# How Does Armor and Magic Penetration Work in LoL?
Learn how flat penetration (Lethality and flat Magic Pen), percentage penetration, and resistance reduction work in League of Legends, including exact formulas, negative armor math, and interactions with minions, monsters, and turrets.
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Guide • October 19, 2026 • Coach Carlo Montana

Percentage penetration strips a fraction of an enemy’s total Armor or Magic Resistance first, and flat penetration (Lethality or flat Magic Pen) subtracts a fixed number afterward. Understanding that calculation order, along with the rule that penetration stops at zero while reduction can go negative, tells you which penetration item to buy against carries, bruisers, tanks, and objectives.

![Serrated crimson-edged assassin dagger and glowing violet crystal staff piercing through a cracked iron breastplate and shattering arcane barrier.](https://baronbuff.com/blog/how-armor-and-magic-penetration-work-lol.jpg)

Every physical and magic damage dealer eventually faces the same shop decision: buy more raw AD or AP, stack flat penetration (Lethality or flat Magic Penetration), or pivot into percentage penetration. Building the wrong type can leave you dealing near-true damage to a support while barely scratching a two-item bruiser.

League of Legends resolves all resistance shred and penetration in a fixed four-step sequence. Once you know the order of operations and the resistance breakpoint where percentage penetration overtakes flat penetration, itemizing against any enemy lineup becomes straightforward math.

## The Four-Step Calculation Order and Exact Formulas

When you hit a target with physical or magic damage, the game computes their Effective Resistance (R_eff) against your hit in four strict steps: (1) Flat Reduction, (2) Percentage Reduction, (3) Percentage Penetration, and (4) Flat Penetration (Lethality or flat Magic Pen). Reductions physically lower the target’s Armor or MR on the scoreboard so every ally benefits; penetrations apply only to your own damage.

Because Step 3 (Percentage Penetration) happens before Step 4 (Flat Penetration), the two stats work together instead of competing. The formula after reductions is: R_eff = max(0, [(R_base − Flat Reduction) × (1 − % Reduction)] × (1 − % Penetration) − Flat Penetration). If a target has 100 Armor and you have 35% Armor Penetration (Lord Dominik’s Regards) plus 18 Lethality (Youmuu’s Ghostblade), the 35% penetration strips 35 Armor first (leaving 65), and your 18 Lethality subtracts from the remainder, leaving the target with 47 Effective Armor. Multiple percentage sources stack multiplicatively rather than additively: 30% Armor Reduction (Black Cleaver) plus 35% Armor Penetration leaves 0.70 × 0.65 = 45.5% of the target’s Armor (54.5% total negation).

### Resistance Reduction & Penetration Calculation Order

Steps 1 through 4 are applied sequentially from top to bottom on every physical or magic hit.

Step & Mechanic | Who Benefits | Can Drop Below 0? | Primary Examples & Target Profile |

1. Flat Reduction | Entire team | Yes (amplifies damage) | Corki E (Gatling Gun), Rengar empowered E; best vs low-resist targets |

2. Percentage Reduction | Entire team | No (stops at 0) | Black Cleaver (up to 30%), Bloodletter’s Curse, Trundle R; best vs frontliners |

3. Percentage Penetration | Attacker only | No (stops at 0) | Lord Dominik’s Regards (35%), Void Staff (40%), Cryptbloom (30%); best at 80+ R |

4. Flat Penetration / Lethality | Attacker only | No (stops at 0) | Youmuu’s Ghostblade, Opportunity, Sorcerer’s Shoes, Shadowflame; best at 35–80 R |

## Flat Penetration (Lethality & Flat Magic Pen) vs. Percentage Penetration

Flat Penetration (Lethality and Flat Magic Pen): Since Patch 14.1, 1 point of Lethality grants 1 flat Armor Penetration immediately from level 1 (replacing the old level-scaling formula). Flat Magic Penetration from Sorcerer’s Shoes (12), Shadowflame (15), or Stormsurge (15) works identically against Magic Resistance. Because each point of resistance equals 1% Effective HP, flat penetration yields its highest percentage damage gain when the target’s resistance is already low. Removing 18 Armor from a 50-Armor marksman drops their physical EHP multiplier from 1.50× to 1.32×, a 13.6% real damage increase. Removing that same 18 Armor from a 250-Armor tank (3.50× to 3.32×) increases your damage by just 5.4%.

Percentage Penetration: Items like Lord Dominik’s Regards (35% Armor Pen), Mortal Reminder (35% Armor Pen), Serylda’s Grudge (30% Armor Pen), Void Staff (40% Magic Pen), and Cryptbloom (30% Magic Pen) scale directly with the enemy’s total resistance. Against a 50-Armor carry, 35% Armor Penetration ignores 17.5 Armor, roughly matching one Lethality item. Against a 250-Armor tank, 35% penetration strips 87.5 Armor in a single slot (equivalent to nearly five Lethality items), increasing your post-mitigation damage by 33.3%.

