Penetration and lethality in LoL: when to build each
Lethality, percent penetration, armor reduction: they sound alike but do different things and apply in a fixed order. Understanding that order tells you exactly when to build lethality and when to build percent penetration.
Resistances and how much they mitigate
Armor (against physical damage) and magic resistance (against magic) don’t “subtract” damage — they divide it. With the formula above, 100 armor means you take half the physical damage (0.5 multiplier), 200 armor a third, and so on. Each point gives diminishing returns in mitigation, but increasing returns in EHP — the classic health vs resistances topic.
Penetration and reduction are the tools to ignore part of that resistance and give the damage back to your champion. But they don’t all work the same, nor apply at the same moment. The order is fixed by Riot and is the key to this whole guide: you can see it step by step, with the formula visible, in the Mitigation Lab and in how damage is calculated.
multiplicador = 100 / (100 + resistencia) | Armor | Reduction | Damage from 1000 | EHP with 2000 HP |
|---|---|---|---|
| 0 | 0% | 1000 | 2000 |
| 25 | 20% | 800 | 2500 |
| 50 | 33.3% | 667 | 3000 |
| 100 | 50% | 500 | 4000 |
| 150 | 60% | 400 | 5000 |
| 200 | 66.7% | 333 | 6000 |
| 300 | 75% | 250 | 8000 |
The mitigation engine with the real formula: damage taken = damage × 100 / (100 + armor).
Reduction vs penetration: order is everything
There are four effects and they apply always in this order: (1) flat armor reduction, (2) percent reduction, (3) percent penetration, (4) flat penetration / lethality. The first two are *reduction* (carried by the attacker, but they lower the target’s armor for everyone); the last two are *penetration* (they only benefit you).
The crucial difference: reduction can push armor below 0 (and negative armor amplifies damage), but penetration never crosses 0 — if armor is already at 0 or less, your penetration does nothing extra. That’s why stacking penetration against a low- or no-armor target is wasted gold.
Example: 100 armor, with 30% penetration and 10 lethality. First the 30%: 100 → 70. Then flat lethality: 70 − 10 = 60 effective armor. Order matters: early reductions could turn it negative, but as penetration they bottom out at 0.
| Effective armor | Damage taken (of 1000) | |
|---|---|---|
| No penetration | 100 | 500 |
| Last Whisper (18% pen) | 82 | 549 |
| Black Cleaver (30% red.) | 70 | 588 |
| Serrated Dirk (10 pen pl.) | 90 | 526 |
Computed with the engine (effectiveResist) on a target with 100 armor. Lethality scales with level.
Lethality (flat) vs percent penetration: against whom
The rule that defines everything: lethality (flat penetration) is better against LOW-armor targets, and percent penetration is better against HIGH-armor targets. There’s a crossover point and it almost always lands around a “normal” target’s armor.
The reason is simple: subtracting 18 armor from someone with 50 (a 36% cut) matters far more than subtracting it from a tank with 300 (barely 6%). Meanwhile, 35% penetration takes the same *proportion* from 50 as from 300 — but in absolute numbers it hits the tank much harder. That’s why lethality is the assassin item that hunts squishy carries, and percent penetration is the anti-tank item.
Since patch 14.1, lethality no longer scales with level: it gives 100% at all levels (1 lethality = 1 flat penetration). And because of the floor at 0, against a no-armor target lethality is “wasted” — don’t over-invest in pure-glass comps; it’s a matter of gold efficiency. The Mitigation Lab lets you plug in the target’s armor and see the exact crossover.
Damage the target takes (out of 1000) by its armor. Flat pen does more against low-armor targets; % pen pulls ahead against heavily armored ones. That’s why you pick based on the enemy: the lines cross.
Magic penetration: the same, for mages
Everything above applies the same to magic resistance, in two flavors: flat magic penetration (like Sorcerer’s Shoes or Torch/Shadowflame-type items) and percent magic penetration (the classic Void Staff). Same logic: flat shines against low-MR targets (most carries), percent against those who stack MR.
The order is the same too (reduction → % pen → flat pen) and penetration doesn’t take MR below 0 either. A mage build usually combines some baseline flat penetration (boots) and, if the enemy builds MR, adds percent penetration only then — buying it against a no-MR team is inefficient.
In practice: check the enemy comp before pen-stacking. If it’s four squishy carries, go flat; if there are a couple of tanks or everyone buys an MR item, go percent. Test your real build against different targets in the Build Lab and Versus.
Lethality ignores a flat amount of armor (shines against squishies); percent penetration ignores a fraction (shines against tanks). Pick by who you have to kill. Tap any to see its page.
FAQ
Lethality or percent penetration against a tank?
Percent penetration. Cutting a percentage off a high armor value (300+) removes far more armor in absolute terms than flat lethality, which barely dents it. Save lethality for hunting low-armor carries.
Does lethality scale with level?
Not anymore. Since patch 14.1, 1 lethality equals 1 flat armor penetration at all levels; before, it gave less at low levels. That’s why lethality is strong from minute one today.
Can penetration make armor negative?
No. Penetration (flat or percent) bottoms out at 0: against a no-armor target it does nothing extra. What can make it negative is armor REDUCTION (Black Cleaver-type effects), which applies earlier and amplifies damage when it crosses zero.