How do Armor Class and Attack Rolls work?

Question

How does attack damage and armor work? The game just says that your armor level "determines whether an attack hits", which is not helpful.

Reading this wiki, it sounds like the attacker rolls a die and adds some modifiers. If that value is lower than the defender's armor class, the attack misses; if it's higher, than that amount of damage is taken.

However, that can't possibly be correct, because that would mean no hits ever deal less than 10 damage, which is not true. It would also mean weaker enemies would always miss, and that armor is completely useless later in the game.

So how does Armor Class actually work? How is attack damage calculated?

Answer 1

You've probably seen ability checks like this throughout the game:

There are three separate factors at work here -- a Difficulty Class (DC), a number of static or semi-variable modifiers (like Guidance), and a 20-sided die to serve as the catalyst for random chance.

Functionally, this is very simple:

• Roll the d20
• add the modifiers
• compare to the DC

Attack Rolls work the exact same way, though rather than "Difficulty Class", use "Armor Class" (AC) instead. Regardless, this value can be viewed via the "Examine" menu. (Right-click an enemy on the PC version)

The modifiers are on the character making their attack. If you open a character's inventory, you will see an "Attack Bonus" that corresponds to their currently equipped weapon(s).

So if Shadowheart were to make a mace attack against Cyrel, we'd roll a d20, add 2, and compare that to Cyrel's AC. If that number is 10 or more, the attack hits, and damage is rolled.

Damage is calculated much the same way -- though instead of rolling a d20, you roll one or more different dice depending on the weapon itself. Shadowheart's mace, for instance, is a 1d6. Comparatively, a Githyanki greatsword, is 2d6

Spell attacks function identically (though make use of a different set of modifiers).

One final note -- rolling a 20 on the attack is a "Critical Hit". In such a case, you double the number of damage dice, whatever they are (2d6 in the case of Shadowheart's Mace, or 4d6 for Lae'zel's Greatsword).

And as a counterpoint to that, rolling a 1 on the attack is a "Critical Miss". Such an attempt will always result in a miss, even if the bonus would otherwise beat the AC.

While the actual calculations can get much more complicated as various buffs and abilities are added on, this is the core of the system, and hopefully helps shed light as to what's been implemented behind the screen (so to speak).

Answer 2

When you make an attack against the enemy's armor class, you first make an attack roll with a 20-sided die. The result plus your attack bonus is compared to the enemy AC, and if it meets or exceeds that the attack hits. Next, the damage dice are rolled, bonus damage (e.g. from strength) is added to that and that is the damage your attack deals. For weapons they have a base damage dice and an enhancement bonus as well as other bonuses, for spells they'll do damage based on the spell's description and some spells can be cast with a higher spell slot for more damage. Some spells might have an attack roll but only deal status effects rather than damage, the attack roll still works the same way.

When you roll a 20 for the attack roll, the attack always hits even if your total attack roll doesn't meet the enemy AC and deals a critical hit, which doubles the damage dice rolled in the damage step (so if your weapon normally deals 1d12 damage, you would roll 2d12). In 5e nothing other than the base weapon dice are increased (so not bonus from strength or extra dice from spell effects, for example), I believe BG3 implements this rule that way. Spells which have an attack roll and deal damage can critically strike, spells which deal damage without an attack roll or don't deal damage cannot critically strike.

When you roll a 1 for the attack roll the attack always misses, regardless of whether it would meet the enemy's AC with bonuses.

To give an example, take a Flail +1. Let's pretend you have 14 strength (+2), +1 from proficiency and nothing else. You attack an enemy with 15 AC. Your total attack roll bonus is +4 (+1 from enhancement, +2 from strength, +1 from proficiency). So, you will hit the enemy if you roll an 11 or better. If you hit the enemy, you then roll the flail's base damage of 1d8, and add +1 from enhancement and +2 from strength. If you roll a 20 on the attack roll, your critical hit damage is 2d8 + 3.