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If a character with the Unarmoured Defence class feature casts Mage Armour, what is their AC? Do they choose which 'base' AC they take, or do they stack somehow? Is there rules text that explains this conflict?

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Both of them set your base AC. So mage armor would supersede Unarmored Defense.

You have this:

  • Normal: AC = 10 + Dex
  • Unarmored Defense: SET AC = 10 + Dex + Stat
  • Mage Armor: SET AC = 13+Dex

Basically, mage armor leaves no room for the second stat for Unarmored Defense so you would not be able to apply the second stat.

The exclusive nature of AC calculations was explained in further detail in a post at the WoTC web site in 2016

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    \$\begingroup\$ I was of the opinion that when you had two valid options as to how to "set" your AC, you got to chose which one took effect. That is, if the extra stat from unarmored defense had a modifier better than 3, you might want to still use unarmored defense. You couldn't for example, reduce someone's AC by casting mage armor on them. I can't find any phrasing that allows this in the basic rules, but I feel like I remember reading that somewhere... \$\endgroup\$ – Tal Aug 25 '17 at 14:22
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    \$\begingroup\$ @Tal is correct. You (in calculating your own AC) get to choose which method to use when you have multiple options. Presumably in almost all cases you'll chose the better one. Basic rules citation here \$\endgroup\$ – mattdm Oct 22 '17 at 22:38
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    \$\begingroup\$ This answer could be improved with tl;dr headings that explicitly answer the quesions. e.g. "Yes. They choose which AC calculation to use." or "The AC calculations are separate and do not stack." or "Yes. There is a rule to cover this." \$\endgroup\$ – GcL Jun 8 '18 at 13:30
  • \$\begingroup\$ @Grosscol this answer is a full answer. The correction should be on the question trying to divide one question into 3 parts. \$\endgroup\$ – Luke Jul 16 '18 at 4:57
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There are two ways to alter your armor class:

  • Set your base AC

  • Add a modifier to AC.

You only get the best base AC available to you, and not many things add a modifier. Mage Armor sets your base AC and Unarmed Defense from the Barbarian and Monk class features set your base AC as well, thus you only get the base AC from the best of them that you qualify for. But they are the same thing despite being worded slightly differently, per this Twitter dev response:

"Is there a reason why the wording is inconsistent between Mage Armor and Unarmored Defenses? Mage Armor uses "Base AC". The difference isn't intentional. -J"

There are very few modifiers for Armor Class (and this is intentional according to WOTC), but notable ones are Shields, Defensive Fighting Style, and the Dual Wield feat.

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This question has been answered by Jeremy Crawford in Sage Advice:

Does Unarmored Defense work with a spell like mage armor? Unarmored Defense doesn’t work with mage armor. You might be asking yourself, “Why don’t they work together? Mage armor specifies that it works on a creature who isn’t wearing armor.” It’s true that the target of mage armor must be unarmored, but mage armor gives you a new way to calculate your AC (13 + your Dexterity modifier) and is therefore incompatible with Unarmored Defense or any other feature that provides an AC calculation.

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DM's Discretion

Mage armor is a magical force with no weight to it. It is not real armor. Unarmored Defense for a monk means being able to freely move, so mage armor does not interfere with Unarmored Defense.

So to agree with the previous post, yes you are changing base AC once to 13 and adding your dexterity modifier as normal for mage armor, then adding your wisdom modifier for Unarmed Defense. Furthermore, if you use the dual wielder trait and are fighting with two weapons, you can get another +1 to AC.

Now as a DM if my player does this and I let him, he is very happy. Also it makes sense to us aesthetically. Now it is up to the DM to come up with ways to make encounters still fun and challenging. No need to be a rules lawyer; just have fun, especially with newer players.

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    \$\begingroup\$ Hi Jon, and welcome to RPG Stack Exchange! Check out our tour to see how we work here. I've made an edit to your post just to fix a few grammatical issues as well as formatting. Thank you for your answer, and we're glad to have you on board :) \$\endgroup\$ – Val Oct 22 '17 at 21:09
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    \$\begingroup\$ "DM discretion" applies to anything in D&D, ultimately, but this reading isn't by the rules, which are very clear; you should note that this is a house rule. \$\endgroup\$ – mattdm Oct 22 '17 at 22:04
  • \$\begingroup\$ I'm sorry, but "DM discretion" is kind of ubiquitous. This is a house rule, not RAW, and should be clearly marked as such to avoid confusing players who are trying to understand RAW. Both Mage Armor and Unarmored Defense set your AC to a value. In the event of conflict, the better AC is applied. The two are not combined. "Some spells and class features give you a different way to calculate your AC. If you have multiple features that give you different ways to calculate your AC, you choose which one to use." Thank @mattdm for me finding the exact wording. \$\endgroup\$ – Sora Tamashii Jan 5 '18 at 14:58

protected by Oblivious Sage Oct 17 '18 at 2:20

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