How to halve the damage on a successful saving throw?

Question

I'm new to DnD, though not new to rpgs in general.

I'm a bit confused on the damage of spells. For example, the Fireball deals 8d6 fire damage on a failed saving throw, and half as much on a successfull one. What exactly is "half as much" in this case?

Is it 4d6 (half the number of dice)? Do I throw 8d6, add them up and halve the sum? Or do I throw 8d3 (half each die)?

Answer 1

The rule is “half damage”, not “half the damage dice” or “half the damage die size”. All the dice are rolled and then the total is halved (rounded down, per PHB page 7).

So in the case of fireball, all 8d6 are rolled and summed, then anyone who succeeded on the saving throw takes half that result.

Answer 2

Damage on area spells has to be rolled all at once.

the spell's damage is rolled once for all creatures caught in the blast.

(PHB p.196)

Because the damage is rolled for all targets at once, and not individually, the answer has to be the one that allows for the damage of a Fireball to be rolled once no matter how many targets are hit.

In this case, roll 8d6. If the target saves, their damage is divided by two. This makes it so that any target that doesn't save knows their damage on the same roll as well. One roll, two possible damage outcomes, depending on save.

Answer 3

Those die rolls you mentioned are not comparable.

8d6 averages 3.5 per die, totaling average 28. It is a very steep bell curve since it is averaging 8 dice, making extreme values very unlikely and most rolls falling in the midrange. Dividing that result in two retains that steep bell curve, with an average 14.

4d6 also averages 3.5 per die, indeed giving an average 14. But the bell curve is flatter - making extreme values more likely.

8d3 averages 2.0 per die, giving an average 16, for 15% more damage than intended. It does provide the very steep bell curve though.

As such, "half as much" must be 8d6/2. Otherwise you get unintended consequences.