What are the ramifications of changing Armor Class to be Constitution-based instead of Dexterity-based?

Question

It's never made much sense to me that your Armor Class is based on your Dexterity modifier, which carries the implication that you're harder to hit because you're somehow nimbly moving out of the way of an attack, whereas armor is about making you harder to injure (which seems right up Constitution's alley, particularly as inanimate objects also have an AC).

I've been considering implementing a rule in my games that changes AC to be calculated based on your Constitution modifier instead of your Dexterity modifier, so per the title, what would be the ramifications of this?

The only thing I can currently think of is that it'd make some classes a little less MAD and making Dexterity a little less of a golden stat; I couldn't see it affecting Unarmored Defense, Mage Armor, a sorcerer's Draconic Resilience, since those provide alternative ways to calculate AC already, but I'm sure there's something I'm missing somewhere.

Answer 1

Nerfs dex classes, buffs everyone else

Well, let's start with the obvious. A rogue will usually try to hit 20 dex, so this change will give them -5 AC. Due to bounded accuracy that's a huge nerf. While many players do prioritise constitution as a secondary stat, it is unlikely they will bring it up to 20 too--and if they do they are making quite a sacrifice. The knock on effect from these sacrifices are not balanced.

For everyone else, it's a buff. Constitution is already a great stat that many people invest in. Why put points into dexterity when you can put them into constitution and double dip survivability. Not only do you get +1 HP per HD, you also get +1 AC. Wizards, sorcerers, warlocks, etc, all have little reason to put points into dex beyond initiative and dex saves.

For builds that are not using dex in the first place, this is a free buff unless they are using heavy armor.

This is an unnecessary change

AC represents your entire defensive ability, the dexterity modifier to AC represents ability to dodge. Positioning is hugely important in real life combat, and that is modeled in D&D using dexterity. That's why the nimble thief can dip, duck, and dodge enemy attacks.

Constitution already does make you able to take more punishment. Your con modifier is added to your HP with each level. There is no need to buff it further.

Answer 2

This is Too Hard to Balance

On its surface, this seems fine. We just move from Dex to Con. Once we dig a little more, though, there are a lot of mechanics touched by this.

1. Makes a Rogue multi-attribute dependent. Rogues need AC to avoid being hit, which they get from Dex in the current rule set. Now they need Dex (for their Rogue Skills) and Con (for both HP and AC). All Rogues will now be -2 or 3 AC.

2. Medium armor currently gives +2 max Dex increase, and Heavy armor none. This makes sense intuitively because in heavier armor one can move less, and the armor takes more of the attack directly. How do we retcon this mechanic with Con? Changing this mechanic would flow everywhere due to class armor restrictions.

3. Assuming we leave the #2 mechanic the same, now we benefit greatly the Medium Armor classes who don't need Dex for attack and damage rolls (or who use their primary stat, like Hexblades).

4. Con is already valuable. Now, Con would be everything for combat - both how often you get hit and how many hits you can take. This mechanic just moves the Golden Stat from Dex to Con, and then we start all over again. It doesn't really seem like progress.

Conclusion: Don't do this.

Answer 3

Weirdly, I think swapping CON for DEX decreases the relevance of armor, and increases the relevance of ranged combat.

The traditional split between DEX and CON is to denote Fast Guys and Sturdy Guys*. If CON is used for AC, a couple of weird things happen:

1. A high CON means never needing to wear heavy armor (the above on Medium Armor summarizes perfectly); Sturdy Guys can wear lighter armor when they need to climb, and (given a 16 CON) will only lose 2-4 to their AC at max
2. Fast Guys are going to need to use skills like Stealth and Acrobatics to use cover and try not to get hit; expect Fast Guys to never enter melee if they can help it, and for sniper fights to become a regular occurrence
3. Monks suddenly become a viable tank option, because they can stop caring about DEX, and stat CON higher, using WIS + CON for their AC, but they won't tumble around as much
4. Everyone values CON more highly; CON is now the only stat you need to stay alive
5. Wizards and others like them will start dump stating DEX and be clumsy academics, instead of dumping WIS or CHA
6. Reflex saves become more deadly because no one (besides Fast Guy) has a good DEX

I agree, CON doesn't need to be more powerful. Flipping the script on AC will make DEX classes worse, except for taking out enemy spell-casters who use AoE Reflex Save spells, where they suddenly shine far more than the rest of the party.

If you wanted to make this change a feat (an expensive option in D&D 5e), it might be a good intermediate test if you want to try the concept. Sturdy Guys can show off a little if they want, but Fast Guys don't get crippled.

*Apologies, using gender neutral version of Guys