What is Bounded AC?

[RyumaruMG]

Basically what the title says. In discussions on 5E I've seen people refer to "Bounded AC" but reading through the PHB, MM, and DMG I haven't found anything about it. What is it, what does it mean, and how do I use it as a DM?

[Naanomi]

Bounded Accuracy is a game design term that basically means efforts are made to make sure numbers in the game don't get too high; which helps keep low-level threats still threatening to higher-level characters... and helps makes the 'balance' game a bit easier (though that isn't the main reason for it).

It isn't a game/rule term

[Slipperychicken]

It's a way of implementing a design principle. Basically it means that armor class doesn't increase very much. The AC of official monsters lies in a range from about 8 to 25, and doesn't go higher or lower than that. It's not a hard rule, but that's what people are referring to when they say "Bounded AC".

The idea is to make sure that armor class doesn't go too high. That would make low-CR enemies nearly irrelevant against enemies with high AC, as their attacks would almost never hit. So in short you don't need to do anything special as a DM to use it; it's just part of how 5th edition was written. I guess you could violate it in a sense by home-brewing monsters with AC exceeding 25, but I don't see why you would want to do that.

[wunderkid]

I cant off the top of my head remember but i think It's basically referring to a soft cap for AC. Because most things don't stack. And your Dex can reach a maximum of 20 it's easier to keep insane ACs under control.

Same with the ability to hit. Bounded accuracy and Bounded ac keep things even.

Of course through shenanigans you can pump them higher.

[TurboGhast]

I just got triply ninja'd. Wow.

[Naanomi]

Note that Bound Accuracy isn't just talking about Hit/Armor Class; it extends to nearly every numerical aspect of the game (except perhaps HP, which is the main stat that still scales drastically)

There are still shenanigans of course; I think the current maximum AC is upwards of 77 or more under very specific, ridiculous circumstances

[treecko]

There are still shenanigans of course; I think the current maximum AC is upwards of 77 or more under very specific, ridiculous circumstances

... Please share how? The highest I remember was some weird barbarian bladesinger build that got around 32. (unarmored defence+bladesing+shield spell + shield of faith)

[Slipperychicken]

There are still shenanigans of course; I think the current maximum AC is upwards of 77 or more under very specific, ridiculous circumstances

That is true. In corner cases that are both extreme and unlikely you can briefly reach an armor class much higher than what the game expects. Of course, AC increases stop being useful after 39, since the highest listed tohit bonus (or at least the highest I know of) is +19, which both the Tarrasque and Tiamat have. At that point every attack in the game needs a nat 20 to hit you anyway, so getting it higher does not further reduce enemies' hit chance.

[Naanomi]

... Please share how? The highest I remember was some weird barbarian bladesinger build that got around 32. (unarmored defense+bladesing+shield spell + shield of faith)

Barbarian with 30 Dex and 30 Con: AC = 30
Shield +3: +5 AC
Defender Weapon: +3 AC
Defender Fighting Style: +1 AC
Ring of Protection: +1 AC
Cloak of Protection +1 AC
Blessing of Protection: +1 AC
Shield of Faith: +2 AC
Haste: +2 AC
Combat Inspiration: +12 AC
Evasive Footwork: +12 AC
Warforged Race: +1 AC
3/4 Cover: +5 AC

This totals to 76

I'd guess working in Bladesinging would increase this but I'm away from my copy of SCAG right now so I can't do the specific math... it would be what, +10 if you had 30 INT, but you'd have to give up the shield and defensive combat style (a total of +6, so a net gain of +4...); but dropping the shield means you can have the Mariner fighting style for another +1... but you'd have to drop warforged to be an elf instead... which I think puts us at a clear 80 AC

Barbarian 1/Fighter 15/Wizard 2 with two levels to spare

This is useless in any monster fight, but may have use in PvP (I think we figured an equally absurd set of circumstances can give up to +119 to hit)

[Pope Scarface]

How do you have a 30 Dex and 30 Con? They stop at 20, unless you're a level 20 Barbarian, then Con stops at 24.

