If an odd damage roll is halved and then doubled, does it result in the same value as before, or one less?

Let's say there's a Dust Mephit at full health that was hit by the Fireball spell and made the save. The damage on the die is low at 17, so the fire damage is halved. But, Dust Mephits are vulnerable to fire damage, so it's doubled again. Does the rounding happen after the final total, and it stays at 17? Or does rounding down happen after each step, reducing it to 8, then double to 16?


The introduction to the basic rules and PHB (p. 7) mentions the general rule "Round Down":

Whenever you divide a number in the game, round down if you end up with a fraction, even if the fraction is one-half or greater.

The rules on damage resistance and vulnerability (PHB, p. 197) also state:

Resistance and then vulnerability are applied after all other modifiers to damage.

You would first divide the damage of fireball by 2 and round down to calculate the damage on a successful save, then multiply that by 2 due to vulnerability. If the initial damage roll is an odd number, you would essentially just subtract 1 to get the final result.


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