D&D Critical Hits Explained

Critical hits are one of the easiest rules in D&D to get excited about and one of the easiest rules to slightly mess up because the table starts celebrating before anyone checks what actually doubles.

A natural 20 on an attack roll is a critical hit. That means the attack hits no matter what the target’s AC is. You could be swinging at something with a ridiculous Armor Class and a stack of smug magical defenses, but if the d20 shows a 20, the attack hits.

This only applies to attack rolls. That part matters.

If you roll a natural 20 on an ability check, that does not automatically mean you succeed. If you roll a natural 20 on a saving throw, that does not automatically mean you beat the save unless a rule says otherwise. A 20 is always nice, obviously, but the automatic hit and critical damage rule belongs to attack rolls.

So no, the bard does not seduce the locked door because they rolled a 20 on Persuasion. I am begging tables to stop doing this unless everyone knowingly signed up for nonsense.

The damage part is where people usually get sloppy.

When you score a critical hit, you roll the attack’s damage dice twice, add them together, and then add your normal modifiers once.

If you hit with a longsword that normally deals 1d8 plus your Strength modifier, a critical hit deals 2d8 plus your Strength modifier. Not 2d8 plus double Strength. Not max damage plus another roll unless your table uses that as a house rule. Just roll the damage dice twice and add the modifier normally.

The same logic applies to spell attacks. If a spell uses an attack roll, it can score a critical hit. If the spell forces a saving throw, it cannot crit because there was no attack roll.

That means something like Fire Bolt can crit. Fireball does not crit. Fireball does not care how dramatically you throw the dice because the target is making a saving throw, not being hit by an attack roll.

The part that gets murky is extra damage dice.

If the attack includes extra damage dice, those dice are doubled too. Sneak Attack is the classic example because rogues live for this moment and will absolutely remind you if you forget. If a rogue crits with a weapon attack and Sneak Attack applies, the weapon dice and Sneak Attack dice are rolled twice.

So if the attack normally deals 1d8 plus 3d6 plus Dexterity, the crit deals 2d8 plus 6d6 plus Dexterity.

Again, the modifier is added once. The dice are the part getting doubled.

This is also why paladins start smiling like criminals when they crit. If they add Divine Smite to the attack, those smite dice are part of the attack’s damage dice and get rolled twice. Yes, it hurts. Yes, the monster deserved better. No, I am not here to protect it from consequences.

Flat damage bonuses are different. If something adds plus 2 damage, that does not become plus 4 just because the attack crit. If something adds your ability modifier, that does not get doubled. If something says you deal extra damage as a fixed number, that number stays fixed unless the feature says otherwise.

The easiest way to remember it is this: dice double, flat numbers do not.

There are still edge cases because this is D&D and apparently we enjoy paperwork. Some features trigger after a hit, some modify the attack’s damage, and some have their own wording. When in doubt, read the exact feature. If it adds damage dice to the attack, those dice usually get doubled. If it creates a separate effect or asks for a separate save, do not just throw it into the crit pile because everyone is excited.

Advantage and Disadvantage also matter here. If you roll with Advantage and one of the dice is a natural 20, congratulations, that is a critical hit. If you roll with Disadvantage and one die is a 20 but the other die is lower, you use the lower die. That means no crit. Painful, but clean.

Critical hits also matter when attacking a creature at 0 Hit Points. Damage taken at 0 already causes a death save failure. If that damage comes from a critical hit, it causes two failures instead. This is why melee attacks against a downed character are not just rude, they are mechanically terrifying.

As a DM, I try not to turn every enemy into a cold-blooded execution machine, but some monsters absolutely would keep attacking a fallen target. A starving ghoul is not carefully considering the party’s emotional investment in character continuity. It is eating.

The DM can also use house rules for critical hits if the table likes them. Some groups use max damage plus rolled damage so crits feel bigger. Some keep the rules exactly as written because swingy damage is already swingy enough. Either is fine as long as everyone knows the rule before the roll happens. Changing crit rules after the paladin picks up eight dice is how you create a courtroom scene.

So, the practical rule is this. A natural 20 on an attack roll is a critical hit and automatically hits. Roll the attack’s damage dice twice, including extra damage dice like Sneak Attack, then add your normal modifiers once. Saving throws and ability checks do not crit by default, and flat bonuses do not double.

That is it. Now let the rogue roll all those d6s before they start vibrating through the chair.