Nope, you can only use one Base AC Calculation at a time.
For example the
The normal base AC calculation is 10 + Dex Modifier
Wearing Armor uses that armors base AC calculation, leather, for example, is 11 + Dex Modifier
However, when you get into racial or class features that adjust the base calculation such as Tortles Natural Armor or a Barbarians Unarmored Defense the calculation takes some changes.
A Barbarians and Monks Unarmored Defense change's your base calculation to be 10 + Dex Modifier + Con (or Wis) Modifier while not wearing armor. Meaning this only applies while you are Unarmored, otherwise, you use the armors base calculation.
Lizard Folks Natural Armor is actually kinda interesting because it will work with armor to an extent. For example, if your Lizard fold is wearing leather armor which has a base calculation 11 + Dex Modifier, you would instead use your Natural Armor because it is higher at 13 + Dex Modifier. But if you were wearing Breastplate and only had a +2 Dex modifier, then you would use the Breastplate armor calculation of 15 + Dex Modifier (max 2) giving you 16. However, if you had a Dex Modifier of + 4 then you would actually still use your natural or natural armor because it would be 13 + 5 giving you a total of 17.
Tortles are actually a lot easier because you ignore any armor you are wearing and instead always use 17 as your base armor calculation. No modifications allowed.
So for this specific case, I would say that you use whichever is higher, your tortles AC of 17 or Unarmored Defense,
Also Unarmored Defense specifically states it's calculation is 10 + Dex Mod + Con Mod. Not your Base AC + Dex Mod + Con Mod
Unarmored Defense While you are not wearing any armor, your Armor Class equals 10 + your Dexterity modifier + your Constitution modifier. You can use a shield and still gain this benefit.
It really makes no sense from a Realism standpoint that a racial AC and a Class based AC don't staack. The racial AC is Biological, which means that you would always have it, and the class one for Monks and Barbs is learned. You would think that being naturally harder to hurt as a species would make you tougher than the other guy who's skin is softer. If I run a home game, That's how I'm gonna run it. Gameplay balance be damned.
So you would happily give a Tortle Barbarian an AC calculation of 17 + Dex + Con? They could easily have 23 AC at level 1 - slowly increasing passed 30 with magic items by max level. Good luck trying to hit them with anything.
Just imagine a Tortle Bear-Totem Barbarian with that AC. I can't imagine much could kill them.
As @korbin_Orion said, you always have to take account that in 5e everything is based around a d20. For example, unlike older systems, AC isn't really designed to extend over 20. Of course, there are ways to do so, but really something you have to specifically build for not something you will happen across by accident.
It really makes no sense from a Realism standpoint that a racial AC and a Class based AC don't staack. The racial AC is Biological, which means that you would always have it, and the class one for Monks and Barbs is learned. You would think that being naturally harder to hurt as a species would make you tougher than the other guy who's skin is softer. If I run a home game, That's how I'm gonna run it. Gameplay balance be damned.
So you would happily give a Tortle Barbarian an AC calculation of 17 + Dex + Con? They could easily have 23 AC at level 1 - slowly increasing passed 30 with magic items by max level. Good luck trying to hit them with anything.
Just imagine a Tortle Bear-Totem Barbarian with that AC. I can't imagine much could kill them.
In my first campaign I had a Tortle Barbarian who used two sheild I only gave him the bonus for one but it is a more logical standpoint.
So here is a question.
Lizardfolk and Tortles have a base AC when not wearing armour.
Does this stack with Monk and Barbarian, Unarmored Defences + Sheild (for barbarian) for stupid numbers?
Thank you guys for you advice and responses.
Nope, you can only use one Base AC Calculation at a time.
For example the
The normal base AC calculation is 10 + Dex Modifier
Wearing Armor uses that armors base AC calculation, leather, for example, is 11 + Dex Modifier
However, when you get into racial or class features that adjust the base calculation such as Tortles Natural Armor or a Barbarians Unarmored Defense the calculation takes some changes.
A Barbarians and Monks Unarmored Defense change's your base calculation to be 10 + Dex Modifier + Con (or Wis) Modifier while not wearing armor. Meaning this only applies while you are Unarmored, otherwise, you use the armors base calculation.
Lizard Folks Natural Armor is actually kinda interesting because it will work with armor to an extent. For example, if your Lizard fold is wearing leather armor which has a base calculation 11 + Dex Modifier, you would instead use your Natural Armor because it is higher at 13 + Dex Modifier. But if you were wearing Breastplate and only had a +2 Dex modifier, then you would use the Breastplate armor calculation of 15 + Dex Modifier (max 2) giving you 16. However, if you had a Dex Modifier of + 4 then you would actually still use your natural or natural armor because it would be 13 + 5 giving you a total of 17.
Tortles are actually a lot easier because you ignore any armor you are wearing and instead always use 17 as your base armor calculation. No modifications allowed.
So for this specific case, I would say that you use whichever is higher, your tortles AC of 17 or Unarmored Defense,
Also Unarmored Defense specifically states it's calculation is 10 + Dex Mod + Con Mod. Not your Base AC + Dex Mod + Con Mod
Unarmored Defense While you are not wearing any armor, your Armor Class equals 10 + your Dexterity modifier + your Constitution modifier. You can use a shield and still gain this benefit.
Here is a thread from last month talking about this and here is Sage Advice from Jeremy Crawford.
From the thread:
As @korbin_Orion said, you always have to take account that in 5e everything is based around a d20. For example, unlike older systems, AC isn't really designed to extend over 20. Of course, there are ways to do so, but really something you have to specifically build for not something you will happen across by accident.
In my first campaign I had a Tortle Barbarian who used two sheild I only gave him the bonus for one but it is a more logical standpoint.
You can only benefit from one shield at a time anyway.
Shield - You can benefit from only one shield at a time.
Tortles only get AC 17 + shield. They don't get Dex or armor mods.
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