Round Down Damage / Damage Rounds Down
https://rpg.stackexchange.com/questions/59008/how-is-a-damage-rounded-for-damage-resistance https://twitter.com/jeremyecrawford/status/857080390535827456?lang=en https://www.reddit.com/r/DnD/comments/28vakg/5e_rules_always_round_up/
Rounded Down
There's one more general rule you need to know at the outset. 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.
When might this matter?
For the most part, one or two points of damage won't matter -- until it does. Certain abilities and traits rely on temporary hit points, like Armor of Agathys and the Druid's Wild Shape. Rounding damage down gives slight bonuses to those abilities because they'll have the opportunity to last slightly longer. Since temporary hit point pools are much smaller than normal hit point pools, rounding damage down (instead of up) has a bigger impact on it.
Other Side Effects
p. 197 - Apply resistance, then vulnerability
https://rpg.stackexchange.com/questions/121086/what-happens-when-a-creature-with-vulnerability-to-a-damage-type-has-resistance
Resistance and vulnerability is a little strange since you apply resistances then vulnerabilities. In a situation where a creature is both resistant and vulnerable to a damage source, the damage is reduced (and rounded down) then doubled. If 7 damage is supposed to be dealt, the damage is halved and rounded down to 3, then doubled to 6.
Saving Throw reduction happens first, then damage reduction (from sources like Heavy Armor Master), then resistances, then threshold (e.g. immune to damage unless it receives X damage).
https://rpg.stackexchange.com/questions/159414/in-what-order-do-damage-resistance-damage-reduction-damage-thresholds-and-hal
Resistances don't stack but they work with other damage mitigation abilities, like Uncanny Dodge.
Exceptions
While this doesn't apply specifically to damage, there are cases where you'll round up (the rules will mention when rounding up applies):
- Hit Dice Recovery after a long rest.
- Druid/Wizard spell slot recovery after rest.