### [Mechanics] How Move Speed and Armor/Magic Penetration Work in LoL

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Anniie

Senior Member

I just had someone read this say "So, MR reduction is calculated last!"

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CooLTanG

Master Recruiter

Bring up an old post for those that still don't understand the mechanics.

Although slow calculations are outdated now.

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TiberiusAudley

Quote:
Originally Posted by CooLTanG
Bring up an old post for those that still don't understand the mechanics.

Although slow calculations are outdated now.
Yeah, a lot of the number tweaks have made this thread outdated, but the way it works is still the same.

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Zoltu

Member

I tested this in a practice game as Ezreal against Sion. He had a magic resistance of 204 and I had 48/49% magic penetration (according to character sheet). I hit him with Ezreal's middle ability that did 403 damage (according to tooltip after AP addition). Instead of hitting him for the expected amount I hit him for only 224 damage.

Magic Resist: 204
Magic Penetration: 48/49%
Damage: 403
Actual Damage: 224
Expected Damage: 258.27

Calculation:
Adjusted Magic Resist: 204 * 0.51 - 48 = 56.04
Adjusted Magic Reduction: 56.04 / (100 + 56.04) = 35.91%
Adjusted Magic Damage %: 1 - 0.3591 = 64.09%
Expected Adjusted Damage: 403 * .6409 = 258.27

This discrepancy seems to be too large for a rounding error as it's a 15% difference between expected and actual damage.

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Zoltu

Member

I did some more number running, this time without runes or masteries, just the three magic pen items. It appears that % based reduction is calculated before flat reduction. Example:

Magic Resist: 84
Magic Penetration: 40 | 39% (Void Staff should have given 40% without runes / masteries)
Listed Damage: 329
Actual Damage: 260

Calculation:
Adjusted Magic Resist: (84 - 40) * 0.6 = 26.4
Adjusted Magic Reduction: 26.4 / (100 + 26.4) = 0.2088
Adjusted Magic Damage %: 1 - 0.2088 = 0.7911
Expected Damage: 329 * 0.7911 = 260.28

If you calculate the 40% reduction first, you end up with quite different numbers. Taking the example from my previous post:
Magic Resist: 204
Magic Penetration: 48/49%
Damage: 403
Actual Damage: 224
Expected Damage: 258.27

Calculation:
Adjusted Magic Resist: (204 -48) * 0.51 = 79.56
Adjusted Magic Reduction: 0.4431
Adjusted Magic Damage %: 1 - 0.4431 = 0.5569
Expected Damage: 403 * 0.5569 = 224.43

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Varnic

Senior Member

itt thread necro and people who dont read patch notes

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T8mNEiLGTw

Senior Member

Quote:
Originally Posted by TiberiusAudley
NOTE: Only *FLAT PENETRATION* works on Towers in regards to armor. September 17th Patch Notes mention "Last Whisper - No longer works on turrets" and Towers are not affected by any debuff. I do NOT know if Void Staff can penetrate tower's Magic Resistance.
Kog'maws % armor pen from his Q affects towers. I'm not sure whether this is a bug or working as intended

NOTE: I know this is a necro, but I think this thread deserves to live.

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Mistikman

Quote:
Originally Posted by Micah71381
I did some more number running, this time without runes or masteries, just the three magic pen items. It appears that % based reduction is calculated before flat reduction. Example:

Magic Resist: 84
Magic Penetration: 40 | 39% (Void Staff should have given 40% without runes / masteries)
Listed Damage: 329
Actual Damage: 260

Calculation:
Adjusted Magic Resist: (84 - 40) * 0.6 = 26.4
Adjusted Magic Reduction: 26.4 / (100 + 26.4) = 0.2088
Adjusted Magic Damage %: 1 - 0.2088 = 0.7911
Expected Damage: 329 * 0.7911 = 260.28

If you calculate the 40% reduction first, you end up with quite different numbers. Taking the example from my previous post:
Magic Resist: 204
Magic Penetration: 48/49%
Damage: 403
Actual Damage: 224
Expected Damage: 258.27

Calculation:
Adjusted Magic Resist: (204 -48) * 0.51 = 79.56
Adjusted Magic Reduction: 0.4431
Adjusted Magic Damage %: 1 - 0.4431 = 0.5569
Expected Damage: 403 * 0.5569 = 224.43
Yea, this was a very explicit change in a patch about a month ago. It used to do % then flat, but now does flat then %.

The OP was made in January, which was obviously more than a month ago so it was accurate at the time of posting, but isn't anymore.

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CnB84

Senior Member

Here's how penetration works in LoL:

"You wasted money on armor/mr? *Buys Last Whisper/Void Staff* "Lol, noob!"

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Zoltu

Member

Sorry about raising a dead thread but when I searched for magic penetration (I think those were my terms) this was the first thread to come up. Since it was laid out in "sticky like" format I figured it was maintained. Also, the various LoL wikis and such have the same information as this thread so I assumed that the common understanding was that it was % first then flat second. I'll go update the various wikis that I use to contain the correct information so hopefully other people won't run into similar trouble.