How high does a mobtrap have to be in order for them to fall and die?

In minecraft, mobs typically take damage from falling if they fall 3 blocks or more. However, to ensure they die from the fall, the drop should be at least 23 blocks high, as this results in enough fall damage to exceed their health points. For certain mobs with higher health, you may need to adjust the height accordingly, but 23 blocks is a safe height for most.