Say you're a homebrew class that uses a stat different than DEX for calculating AC. Something like the 3rd party Mordenkainen's Codex of Allies' Arcane Tradition Invention or Racial Feats Expanded's Dao Legacy feat. These two examples replace the DEX modifier with INT or CON. Why do they replace instead of add to the DEX modifier? Hell if I know, but they do and I figured out how to recreate the effect. Here's the two examples I've figured out so far:

From Mordenkainen's Codex of Allies' Arcane Tradition Invention:

Starting at 2nd level, you gain proficiency with light armor and gain a suit of alchemical armor— a magic item that only you can attune to. While you are attuned to it and wearing it, it grants you resistance to force damage and you can use the armor as a spellcasting focus by extending one of your hands, so long as that hand is free.

The armor is light armor and provides an AC of 11 + your Intelligence modifier. It weighs 8 pounds.

We can reproduce this in a homebrew magic item:

RARITY: Whatever you want. The more rare, the further up in your inventory it'll show up
BASE ITEM TYPE: Item
MAGIC ITEM TYPE: Wondrous Item
REQUIRES ATTUNEMENT: Checked
ATTUNEMENT DESCRIPTION: by a Wizard in the Arcane Tradition of Invention

We have to make this an Item instead of Armor because Armor adds DEX to your AC no matter what

Modifiers:

MODIFIER TYPE:Resistance
MODIFIER SUBTYPE: Force

MODIFIER TYPE: Set
MODIFIER SUBTYPE: AC Max Dex Modifier
FIXED VALUE: 0

MODIFIER TYPE: Bonus 
MODIFIER SUBTYPE: Armor Class
FIXED VALUE: 1

MODIFIER TYPE: Bonus 
MODIFIER SUBTYPE: Armor Class
PRIMARY STAT: INT

 Resistance Force is there because of the text. It is not important for what this thread is about

1. First you have to set the AC Max DEX Modifier to 0
2. Second you have to set the base AC for the item. Dndbeyond already says your AC is 10 plus stuff. For this magic item we want 11 + stuff. Subtract 10 from what you want the new base to be and use that number here as a Bonus to Armor Class. In this case we're adding 1
3. Third you need to add a bonus to your AC using your INT modifier.

Equip and attune this item and BAM, your AC is now 11 + your INT modifier. 

Let's make another example. From Racial Feats Expanded's Dao Legacy:

When not wearing armor, your AC is calculated as 13 + your Constitution modifier.

 We can reproduce this in a homebrew feat. Modifiers:

MODIFIER TYPE: Set
MODIFIER SUBTYPE: AC Max Dex Unarmored Modifier
FIXED VALUE: 0

MODIFIER TYPE: Bonus 
MODIFIER SUBTYPE: Unarmored Armor Class
FIXED VALUE: 3

MODIFIER TYPE: Bonus 
MODIFIER SUBTYPE: Unarmored Armor Class
PRIMARY STAT: CON

1.  First we're making changes to your Unarmored AC, so we pick the appropriate Modifier Subtype
2. Second we subtract 10 from 13 to get 3 and use this as the bonus for Unarmored Armor Class
3. Third we set the Bonus to Unarmored Armor Class to your CON modifier.

Take the feat and BAM your Unarmored AC is now 13 + CON and you're done.

I make no claims about how these changes affect other AC or Unarmored AC rules. I would defer to official rules and sage advice on choosing the different ACs in that you pick 1 and use that. Why would anyone want to do this? I don't know. Why did I do this? Because I wanted to and I wanted to show you how.

This is awesome help! Exactly what I was trying to accomplish!

-Thx

I'm trying to do this with Charisma for one of my players. They're a bard, with a +3 CHA mod, I followed the steps above, made a +1 armor that ignores Dex and adds a bonus from CHA instead, except the AC is just 12, when it should be 14. Not sure what I'm doing wrong, but it seems like it's just ignoring the CHA modifier.

Quote from ThatEnglishChap >>

I'm trying to do this with Charisma for one of my players. They're a bard, with a +3 CHA mod, I followed the steps above, made a +1 armor that ignores Dex and adds a bonus from CHA instead, except the AC is just 12, when it should be 14. Not sure what I'm doing wrong, but it seems like it's just ignoring the CHA modifier.

 
How exactly did you set it up? After you added the Modifiers, did you remember to save the entire magic item using the main [SAVE CHANGES] button under Basic Information? If not, the system won’t pick up your changes.

I saw this post trying to make something similar. But was wondering if it is possible to make a race that calculates its AC with both Strength and constitution instead of dexterity. As an unarmored bonus?

Quote from Yurit1337 >>

I saw this post trying to make something similar. But was wondering if it is possible to make a race that calculates its AC with both Strength and constitution instead of dexterity. As an unarmored bonus?

 Absolutely. You would use the following combination of Modifiers:

• Modifier: Set->Subtype: Unarmored Armor Class->Ability: [Str/Con]
• Modifier: Bonus->Subtype: Unarmored Armor Class->Ability: [Con/Str]
• And either one of the two following:
  • Modifier: Set->Subtype: AC Max Dex Unarmored Modifier->Fixed Value: 0
  • Modifier: Ignore->Subtype: Unarmored Dex Natural AC Bonus

Whichever ability score you choose to Set, either Str or Con, just be sure to use the other one for the Bonus.

Of the two that mention Dex, either one should accomplish what you want and give you the result you’re looking for. However there is some debate as to which is actually more appropriate to use. I have always used the Ignore Modifier and never had any problems with it, it has always delivered consistent results for me unless I was trying to do something funky like how Tortles Armored Defense works. Someone else who is pretty much just as proficient with the system as I am prefers to use the Set modifier in all instances. The only ones who could tell us definitively and were engaged with the community have all been let go by WotC and I don’t want to bug them about it since they are no longer employed here. So it’s really up to you which you use, and if it turns out to not be performing as expected then switch, but please also let me know too. I’d appreciate that.

HI All,

I am currently trying to make a piece of armour that grants the user a bonus to their AC using a fixed number from their spellcasting ability. I currently have an item with a fixed bonus of three to the AC using their charisma. However, when equipped it gives them a +7 bonus to their AC as they have a +4 as their charisma modifier. How do I fix this?

Thanks in advance.

Quote from lachydean1999 >>

HI All,

I am currently tryign to make a piece of armour that grants the user a bonus to their AC using a fixed number from their spellcasting ability. I currently have an item with a fixed bonus of three to the AC using their charisma. However, when equipped it gives them a +7 bonus to their AC as they have a +4 as their charisma modifier. How do I fix this?

Thanks in advance.

Did you make it armor or a wonderous item?

I made it an armour

Make it a wonderous item. If you make it armor, it will either always add your DEX mod (base item some light armor), either +2 or +3 of your DEX mod (base item some medium armor), or just not work correctly (base item some heavy armor).

I'm trying to change a set of Leather armor from 11+Dex. Mod. To 14+Dex. Mod. Any ideas? This has thwarted me for about a week. I'm sure it's super simple and I've just overlooked it about a thousand times. But I need help all the same. Lmao

Quote from rainebauman >>

I'm trying to change a set of Leather armor from 11+Dex. Mod. To 14+Dex. Mod. Any ideas? This has thwarted me for about a week. I'm sure it's super simple and I've just overlooked it about a thousand times. But I need help all the same. Lmao

 
Modifier: Bonus->Subtype: Unarmored Armor Class->Fixed Value: 3

Much appreciated