For example, if your champion has 200 magic resistance / armor and your enemy has some crazy AP/AD damage that can be dealt with one single spell/attack, say 800 damage at once...
Considering the enemy doesn't have any magic/armor penetration (not even MPen/APen runes), how much damage would your champion receive?
31 Answer
According to League of Legends wikia, both armor and magic resistance "damage multiplier" is calculated with the following formula (replace mr with armor the same):
if enemy MR is greater or equal to 0 (MR >= 0) calculation is like below;
100 / (MR+100)
which means;
for 100 magic resistance → × 0.5 incoming magic damage (50% reduction)
so for a single spell with magic damage of 800 will deal 400 damage (considering your have no magic penetration).
if enemy MR is less than 0 (MR < 0) calculation is like below;
2 − (100 / (100-MR))
which means;
for −25 magic resistance → × 1.2 incoming magic damage (20% increase)
so for a single spell with magic damage of 800 will deal 960 damage (considering your have no magic penetration).
