Poison Damage can be difficult to calculate in Diablo 2, because multiple poison items and skills do not stack directly. Poison damage is not actually calculated in damage per second, but damage per frame. Diablo 2 is set to run at a constant 27 frames per second. An item that does 27 poison damage, over 1 second, is actually doing 1 damage per frame. To calculate the total poison damage done from all your items and skills, there are two steps. First, calculate the damage per frame for all your items and and them together. Second, average the duration in frames for all the sources of poison. Finally multiply these numbers together to get the total damage done.