How exactly do backpacks work? [closed]

So I’ve always assumed that you just add the backpack weight (5 lbs not the carry weight which is 30 lbs) to my cleric that has a carry weight of 255 lbs if my character was carrying nothing but the backpack I would be carrying 5 lbs of my total 255 lbs but do I leave out the weight of the full backpack or do I add that to how much I’m carrying (like 35 lbs out of 255 lbs)