How much damage do you gain from x amount of magic penetration? I can't figure out how to math it.

I'm pretty sure Magic Penetration relates to Magic Resistance.

So if you have 10% Magic Penetration and your opponent has 100 Magic Resistance, it'll be like they have 90 Magic Resistance. I THINK that's how it works, though I may be wrong.

Krath is right. You can only penetrate up to the amount of magic resistance your opponent has.

That still leaves room for how much it will increase your damage by, assume that it doesn't take them below 0.

There's no direct ratio where x penetration equals y amount of extra damage.

Every 1 point of magic resist translates to 1% extra effective health against spells. The amount of pure damage reduction then depends on how much magic resist they have. Penetration alone cannot drop Magic Resist below zero. Magic Resist reduction (abyssal scepter, amumu, and fiddle passives) can, but the reduction from that happens before penetration kicks in AFAIK.

So if Champion A has 2000 health, and 100 Magic Resist their effective health against magic damage is 4000 - or a 50% reduction, causing an ability that deals 200 damage normally to do only 100. If you have 40 penetration, that 100 magic resist drops to 60, making the effective health of the champion 3200 against spells - or a 37.5% reduction in damage, which would cause the same 200 damage ability from before to deal 125. In this case 40 penetration means 75 damage, but that will change based on the level of Magic Resist you started with. 40 MR being reduced to 0, for example, only nets ~42 extra damage.

Fiddle, Amumu, and Abyssal all can reduce past 0. hence why i see a lot of -50 MR players... yummm.

damage from spell = x
damage taken by hero with mr magic resist = x*(100/(100+mr))
damage taken by hero when you have y penetration = x*(100/(100+mr-y)

It is like a pizza. Damage = tastiness. Someone can add anchovies (mr) to make it less tasty. You counter this by removing some of the anchovies (magic penetration). Or you can just add more tastiness by putting on pineapple, bbq sauce, chicken, and salami (AP). Which one is better to do depends on how many anchovies were added.

Or how tasty ur pizza was to start off with.

I must know the rate for how much pineapple, BBQ sauce, and chicken raises the tastiness. It might even depend on each person.