \$\endgroup\$ – kviiri Feb 27 '17 at 7:02
  • \$\begingroup\$ @daze413 The answer mentions defensive CRs and AC but not hit points - assume you've never seen the table in the DMG, it's going to seem like defensive CR is tied to armor alone. As for the point about armor, yes, it's true that humanoids tend to have their AC based on pieces of equipment but it has no difference to a natural armor from a balance point of view (except vulnerability to spells like heat metal, but they are exempt from the CR calculations). \$\endgroup\$ – kviiri Feb 27 '17 at 9:31
  • \$\begingroup\$ @kviiri Yeah, I guess I can see a problem if a reader hasn't read the DMG's rules on creating monsters. So it might be worth putting in a few more words to clarify that. \$\endgroup\$ – daze413 Feb 27 '17 at 9:35
  • \$\begingroup\$ I've added in a bit more detail about the defensive CR calculation. Does this address the issues here? \$\endgroup\$ – Icyfire Feb 27 '17 at 15:57
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Aside from DMG suggestions, a statistical analysis can also be useful. For example, analyzing the creatures in the SRD, gives you this chart

enter image description here

And you can see it matches the guidelines from the accepted answer.

Just keep in mind that AC is just a part of a CR, and can vary wildly from monster to monster, or from encounter to encounter (lots of tiny minions can be just as dangerous as a big monster!)

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  • \$\begingroup\$ What is ruining the trend at CR12? \$\endgroup\$ – András Nov 18 '19 at 16:08
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    \$\begingroup\$ There's a much higher variance at CR12. Maybe monsters there either have a lot of attack and low defenses, or the opposite, both of which raise CR. \$\endgroup\$ – BlueMoon93 Nov 19 '19 at 13:18
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As mentioned in the accepted answer, the DMG offers a table with appropriate AC corresponding to the challenge rating (CR) of a creature. However, the question mentioned that this AC should be used as a reference for the sake of comparing character builds. Therefore, an average/reference AC for a given level of a PC could be useful (even though the question specifically asked for CR). Now the correspondence between the level of a PC and the CR of a typical enemy at this level is unclear (see discussion here), the DMG just mentions that

especially at lower levels, [the DM should] exercise caution when using monsters whose challenge rating is higher than the party’s average level.

When it comes to comparing different builds mechanically, I would suggest to use the following formula to determine a reference enemy AC for a given PC level:

AC = 8 + PB + AM

where PB denotes the proficiency bonus of the PC and AM is the ability modifier that any PC focusing on increasing a certain ability score usually has at that level. That is, AM=3 at levels 1-3, AM=4 at levels 4-7 and AM=5 at levels 8-20 (starting with a score of 16 and taking ASIs at levels 4 and 8).

Advantages of this way to determine the AC are that

  • for calculations, the proficiency bonus cancels out (as in the formula occuring in the question),
  • this formula also determins the spell save DC of a spellcaster, so there is a nice correspondence,
  • this gives the same ACs as the ones given in the accepted answer (assuming CR=PC level and except for level 9) and
  • any PC that increases its ability score used for attack rolls as soon as possible will have a 65% chance to hit against the reference AC at every level; this might simplify calculations.

Note that there is also the approach to fix the "baseline-chance to hit" (which implicitly determins the reference AC) and (as just mentioned) the formula above corresponds to fixing this baseline to 65%. Another prominent choice is fixing it to 60%, corresponding to the formula AC = 9 + PB + AM that would yield an AC of an enemy with CR slightly above the PC's level.

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In 5e there is no real connection between CR and AC. This was a deliberate step to move away from the number treadmills of previous editions.

Yes, increasing a creature's AC can make it tougher (which might justify a higher CR), but that doesn't go in reverse (i.e. having a higher CR does not necessitate a higher AC).

Take the Balor, it has a CR of 19, yet its AC is also 19. You know who else has an AC of 19? A level 1 fighter using Chainmail, a Shield, and the Defense fighting style (the +1 AC one).

Let's look at some others:

  • CR 1/2 Hobgoblin: 18 AC

  • CR 1/4 Goblin: 15 AC

  • CR 1/8 Noble: 15 AC (blame nepotism)

Anyway, basically in 5e AC is used to represent how hard things are to hit, not how high of a level you need to be to hit them.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – nitsua60 Feb 27 '19 at 1:37

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