When a rune or item says +x magic penetration, how is that calculated?

Question

This is something that has been bothering me for a while now. Insight runes and Sorcerer's Shoes provide magic penetration. Is this a flat-rate penetration? Or is it a percentage?

For example:

9 Greater Marks of Insight provide +8.55 magic penetration. If I'm Shen, my level 2 Vorpal Blade does 95(+0.2AP). For simplicity, say I have 0AP. If I'm facing a level 3 Akali who has 54.36 magic resist (35% reduced damage), when I use Vorpal Blade on her, does it do damage based on:

• Her magic resist minus my magic penetration: (45.81 magic resist = 31% reduced damage = 65.55 damage)
• Her magic resist minus a percentage of my Vorpal damage: (54.36 magic resist - (8.55% * 95) = 46.23 magic resist = 32% reduced damage = 64.6 damage)
• Her magic resist minus a percentage of her magic resist: (54.36 * 91.45% = 49.71 magic resist = 33% reduced damage = 63.65 damage)

The reason this confuses me is that the Void Staff provides 40% magic penetration. The Archaic Knowledge mastery reads "penetrates 15% of the target's magic resistance". There doesn't seem to be any standardized wording.

Answer

Runes, as well as items like Sorcerer's Shoes and Haunting Guise offer FLAT penetration (i.e., you ignore up to X) of your opponent's MR.

The "average" champion has a base MR of 30, that doesn't increase with levels. With ~9 MR from runes, sorcerer's shoes are enough to bring that down to nearly zero.

The other options are reduction (triggered by champion abilities like Amumu, Ryze, or Fiddlesticks, or the Abyssal Scepter) which actually decrease your opponent's MR (and so stacks with penetration) and % based (the talent and the void staff). Of these, Reduction is the only way one can bring your target's MR below zero.

When a single character benefits from one or more of these, they are applied in this order:

1. % Based Armor/MR Reduction
2. Flat Armor/MR Reduction
3. Flat Armor/MR Penetration
4. % Based Armor/MR Penetration

Some of the above information can also be found in the game mechanics thread on the official LoL forums, here: http://www.leagueoflegends.com/board/showthread.php?t=3287

The answer to your question would then become:

54.36 MR * .85 = 46.206 MR - 8.55 = 37.656 effective MR. (I don't know the formula off hand to convert that to % damage reduced)

LoL has some inconsistent wordings, I'll agree.

Check more discussion of this question.

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