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I just purchased the D&D edition 5 starter set, and the instruction manual/Rule book doesn't go in depth about the effect of AC and how armor effects total AC.

So how does AC affect the enemies damage/ability to hit the character and how does armor affect AC?

There are pregenerated characters that come with this set. Why does the character sheet have a preset AC at the top of the page, and how does armor effect the total amount? I have a fighter with AC 16, a +2 proficiency bonus, and a goblin helm with +1 AC that was given to us as loot following the second boss fight. What would my AC be with chainmail, that goblin helmet, and my proficiency on all armor?

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When using pregenerated characters, use the Armor Class as depicted

The Starter Set has precalculated the armor class for ease of use to make it easy to start to play with a minimum of preparation time. The deeper explanation will be found in the Basic Rules that are available for download for free at this site.

What is my armor class?

I have a fighter with AC 16, a +2 proficiency bonus, and a goblin helm with +1 AC that was given to us as loot following the second boss fight. What would my AC be with chainmail, that goblin helmet, and my proficiency on all armor?

Your armor class would be 17, since your proficiency does not add to armor class. While helmets aren't normally calculated separately, a magic item can add to armor class, and this one does.

What about proficiency?

Armor Proficiency. Anyone can put on a suit of armor or strap a shield to an arm. Only those proficient in the armor’s use know how to wear it effectively, however. Your class gives you proficiency with certain types of armor. If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells. (p. 44 Basic Rules, also p. 16 in Starter Set Rules)

On page 44-46 of those rules is the explanation of how armor class works.

Other examples of AC calculation

For example, there is a first level pre generated dwarf cleric made with the standard array who had chain mail and a shield. The listed armor class is 18.

  1. When we look at the table on page 44 of the Basic Rules, we note that chain mail is given an AC of 16, and a shield is given an AC of 2.

    If the cleric uses his Warhammer one handed, he has AC 18.
    If he uses the war hammer two handed, he can't use the shield and has AC 16.

As a second example, there is a pregenerated Halfling rogue with an armor class of 14. He starts with leather (AC 11) and add his +3 dexterity modifier to his AC to make 14 (since it is light armor, see the chart on p. 44 of the linked Basic Rules).

For medium and lighter armor, a character's dexterity bonus is added to armor class depending upon the score and the kind of armor. It is all explained in pages 44-46 of the Basic Rules. I strongly recommend that you download them for free and use them as a reference. They explain a great deal about things like this.

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  • \$\begingroup\$ You may want to mention that proficiency doesn't add to armor class in this answer as you did in your comment on the question, since the answers on the comment are almost certainly going to need to get purged again soon. \$\endgroup\$ – doppelgreener Jan 18 '18 at 14:42
  • \$\begingroup\$ @doppelgreener Yeah, I edited some more in as the question changed. thanks. \$\endgroup\$ – KorvinStarmast Jan 18 '18 at 14:47
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When you make an attack roll (either using a melee weapon, such as a sword, or a ranged weapon, such as a bow, or when you use a spell that tells you to make a melee/ranged spell attack), you take the AC of the creature as the threshold you have to reach or surpass to hit the target.

Example:

You use a longsword to attack a creature. Your strength is 14, i.e. modifier +2, your proficiency bonus is +2, making your attack bonus +4.

Therefore, you now roll a d20, and add 4. Then, you compare this value to the AC of the targeted creature, and if you have scored at least the same value, you hit and can roll for damage.

Example values:

  • you rolled a 7+4 = 11, versus an AC of 15. The attack does not hit
  • you rolled an 11+4 = 15, versus an AC of 15. The attack does hit
  • you rolled anything higher than an 11+5, versus the same AC of 15 → you also hit.

The way AC works is, you basically have armor and your dexterity. So if your AC is higher than the attack, you eigher dodge out of the way, or you deflect it with your armor.

Mind you, there are other kinds fighting your enemies, such as casting spells, which often require saving rolls. In that case, the target has to roll a d20, adding the value of the required saving throw, and score at least the same value as your spell save DC (which is 8 + proficiency modifier + spellcasting modifier). That wasn't your question, though, so for more details, read the spellcasting chapter in the Player's Handbook.

The AC on the pregenerated sheets is calculated by taking the base AC of the worn armor (leather 11, studded 12, etc.) and adding the creatures dexterity value. Heavier types of armor let you add less (medium armor, max +2) or even no dexterity bonus (heavy armor). With light armor, you can add as much dexterity bonus as you want. If you aren't wearing any armor, your AC is simply 10 + your dex. Mind you, negative dex also reduces AC, unless you are wearing heavy armor.

What does AC do?

AC makes it harder to hit the creature. To reduce the damage, you need resistence (1/2 damage) or immunity to the type of damage that the attack deals (slashing, bludgeoning, and piercing for regular weapons, or fire, cold, etc. for other effects). There are also class features (such as the rogue's "Uncanny Dodge") that allow you to reduce the damage regardless of resistence. You can stack such a feature with resistence for 3/4 damage reduction, but gaining resistence from multiple sources doesn't help any more than just from one source.

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AC affects enemies ability to hit by raising the target number they have to achieve / beat to hit the target.

For example, unarmored person without any dexterity, etc bonuses to AC would have an AC of 10.

An armored knight in half plate (15 AC) and with a little dexterity (lets say +1) would have an AC of 16.

A hard skinned daemon with magical protection might have AC of more than 20.

So what kind of armor you have affects how much AC you have, as does your dexterity, magical bonuses, etc.

The enemy trying to hit would use their strength- or dexterity bonus (for ranged weapons), add their proficiency and other assorted bonuses (from a magical weapon or a spell, for example).

Their result would have to beat, or equal, the target's AC, or it would not hit.

So, the higher the AC, the harder it is to hit the target.

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    \$\begingroup\$ This answer has a fundamental misunderstanding of the game. In D&D5E, Armor does not provide a bonus to AC, it provides a way to calculate AC - a character can have more than one calculated AC, and picks the best one. \$\endgroup\$ – T.J.L. Jan 18 '18 at 14:34
  • \$\begingroup\$ Old habits die hard when game companies change terms and descriptions (I come from 1st Edition to 5E background). Bonus might be the wrong term today, I give you that much - though. :D (Edit: edited to remove "bonus" from armor example). \$\endgroup\$ – DocWeird Jan 18 '18 at 14:36

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