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

Anniie

Senior Member

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

CooLTanG

Recruiter

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

Although slow calculations are outdated now.

TiberiusAudley

Quote:
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Although slow calculations are outdated now.

Yeah, a lot of the number tweaks have made this thread outdated, but the way it works is still the same.

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.

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 Damage %: 1 - 0.4431 = 0.5569
Expected Damage: 403 * 0.5569 = 224.43

Varnic

Senior Member

T8mNEiLGTw

Recruiter

Quote:
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.

Mistikman

Quote:
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 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.

CnB84

Senior Member

Here's how penetration works in LoL:

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