[Naanomi]

How do you have a 30 Dex and 30 Con? They stop at 20, unless you're a level 20 Barbarian, then Con stops at 24.
Repeated readings of Magic Tomes... as I said getting that high of stats is ridiculous (and the updated version has 30 Intelligence as well)

[Mellack]

Reading magical tomes can give a permanent bonus. Note that to get to 30 you would need several books.

edit: damn ninjas :)

[NewDM]

Reading magical tomes can give a permanent bonus. Note that to get to 30 you would need several books.

edit: damn ninjas :)

The likely hood of finding that many tomes is insanely low. Your chance of finding 1 or 2 is in the single digits.

[comk59]

The likely hood of finding that many tomes is insanely low. Your chance of finding 1 or 2 is in the single digits.

And? This is a theoretical build, not something people will actually be playing with. Its sole purpose is to test the limits of the system.

[NewDM]

And? This is a theoretical build, not something people will actually be playing with. Its sole purpose is to test the limits of the system.

You aren't testing the limits of the system if you are going outside the rules and guidelines used for the common game.

[comk59]

You aren't testing the limits of the system if you are going outside the rules and guidelines used for the common game.

Yes. We are going outside the guidelines and choosing the most shenanigany way to get the highest possible AC within the 5e system. It's not supposed to be representative of actual play, it's theorycrafting.

[DeAnno]

At that point every attack in the game needs a nat 20 to hit you anyway, so getting it higher does not further reduce enemies' hit chance.

Hmmmm. An Archer with 20 Dex, Archery Style, a d12 Superiority die, a +3 Bow and a +3 Arrow, +6 proficiency, and a 19 rolled would be:

5+2+12+3+3+6+19=50

So 51 is still a "valid" AC :smallcool:

[Slipperychicken]

The likely hood of finding that many tomes is insanely low. Your chance of finding 1 or 2 is in the single digits.

It's just taking the system to the most extreme corner case to see how high AC can possibly go. It's not intended for real games, and no-one is trying to argue that those ability scores or AC are realistic to achieve in actual play.


So 51 is still a "valid" AC :smallcool:

That is true if indeed you face such an enemy. I suppose it was folly to restrict myself to enemies in the MM, rather than including possible PC-classed enemies. So I guess I should qualify that statement: 39 is the point at which further AC increases are not useful against creatures listed in the monster manual, but that point increases to 51 or more when you include enemies with PC classes, especially with buff spells or very powerful equipment.

[RyumaruMG]

Awesome, thanks for the help, all. I feel much more clear on a lot of the system now!

[pwykersotz]

A quote from the Legends and Lore article regarding bounded accuracy:


The basic premise behind the bounded accuracy system is simple: we make no assumptions on the DM’s side of the game that the player’s attack and spell accuracy, or their defenses, increase as a result of gaining levels. Instead, we represent the difference in characters of various levels primarily through their hit points, the amount of damage they deal, and the various new abilities they have gained. Characters can fight tougher monsters not because they can finally hit them, but because their damage is sufficient to take a significant chunk out of the monster’s hit points; likewise, the character can now stand up to a few hits from that monster without being killed easily, thanks to the character’s increased hit points. Furthermore, gaining levels grants the characters new capabilities, which go much farther toward making your character feel different than simple numerical increases.

Now, note that I said that we make no assumptions on the DM’s side of the game about increased accuracy and defenses. This does not mean that the players do not gain bonuses to accuracy and defenses. It does mean, however, that we do not need to make sure that characters advance on a set schedule, and we can let each class advance at its own appropriate pace. Thus, wizards don’t have to gain a +10 bonus to weapon attack rolls just for reaching a higher level in order to keep participating; if wizards never gain an accuracy bonus, they can still contribute just fine to the ongoing play experience.

This extends beyond simple attacks and damage. We also make the same assumptions about character ability modifiers and skill bonuses. Thus, our expected DCs do not scale automatically with level, and instead a DC is left to represent the fixed value of the difficulty of some task, not the difficulty of the task relative to level.

[AvatarVecna]

The likely hood of finding that many tomes is insanely low. Your chance of finding 1 or 2 is in the single digits.

To be fair, we are talking about an Elf here, and the tomes become re-readable after a century. The odds that a 20th level Elf who's been alive forever having one of each book that they read every century to get all their stats up to 30 is about as ridiculous as the rest of this scenario, but it's only ridiculous because of the exact scale things are being taken to (having three books that you've read once a century five centuries in a row).

