5e SRD 5.2.1 · 5.2.1
Lose Temporary Hit Points First
Playing the Game > Lose Temporary Hit Points First
If you have Temporary Hit Points and take damage, those points are lost first, and any leftover damage carries over to your Hit Points. For example, if you have 5 Temporary Hit Points and take 7 damage, you lose those points and then lose 2 Hit Points.