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The Fighting of Large Groups: Expected Damage

A battle with too many monsters and PCs can be tough.  A couple of weeks ago I had an encounter where a group of 5 PCs and a companion animal were caught in a cemetery with around 30 undead.  How am I supposed to keep track of 30 monsters?


So here is a proposed solution that I am calling "Expected Damage".


Simple formulas:  


Probability of Hit = (20-(AC of target - attack of attacker))/20  (Minimum is always 1/20, maximum is always 19/20)


Expected damage = Probability of Hit * Half Maximum Damage


So now, given a defender and an attacker, I can estimate the amount of damage the defender will take per round.


(Note that this formula does not take into account criticals.  The min and max values take into account miss on natural 1, hit on natural 20)




Back to the scenario -- 30 undead of the same class.  Given a PC AC, I can estimate the amount of damage they get per round and ignore having to run individual hits from each undead.


Example:


PC AC: 14
Monster Attack: +3
Damage: 1d6


Expected Damage Per Round: 9/20*3 = 1.35


If the PC is surrounded by 3 attackers, they take 3 * 1.35 = 4 damage.





I would also use this formula if battling two large groups of monsters or NPCs against each other.  This might be good in a scenario where PCs are caught in a large battle, either directly or in crossfire.
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