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LoL Armor/Magic Penetration: Part II: Strategy

January 22, 2011

Penetration strategy



Once you have a firm grip on how each flavor of armor or magic penetration functions on the mathematical level, the next logical step would be to figure out which type of penetration will best suit your damage-dealing needs.

The following sentence is likely to either blow your mind, or seem very obvious:

Percent penetration is most effective against tanks, flat penetration is most effective against squishies, and these two types of penetration should not be used together.

If you’re reading this post in search of a hard-and-fast guideline to blindly follow for how to choose which type of penetration to build, take the above sentence and be on your way. If you’re interested in finding out why it’s is true, continue reading!

Note: the blue line is a REDUCTION in damage by armor. The red and green lines are damage BONUS from armor penetration, which can be thought of as a percent bonus applied as true damage.


Percent penetration beats tanks



You’re playing a normal game. You select your champion, Ashe, and lock in. The game launches, and as all the players load, you notice the other team consists of five tanks.You’re filled with dread as you realize that all of their champions are capable of shrugging off your physical damage as if you weren’t even attacking.

Then, in a flash of brilliance, you remember that you know how to counter a high-armor target: percent penetration. Last Whisper (40% armor penetration). Every point of armor your target acquires as the game progresses directly boosts the effectiveness of that one item.

The math:

Instead of thinking about this in terms of penetration, think of Last Whisper as an item that grants a percentage boost to your total physical damage that scales with the amount of armor on your target.  Against an individual target with X amount of armor, the  percentage of bonus damage granted by (the armor penetration component of) this one item can be calculated as [AR(X) – AR(.6x)], where the function [AR(X) = X / (100 + X)] represents armor damage mitigation.

Still unsure why percent beats flat penetration against a tank? Read on, my friend!

Flat penetration beats squishies



“He’s got a ton of magic resist, so I’ll build Haunting Guise and Sorcerer’s Shoes.”

No! No no no no no!!! NO! Bad! VERY BAD!

The idea of building flat magic penetration to counter a high magic resist target may seem to make sense at first glance, but when you analyze that decision a little bit further, you realize it’s an error.

The key is that armor and MR returns diminish, as depicted in my graph two posts ago. Each armor point is successively less important than the one before it, until the point where extra armor points do virtually nothing. Therefore, flat armor penetration against a low-armor target is negating critical armor points, where that same amount may add virtually no damage bonus against a high-armor target.

The math:

To calculate the bonus damage percent to a target with X armor from Y armor penetration, use [AR(X) – AR(X-Y)], where the function [AR(X) = X / (100 + X)] represents armor damage mitigation.

To illustrate, two examples:

1. You have 30 armor penetration. Your target has 50 armor. You deal 16.67% more damage because of armor penetration.

2. You have 30 armor penetration. Your target has 200 armor. You deal only 3.7% more damage from the armor penetration!

There can only be one

In a game, you should avoid building both flat and percent penetration. Choosing to build both leads to a very inefficient build because flat penetration is calculated before percent penetration, creating an anti-combo. Every point of flat armor penetration reduces the scaling of your percentage armor penetration. Decide early who you’ll be targeting with your damage, and assess how much resistance they’ll likely have against it. For best results, pick either flat or percent penetration and forget about the other completely for the game.


From → Games

  1. sick post dude

  2. jasonfu9966 permalink


  3. mappum permalink

    Yes, but what about the name of the site?

  4. Clocktown permalink

    Well. “There can only be one” is simply wrong. Taking both WILL reduce the armor more than just taking one. Additionally, physical % armor penetration doesnt apply to skills that deal physical damage, only to autoattacks, so if you are a physical caster, flat armor penetration is important.
    Example: target has 300 armor, you have 51 flat and 40% armor penetration:
    (300-51)*0.6 = 149.4 armor
    If you only have 40% armor pen, it’s 300*0.6 = 180 armor. it’s the same for tanks an squishies, taking both will always result in being able to take down both tanks and squishies, and it isnt less effective.
    (51 armor pen can be achieved with runes, masteries and youmuus)
    As for Magic pen, it’s the same(but there you can have 50+ flat and 55%)

    • You’re right. No doubt about it, more armor penetration is always good. What I’m arguing, though, is that it’s comparably inefficient to stack both types of penetration (flat and percent), rather than just one. The reason for this is that flat pen is calculated first, creating an anti-combo. If the order were reversed, however, then I’d be completely off base with that assertion.

      I appreciate you pointing that out– my wording is too strong for the situation. More penetration is always more effective, no matter the source or target, and I’d be a fool to disagree with that. 😛

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