## Who Should Build Each, and Against Whom?

Build Flat Penetration when your job is bursting squishy targets (35 to 80 Armor/MR): AD assassins (Zed, Talon, Qiyana, Rengar), lethality marksmen (Varus, Miss Fortune, Caitlyn), burst mages (Syndra, LeBlanc, Vex), and AP assassins (Akali, Evelynn, Ekko) spike hardest on 1st- and 2nd-item flat penetration because enemy carries have not yet gained late-game base armor growth or defensive items. Stacking Sorcerer’s Shoes with Shadowflame (27 flat Magic Pen) leaves a 42-MR ranged carry at just 15 Effective MR, which is nearly true damage.

Build Percentage Penetration as soon as priority targets cross 80 to 90 Resistance: At 80 Armor, 35% Armor Penetration ignores 28 Armor, already outperforming every single flat Lethality item in the game. At 75 MR, Void Staff’s 40% Magic Penetration ignores 30 MR, beating two flat Magic Pen items combined. Because every champion in League of Legends reaches 80 to 115 base Armor by levels 14 to 18 without buying a single tank item, ADCs and physical casters almost always need a percentage penetration item by their 3rd or 4th slot, and mages need Void Staff or Cryptbloom the moment enemies buy a single Null-Magic Mantle, Mercury’s Treads, or Kaenic Rookern.

Build Percentage Reduction when your team shares your damage type: Bruisers and ability fighters like Vi, Wukong, Jarvan IV, and Garen build Black Cleaver not only for their own damage, but because carving 30% off a frontline tank’s Armor boosts their allied ADC’s damage as well and stacks multiplicatively with the ADC’s own Lord Dominik’s Regards.

- [See the defensive side of the math: How Much Armor and Magic Resistance Should I Build in LoL?](https://baronbuff.com/blog/how-much-armor-and-magic-resistance-to-build-lol/)

## How Negative Armor and Magic Resistance Work

Penetration can never reduce a target’s Armor or MR below 0. If an enemy mage has 38 MR and you have 42 flat Magic Penetration, your spell calculates damage at 0 MR (100% of tooltip damage), and the extra 4 points of penetration do nothing. Only Flat Resistance Reduction, applied in Step 1, can push a target’s Armor or MR into negative numbers.

When flat reduction drops a target’s resistance below zero (R < 0), the game switches to an amplification formula: Damage Multiplier = 2 − 100 / (100 − R). Unlike positive resistance, negative resistance has diminishing returns capped at 2.0× damage: a target reduced to −25 Armor takes 2 − (100 / 125) = 1.20× damage (+20% bonus damage), and a target at −100 Armor takes 1.50× damage (+50% bonus damage).

## Do Penetration and Reduction Affect Minions, Jungle Monsters, and Turrets?

Lane Minions: Melee, caster, siege (cannon), and super minions all have 0 base Armor and 0 Magic Resistance. Because penetration cannot drop resistance below 0, buying Lethality, flat Magic Penetration, or percentage penetration adds zero bonus damage to lane minions. Only raw AD, AP, attack speed, or flat-reduction debuffs increase waveclear damage.

Neutral Jungle Monsters and Epic Objectives: Yes. Jungle monsters have positive Armor and MR, so both flat and percentage penetration (as well as reduction) increase your damage until the monster’s resistance hits 0. Standard large monsters (Blue Sentinel, Red Brambleback, Gromp, Ancient Krug, Crimson Raptor, and Greater Murk Wolf) have 42 Armor and 42 MR (smaller camp monsters have 20 Armor and 20 MR), while epic objectives like Baron Nashor carry 120 Armor and 70 MR. Early Serrated Dirk or Sorcerer’s Shoes noticeably speed up camp clears and Dragon or Voidgrub takes, and percentage penetration significantly boosts Baron and Atakhan DPS.

Turrets: Turrets have positive Armor and Magic Resistance. Both flat penetration (Lethality and flat Magic Pen on basic-attack magic damage) and percentage penetration apply to your damage against turrets, helping carries melt plates and late-game towers faster, whereas armor and magic reduction debuffs (like Black Cleaver) do not apply to structures.

## Related guides

- [How Much Armor and Magic Resistance Should I Build in LoL?](https://baronbuff.com/blog/how-much-armor-and-magic-resistance-to-build-lol/)

- [How to Adapt Your LoL Item Build to the Match](https://baronbuff.com/blog/how_to_adapt_lol_item_builds/)

## Sources

- [League of Legends Wiki: Armor Penetration & Lethality](https://wiki.leagueoflegends.com/en-us/Armor_penetration)

- [League of Legends Wiki: Magic Penetration](https://wiki.leagueoflegends.com/en-us/Magic_penetration)

- [League of Legends Wiki: Monster Stats & Resistances](https://wiki.leagueoflegends.com/en-us/Monster)

[Baron Buff calculates enemy resistance breakpoints in real time so you know the exact moment to buy flat or percentage penetration.](https://baronbuff.com/)

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