[Pope Scarface]

Pretty sure reading it twice would do nothing, as it says somewhere that having the same effect from two sources does not stack, and this would seem to qualify, Maybe I'll look for it after work, if nobody beats me to it.

[AvatarVecna]

Pretty sure reading it twice would do nothing, as it says somewhere that having the same effect from two sources does not stack, and this would seem to qualify, Maybe I'll look for it after work, if nobody beats me to it.

I know that's true for spells, and that items giving bonuses to things can't be stacked with themselves (for example, you wouldn't get benefit from carrying two Luckstones any more than you would from carrying one), but the books don't give bonuses, they alter base stats; losing the book doesn't take away your bonuses, because the books are altering your abilities permanently. And the books increase both your attribute and the maximum by two points, up to an absolute maximum of 30, and then they can't be used again for a century by anybody.

I'm pretty sure that's how it works.

[Takewo]

Pretty sure reading it twice would do nothing, as it says somewhere that having the same effect from two sources does not stack, and this would seem to qualify, Maybe I'll look for it after work, if nobody beats me to it.

But the same rule wouldn't allow you to benefit from two different books either, then.

[Pope Scarface]

I know that's true for spells, and that items giving bonuses to things can't be stacked with themselves (for example, you wouldn't get benefit from carrying two Luckstones any more than you would from carrying one), but the books don't give bonuses, they alter base stats; losing the book doesn't take away your bonuses, because the books are altering your abilities permanently. And the books increase both your attribute and the maximum by two points, up to an absolute maximum of 30, and then they can't be used again for a century by anybody.

I'm pretty sure that's how it works.

I would rule differently, but that seems like an equally valid way to interpret that.

[Naanomi]

But who can hit this unstoppable man?
Dexterity 30: +10
Proficiency Bonus: +6 Hit
Ioun Stone of Mastery: +1 Hit
Archery Combat Style: +2 Hit
Battle Inspiration: +12 Hit
Precision Attack: +12 Hit
Channel Divinity: War Domain: +10 Hit
Bless: +4 Hit
Arrow +3: +3 Hit
Bow +3: +3 Hit
Bend Luck: +4 Hit
Precision Attack+ 12 Hit
Sacred Weapon: +10 Hit
Epic Boon: Peerless Aim: +20
Epic Boon: Boon of Luck: +10

A Devotion Paladin/Battlemaster Fighter with a Valor Bard, War Cleric, and Wild Sorcerer buddy and a host of magic items can get... +119 to hit, and thus with a 20 hit AC 139!

[EvanescentHero]

Since a 20 automatically hits regardless of AC, shouldn't we only consider what a build can hit with a maximum roll of 19?

Not a huge difference, but when you roll a 20, the game doesn't care if you have a +100 or a -39.

EDIT: Likewise, you automatically miss on a 1, so the range of rolls to be considered is 2 to 18.

[Dizlag]

A 3rd level Fighter/Champion will hit that 76 AC on a roll of 19 or 20, so a 10% chance and it will be a critical hit. At 15th level it will be a roll of 18, 19, or 20 so 15% chance to hit.

Dizlag

[Slipperychicken]

A 3rd level Fighter/Champion will hit that 76 AC on a roll of 19 or 20, so a 10% chance and it will be a critical hit. At 15th level it will be a roll of 18, 19, or 20 so 15% chance to hit.

Dizlag

Only nat 20s are automatic hits on attack rolls. This is distinct from critical hits (PHB 194, "Rolling 1 or 20"), but is often confused because for most characters they both occur on nat 20s. When a champion can roll a critical on a 19, he still only hits if the attack would otherwise hit.

You can consider it similar to attacking a paralyzed creature. Those attacks are criticals when they hit, but that does not mean they automatically hit.

[Dizlag]

Only nat 20s are automatic hits on attack rolls. This is distinct from critical hits (PHB 194, "Rolling 1 or 20"), but is often confused because for most characters they both occur on nat 20s. When a champion can roll a critical on a 19, he still only hits if the attack would otherwise hit.

