Calculating Damage. Each weapon, spell, and harmful monster ability specifies the damage it deals.
1. Roll the appropriate damage die or dice. The damage will be a certain type.
2. Add any applicable modifier. For weapons, this is the modifier of the ability used for the attack. The damage type is the same as above.
3. Add damage due to any magical properties of any applicable weapon or item. The damage type depends upon the magic.
Damage can also be affected by:
Round down on the damage total, even if the fraction is one-half or greater.
Damage is subtracted from the target's hit points. This has no effect on its capabilities until it drops to 0 hit points.
Damage from an improvised weapon or unarmed strike.