League-of-legends – How does Armor Penetration works with Critical Hits

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 penetration from runes, sorcerer's shoes are enough to bring that down to basically 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. Flat Armor/MR Reduction
2. % Based Armor/MR Reduction
3. % Based Armor/MR Penetration
4. Flat 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.

First, let's clear up a misconception.

Armor Penetration gives you a constant %damage bonus, regardless of how much armor the enemy has see here.

It doesn't matter if your enemy has 300 armor or 15 -- penetrating 10 of that will have the same effect on total DPS increase.

That said, let's take a look at why Armor Penetration is preferred, even with its linearity.

Attack Speed is easy to acquire via items.

Armor Penetration is not.

You can more than match the effects of attack speed runes with a single dagger - 420 gold.

The cheapest item with Armor Penetration is the Brutalizer at 1337 total gold. By this metric alone, Armor penetration runes are worth approximately three times that of attack speed runes.

It is easier to use items to stack Attack Speed over Armor Penetration -- but to get the highest DPS, you want to stack both.

Given that the amount of Armor Penetration available via items is strictly limited, if one's goal is to achieve maximal values for both Armor Penetration and Attack Speed, runes for Armor Penetration are much better than that of Attack Speed.