Magic Penetration

[quote=Goregrim]I mainly play AP characters so obviously magic penetration is a big part of my act. In the champion statistics screen (C), it has both a number and a percentage. I was curious if these two are added together, or if the higher of the two is chosen. I have runes for penetration that give me 9.3 currently, but when I get a void staff along with my mastery talent, I have a 49% magic penetration, which seems significantly better than 9.3. It seems like a waste of rune slots if it is simply replaced by item effects. Also, how does magic penetration calculate with champion magic resistance? For example, if an enemy champion had 63% reduced magic damage and I have 49% penetration, does that mean their resistance only offers 14% reduction, or 32% reduction, or some other equation?[/quote]

[quote=Lugignaf]You do get both. I can't explain it too well though so, I'm going to wait until one of the higher-ups sees this and they'll hopefully explain more.[/quote]

[quote=Jebus McAzn]You calculate magic resist based on a flat amount of magic resist, which translates to a percentage reduction. If an enemy has 100 MR, they have 100/200 reduced damage or 50%. An enemy with 200 MR will have 200/300 reduced damage or 67%. Your enemy champion with 63% reduced damage actually has 170 magic resist (because 170/270=63%). That 170 magic resist will be reduced by your 49% penetration to 86.7 MR, offering 86.7/186.7=46.4% reduced damage. Magic penetration is calculated in terms of percentage reduction, flat reduction, flat penetration, and then percentage penetration. Your runes, which provide flat penetration, apply before your Void Staff and masteries, which applies percent penetration. A build which runs full magic penetration marks will have approximately 9 magic penetration. Let's say you have 49% penetration again. Against a target with 50 MR, you will first penetrate 9 of that MR, lowering it to 41, and then penetrate 49% more, lowering 41 to 20.91, meaning that your enemies will be treated as if they had 20.91 magic resist for your spells only.[/quote]

[quote=Goregrim]Wow, thank you very much, that was incredibly detailed and exactly what I needed.[/quote]

[quote=Eihn]nice didnt know all of that[/quote]

[quote=DEWO]Resist reduction and resist penetration are applied in this order: 1. Percent reduction 2. Flat reduction 3. Flat penetration 4. Percent penetration They add but they are calculated in this sequence.[/quote]

[quote=Lord Rygor]I have a question. Say you have 20 magic penetration from [[Boots of Sorcery]] and the enemy have 0 MR items. Your spell says it hits for 100 damage. How much damage would you actually do? Or rather, do enemies take increased damage if a champion has magic penetration items on and they are not actively raising magic resists?[/quote]

[quote=DEWO]Penetration cant go on minus values. So with 0MR you will deal 100 damage. But flat reduction can get your target to minus values for bonus damage (correct me here if i am wrong) In your maths If your target has 20MR and you have 20Mpen you will deal 100 damage If your target has 0MR and you have 2834592837592875928Mpen you will deal 100 damage.[/quote]

[quote=Lord Rygor]Dang, I may need to re-evaluate the way I build my AP characters then. I tend to go all out on magic pen because I thought it did extra damage if they didn't stack MR! Thanks for this.[/quote]

[quote=TheJohn]See if I understood it: Flat: It's MR-MagicPen/(MR-MagicPen)+100, then I find a %. %: It's like the flat, but already reduces the %, not the flat. And The enemy has 100 MR, so the magic dmg taken is reduced by 50%. I have 50 MagicPen, so I can "pass" by 33% of his MR. That means that I have 33% MagicPen? And the damage reduce will decresease to 17%? It's better to get flat or % MagicPen?[/quote]