There is a new Sage Advice article on the Wizards of the Coast D&D website by Jeremy Crawford. The focus of this month’s Sage Advice is Armor Class (AC).
Below is the excerpt of the article for your convenience:
How do you calculate a creature’s Armor Class (AC)?
Chapter 1 of the Player’s Handbook (p. 14) describes how to determine AC, yet AC calculations generate questions frequently. That fact isn’t too surprising, given the number of ways the game gives you to change your AC!
Here are some ways to calculate your base AC:
- Unarmored: 10 + your Dexterity modifier.
- Armored: Use the AC entry for the armor you’re wearing (see PH, 145). For example, in leather armor, you calculate your AC as 11 + your Dexterity modifier, and in chain mail, your AC is simply 16.
- Unarmored Defense (Barbarian): 10 + your Dexterity modifier + your Constitution modifier.
- Unarmored Defense (Monk): 10 + your Dexterity modifier + your Wisdom modifier.
- Draconic Resilience (Sorcerer): 13 + your Dexterity modifier.
- Natural Armor: 10 + your Dexterity modifier + your natural armor bonus. This is a calculation method typically used only by monsters and NPCs, although it is also relevant to a druid or another character who assumes a form that has natural armor.
These methods—along with any others that give you a formula for calculating your AC—are mutually exclusive; you can benefit from only one at a time. If you have access to more than one, you pick which one to use. For example, if you’re a sorcerer/monk, you can use either Unarmored Defense or Draconic Resilience, not both. Similarly, a druid/barbarian who transforms into a beast form that has natural armor can use either the beast’s natural armor or Unarmored Defense (you aren’t considered to be wearing armor with natural armor).
What about a shield? A shield increases your AC by 2 while you use it. For example, if you’re unarmored and use a shield, your AC is 12 + your Dexterity modifier. Keep in mind that some AC calculations, such as a monk’s Unarmored Defense, prohibit the use of a shield.
Once you have your base AC, it can be temporarily modified by situational bonuses and penalties. For instance, having half cover gives you a +2 bonus to your AC, and three-quarters cover gives a +5 bonus. Spells sometimes modify AC as well. Shield of faith, for example, grants a target a +2 bonus to AC until the spell ends.
Magic items can also enhance your AC. Here are a few examples: +1 chain mail gives you an AC of 17, a ring of protection gives you a +1 bonus to AC no matter what you’re wearing, and bracers of defense grant you a +2 bonus to AC if you’re not wearing armor or using a shield.
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.
How does barkskin work with shields, cover, and other modifiers to AC?
Barkskin specifies that your AC can’t be lower than 16 while you are affected by the spell. This means you effectively ignore any modifiers to your AC—including your Dexterity modifier, your armor, a shield, and cover—unless your AC is higher than 16. For example, if your AC is normally 14, it’s 16 while barkskin is on you. If your AC is 15 and you have half cover, your AC is 17; barkskin isn’t relevant in this case, because your AC is now higher than 16.
Can you extend the duration of armor of Agathys by gaining more temporary hit points?
The spell is meant to work only as long as you have the temporary hit points that the spell grants. When those temporary hit points are gone, the spell is done.
Keep in mind that temporary hit points aren’t cumulative (see PH, 198). If you have temporary hit points and receive more of them, you don’t add them together, unless a game feature says you can. You decide which temporary hit points to keep. As an example, let’s say you’re a warlock with the Dark One’s Blessing feature, which gives you temporary hit points when you reduce a creature to 0 hit points. You currently have 2 temporary hit points from armor of Agathys, you just slew a monster, and your Dark One’s Blessing can now give you 4 temporary hit points. If you take those temporary hit points, they replace the ones from armor of Agathys and end that spell, so you might not want to take them and keep the spell going.
Do the temporary hit points from heroism accumulate each round?
These temporary hit points aren’t cumulative. The spell would tell you if you were meant to add them together. At the start of each of your turns, the spell, effectively, refreshes the number of temporary hit points you have from it; if you lost some or all of the temporary hit points, the spell gives them back to you.