You can consider it similar to attacking a paralyzed creature. Those attacks are criticals when they hit, but that does not mean they automatically hit.

Not according to the developer's here ...

http://www.sageadvice.eu/2014/09/18/only-20-hit/

The Improved Critical feature IS an autohit as well on 19 and 20 at 3rd level and 18 at 15th.

Dizlag

[AvatarVecna]

A 3rd level Fighter/Champion will hit that 76 AC on a roll of 19 or 20, so a 10% chance and it will be a critical hit. At 15th level it will be a roll of 18, 19, or 20 so 15% chance to hit.

Dizlag

Incorrect. The Champion ability indicates that a roll of 19 (or 18, if 15th level or higher) is a crit. Auto-hitting is not part of getting a critical hit.

Rolling a natural 20 means you automatically hit (regardless of target AC) and you have scored a critical; scoring a critical and auto-hitting are both functions of rolling a natural 20, but that does not mean that any crit is an auto-hit (as is the case with the Champion Fighter) or that any auto-hit is a crit (as is the case with Magic Missile).

EDIT: Wow, ninja'd by both the reply and the rebuttal. That's certainly interesting about auto-hitting.

[Dizlag]

Yeah, pretty interesting that this lonely 3rd level Fighter has the same chance to hit an AC 138 (by rolling a 19 ... everyone can hit the AC 139 with a nat 20) as the build Naanomi put forth needing 3 other buddies for the buffs. =)

Dizlag

[Slipperychicken]

Yeah, pretty interesting that this lonely 3rd level Fighter has the same chance to hit an AC 138 (by rolling a 19 ... everyone can hit the AC 139 with a nat 20) as the build Naanomi put forth needing 3 other buddies for the buffs. =)

Dizlag

He is a champion, after all.


I see why Crawford made that ruling, though. In terms of its effect on real games where 18-20 should be hitting anyway, I think he made the right call.

[RickAllison]

Yeah, pretty interesting that this lonely 3rd level Fighter has the same chance to hit an AC 138 (by rolling a 19 ... everyone can hit the AC 139 with a nat 20) as the build Naanomi put forth needing 3 other buddies for the buffs. =)

Dizlag

That's kind of what I saw the champion as being about, overcoming impossible odds through sheer bravado/cussedness. My crit-fishing Marilith super-beast loves her Champ 3 with her Oathbreaker levels because it made the odds of her NOT getting a critical Smite in a round as 47.8% and with advantage it became 22.9%.

My players will hate her sooooooo much...

[Demonslayer666]

It's the nerf bat that hit Pathfinder so hard that it halved everyone's bonuses.

[Knaight]

You aren't testing the limits of the system if you are going outside the rules and guidelines used for the common game.

It's not outside the rules and guidelines, it's just exceptionally unlikely due to needing an absurd level of luck on random item rolls. Are there enough games total that it is likely to ever show up in even one of them? No. It just provides the currently known theoretical maximum.

[RickAllison]

It's not outside the rules and guidelines, it's just exceptionally unlikely due to needing an absurd level of luck on random item rolls. Are there enough games total that it is likely to ever show up in even one of them? No. It just provides the currently known theoretical maximum.

Indeed, it's like those theory-craft builds that have outlandish ability requirements and so only work for exceptionally lucky rolls, yet are considered valid for the purpose of creating such a ridiculous build. Also, so we can hijack these characters to use as antagonists. I might steal the whole Dex/Con/Int 30 build...

[Foxhound438]

take a normal hunter ranger with no shenangians and have multiattack defense feature

get hit 100 times by the same creature in the same turn somehow

AC over 400

flip off DM who made a creature that attacks 100 times in a turn

[Dizlag]

And the 3rd level Champion will still have a 10% chance of hitting it and even more if he/she has advantage. :smalltongue:

[Slipperychicken]

It's not outside the rules and guidelines, it's just exceptionally unlikely due to needing an absurd level of luck on random item rolls. Are there enough games total that it is likely to ever show up in even one of them? No. It just provides the currently known theoretical maximum.

In programming, we call these "edge cases" or "corner cases"; scenarios that take one or more variables to extreme values, often far beyond what you'd expect in normal use. They might seem unlikely to happen in reality, but it's a common way to look for bugs in a system.