Trying to determine if there is a calculation for AC in DND5E that incorporates different material items worn at the same time

Question

In D&D5eE - I want some homebrew rules involving calculating AC and their modifiers when mixing different materials of armor.

If a player has on a plate mail chest piece and studded leather gauntlets, maybe they find a set of boots that are of a different material make up. What are some good homebrew rules for calculating the overall AC of the player and their different gear?

For example:

A fighter starting out has Studded Leather armor on. Should this fighter run across plate gloves that give some bonus to AC, they should be able to wear those, along side say plate helm or scale boots, along with the rest of their armor being Studded Leather.

The standard rules only give calculated AC for overall armor type sets ONLY, not individual types. What are some good homebrew rules for mixing armor types?

Answer 1

The space you’re looking for is occupied by magic items.

This doesn’t really exist for mundane armors. Without magic items, your AC is determined by your armor, and whether or not you are wearing a shield, and that is all. Mundane armor is a “just pick one and move on” sort of exercise. There is no going to the blacksmith to upgrade your pauldrons or to add steel gauntlets to your leather kit.

What 5e does have are magic items of various kinds that provide a bonus to your AC. There are several magic items that you can wear in addition to your standard armor that provide a bonus to your AC, such as the ring and cloak of protection, or the watchful helm. And of course, there are also +X versions of most mundane armors that provide some bonus to AC. Whether or not these items are common, rare, or in your game at all is up to your DM.

And because this space is reserved for magic items, implementing this sort of system where AC bonuses are cheap and easy to acquire is inherently unbalanced. I elaborate on this in my answer to a similar question about stacking different armors for AC benefits (Can a character wear both chain mail and leather armor to get a higher AC?):

If a ruling trivializes magic items that cost thousands of gold, the ruling is suspect.

First, I must mention that Bardic Wizard's answer is exactly correct, so I won't be repeating the rules here - just read and upvote their answer for that. However, I do want to approach this from another direction by applying an interpretation principle that I have found quite useful as I read the rules and make rulings.

The idea is this: does this ruling make something else in the game entirely pointless?

With this question, it absolutely does. Chain Mail gives an Armor Class of 16 and costs 75 gp. Leather armor costs 10 gp. This question is really asking something like "Can I get Chain Mail +1 for 85 gp?" And the answer to that question should be obvious: of course not. +1 Armor is classified as "Rare", and going by the Magic Item Rarity table in the Dungeon Master's Guide has a value of 500 gp on the low end, up to 5000 gp on the high end. If wearing some cheap leathers under your chain shirt were just as good as enchanted armor that fetches thousands of gold at the market, there is no point to having magic armor at all.

The space you’re looking for is already filled with magic items, and implementing cheap and easily accessible boosts to AC trivializes those magic items, which is wrong, it’s just something to be aware of should you choose to do so in your games. There is a reason AC bonuses are very expensive: they’re deceptively powerful.

Answer 2

Is this even allowed in DND5E?

"Allowed" is maybe not the right category.

There are no rules for this. There also is no need for this in the rules. Because for different types of armor protecting different body parts to be useful, there must be rules for hit zones. There is no point in a character wearing a loincloth and a plate helmet, when there is no system that indicates whether something struck the head or the body. D&D is no simulation and one of the many simplifications to make the game easier to play is that every hit is just "a hit" and every part of the body is protected by the armor equally. This is not only in the current edition, but has been a theme through all editions of D&D.

I personally don't know any addons, rules editions or magazine articles that would introduce hit zones or partial armor. Other RPG systems have this, but it was never something that people felt was "missing" from (A)D&D of any edition. At least not enough to publish a successful alternative.

If you think it would be fun for your group, you can add it. There is nothing "forbidden" about it. However, I would suggest you look at other systems that do that already and decide what is "more fun" about them. And then port those parts over. Many people like the simplicity of D&D compared to systems that need more bookkeeping and dicerolling.

Just to send you down a rabbit hole: If a plate helmet gives +1 AC and plate legs give +2 AC, can an Ogre with two heads (commonly called Ettin) wear two plate helmets, for +2 AC? How about a giant centipede with 100 leg armors? So... what I want to say is, this is not only about a leather glove for a PC. This is a system. You would need to reinvent the whole system. You can. But it will be a lot of work.