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During a D&D campaign, we were fighting a Mummy Lord. It’s vulnerable to fire, so I used Fireball against it. It succeeded on the saving throw to halve the damage. Would it just take whatever damage I rolled, or would what I rolled be halved?

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    \$\begingroup\$ Are you sure you used scorching ray? That spell involves attack rolls and not saving throws \$\endgroup\$ – Medix2 Dec 22 '19 at 11:12
  • \$\begingroup\$ Related (though not quite the same): "How do saving throws for half combine with damage resistance?" \$\endgroup\$ – Medix2 Dec 22 '19 at 11:13
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    \$\begingroup\$ Ah but where are my manners. Welcome to the site! Take the tour if you haven't already and you can visit the help center if you have more questions. Good luck and happy gaming! \$\endgroup\$ – Medix2 Dec 22 '19 at 11:26
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    \$\begingroup\$ @medix I'm going to swap out the name of the spell. It is irrelevant for the main point. \$\endgroup\$ – Mindwin Dec 23 '19 at 11:31
  • \$\begingroup\$ @Mindwin Yup, that works. I'm unsure whether that one is a dupe or not since it involves resistance meaning the order of application doesn't matter while in this case it technically does \$\endgroup\$ – Medix2 Dec 23 '19 at 12:51
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There would be rounding errors. Player's Handbook page 7 tells you to always round down. Thus, if you rolled 7 and the Mummy Lord succeeded at the saving throw to halve damage, it would be halved to 3. Then, this 3 would be doubled due to vulnerability, for a total of 6 damage.

Many DMs I know would just rule that halved, then doubled means it just stays unchanged, but strict RAW it is only true for even values. Odd values are lowered by one in that process.

As a side note, scorching ray doesn't seem to allow a saving throw for half damage. It only requires the attack roll for each ray to hit in order for a ray to do its damage.

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    \$\begingroup\$ You can get around the rounding error by doubling first, then halving. I don't think the rules specify the order. \$\endgroup\$ – Jorn Dec 22 '19 at 22:09
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    \$\begingroup\$ @Jorn they do. Spelsl say somethinglike "XdY [type] damage, or half as much on successful save", so that's the damage taken. Vulnerability or resistance is applied after the spell damage is figured out. Can't find exact reference now, it's about midnight here. \$\endgroup\$ – Mołot Dec 22 '19 at 22:51
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    \$\begingroup\$ @Jorn: There is an order to resistance and vulnerability: "Resistance and then vulnerability are applied after all other modifiers to damage." Not sure if "half damage on a successful save" is considered a modifier to damage, but Mołot's interpretation of "half on a successful save first" has always been my interpretation, because that half-damage is the starting point of the amount of damage that's even relevant to them before even considering resistance/vulnerability. \$\endgroup\$ – V2Blast Dec 22 '19 at 22:53
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Technically the damage you rolled would be halved from the saving throw and then doubled from the vulnerability

The fireball spell states:

[...] Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one [...]

The saving throw represents whether the Mummy Lord takes the full force of the spell or manages to avoid some of itm Thus the Mummy Lord has evaded some of the damage, and will now only be taking half. However, this all happens before resistance and vulnerability apply.

The section on "Damage Resistance and Vulnerability" states:

[...] If a creature or an object has vulnerability to a damage type, damage of that type is doubled against it [...]

This tells us that we would now double the damage we just halved from the saving throw success. You could simply have the Mummy Lord take the damage you actually rolled but this would be slightly incorrect 50% of the time.

Imagine the fireball dealt 33 damage, this would half to 16 due to rounding down and then double to 32, different from the initial roll. This happens with any odd numbered damage roll. Though the whole halving then doubling thing is merely the strict RAW, many GMs would simply not modify the damage at all.

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I just simplify the result by the vulnerability and successful save cancelling out. doubling and halving anything leaves it unchanged

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    \$\begingroup\$ Welcome to RPG.SE! Take the tour if you haven't already, and check out the help center for more guidance. This seems like a partial answer (both answers already say that most DMs would resolve that specific part of the situation as you suggest), and doesn't really explain the relevant rules for OP's question. Could you elaborate on your answer to more directly address the question (and support your answer/recommendation by citing evidence/experience)? \$\endgroup\$ – V2Blast Dec 22 '19 at 22:56

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