I'm already aware of the interaction between Mage Armor and features that grant an alternative AC calculation such as a Monk or Barbarian's Unarmored Defense, or simply wearing Armor. However I haven't been able to find any definitive answer on how it interacts with a Tortle's Natural Armor or with any other creatures whose Base AC is also derived from a flat Natural Armor value in the same way.

Relevant rules excerpts:

Tortle (MPMM, p34)

Natural Armor. Your shell provides you a base AC of 17 (your Dexterity modifier doesn't affect this number). You can't wear light, medium, or heavy armor, but if you are using a shield, you can apply the shield's bonus as normal.

Mage Armor (PHB, p256)

You touch a willing creature who isn't wearing armor, and a protective magical force surrounds it until the spell ends. The target's base AC becomes 13 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action.

Emphasis mine in both cases. Since both of these effects specify an override to the typical Base AC calculation of 10+DEX it's unclear to me which effect takes precedence. Does the term "Base AC" even have any special meaning? I'm curious as to whether there's any rules text that covers this scenario. RAW-only answers please, I'm not looking for DM rulings but for some actual material I can refer to or a definitive statement that such rules text does not currently exist.

For the curious, a player jokingly mused on whether you could charm a Monster with low DEX but fantastic Natural Armor, make it a willing target of Mage Armor in doing so and thus cripple its AC by overriding it and I actually have no idea whether this works RAW. In terms of my personal DM ruling, absolutely not, but the RAW justification revolving around the term "Base AC" struck me as an interesting subject.

  • \$\begingroup\$ Related on Can you make an unwilling creature willing? \$\endgroup\$
    – NotArch
    Oct 6, 2022 at 13:47
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    \$\begingroup\$ "base AC" probably just means "before things that explicitly modify your AC, like equipping a shield". \$\endgroup\$
    – Erik
    Oct 6, 2022 at 14:30
  • \$\begingroup\$ This indirectly answers my question especially with the clarification from Crawford that there's no inherent special meaning to the term "Base AC" but I'll leave this one open anyway. Though there's no source provided for the quote from Crawford and getting the same information from a more authoritative source would be ideal. \$\endgroup\$
    – Aiken
    Oct 7, 2022 at 8:28
  • \$\begingroup\$ @Aiken I think the bit about using a charm effect to make this work is a unique feature of this question that isn't addressed in any other questions, so this is definitely not a duplicate. \$\endgroup\$ Oct 7, 2022 at 12:21

1 Answer 1


The creature chooses.

The rules for Armor Class state:

If you have multiple features that give you different ways to calculate your AC, you choose which one to use.

However, these rules are somewhat meta - I cannot articulate what it would mean in-universe for a creature with natural armor to choose to use the bonus from mage armor instead. It might be possible, as in the rules technically allow it, but you will have to rely on the DM to bring that ruling to the table and narrate what that looks like, which I would find challenging as a DM.

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    \$\begingroup\$ Regarding the narration: the character is choosing in which manner they're avoiding damage. Either by moving in such a way where blows are deflected by their natural armor or by maneuvering so that incoming attacks are intercepted by the mage armor. There is no additive effect, so using one is exposing the weakness of the other. \$\endgroup\$
    – GcL
    Oct 7, 2022 at 6:06
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    \$\begingroup\$ @GcL: Narratively, attempting to dodge so your Mage Armor works shouldn't mean all the blows that don't get dodged ignore your natural armor. You'd expect that being armored instead of being a human wearing street clothes would be strictly beneficial in getting hit less. Maybe you could say that since you're not experienced with using your Dex as part of your AC, it comes less naturally, and the underlying shell still helps with some blows you don't avoid. It doesn't exactly match the mechanics, but Mage Armor wasn't written for targets that were armored in any way. \$\endgroup\$ Oct 7, 2022 at 7:27
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    \$\begingroup\$ That rule makes more sense narratively for features like the one monks have, which is more of a thing you do than a thing you have "on" (and was probably written with that in mind). Anyway, "you choose" is a fancy way of saying "whichever is higher", which poses less narrative problems. \$\endgroup\$
    – Rad80
    Oct 7, 2022 at 8:15
  • \$\begingroup\$ Does "you choose" refer to "you, the player" or "you, the character"? The distinction being that the player would be free to use out-of-character knowledge plus wouldn't be vulnerable to things like mind-control. \$\endgroup\$
    – Nat
    Oct 7, 2022 at 19:05
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    \$\begingroup\$ @Nat Since characters don't have "features" or "ways to calculate your AC" or "AC", the sentence is addressing player,, or in the case of an NPC, the DM. \$\endgroup\$ Oct 7, 2022 at 19:07

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