How do critical hits work with static monster damage?

Question

If I use static damage from monsters, how do I calculate the value on critical hits? I can't just double the value, because that includes the static modifier. Should I recalculate the damage on the fly (so, if the monster usually has (7) d8 + 3, I make it (11) 2d8 + 3)? That's kind of pain in the ass. Or do the monsters not double their static damage on critical hits and only get the benefit of the guaranteed hit? Alternatively, I could decide that I roll on crits.

Are there official rules the govern this?

Answer 1

The Dungeon Master's Guide has rules for handling this case:

On the Monsters and Critical Hits section (DMG page 248), it states what to do in the case of a monster critical hit when using average damage:

When the monster scores a critical hit, roll all the damage dice associated with the hit and add them to the average damage. For example, if a goblin normally deals 5 (1d6 + 2) slashing damage on a hit and scores a critical hit, it deals 5 + 1d6 slashing damage.

So the suggestion is simply to roll the damage dice and then add the average damage. In your case, the damage of (7) d8 + 3 simply becomes 1d8+7 when a mosnter scores a critical hit.

Answer 2

Are there official rules on this?

Yes, per Sdjz's answer..
But it requires a die roll.
You can do it without any dice rolling if you like.

What have I seen done that is fast and simple?

A critical hit when using average damage is "Max roll +1"
If I am doing 1d4, it becomes 5 + modifier.
If I am doing 1d6, it becomes 7 + modifier.

Easy to keep track of in one's head:
When monster rolls a "Critical Hit, I add one to the max die value"
... you have to add the modifier anyway, critical hit or not.

No die roll required.

What do I do? I roll the dice. I got addicted to the dice rolling bit when I started playing, so even though the above method is faster, I still roll the dice.

---

_{As the number of dice rolled increase, you run into the matter of even number versus odd number. If monster damage is 2dx, 4dx, 6dx just double the average + modifier. If you have an odd number of dice, 3dx, 5dx, it's double the average +1 + modifier ... because the posted average in the books rounds fractions down}