Penetration Calculator

Armor penetration is a core offensive mechanic in MOBAs, action RPGs, and many other game genres that feature a physical defense (armor) system. When you deal physical damage to an enemy, their armor stat reduces the amount of damage they actually receive. Armor penetration works against that reduction, allowing more of your raw damage to land.

In most games that use this system, armor reduces incoming physical damage by a percentage determined by the formula 100 / (100 + Armor). A target with 100 armor takes only 50% of incoming physical damage. A target with 300 armor takes only 25%. Because armor's diminishing return curve makes high-armor stacking increasingly cost-inefficient, penetration items become disproportionately valuable against such targets.

There are two primary forms of armor penetration: flat (additive) penetration and percent (multiplicative) penetration. Most games also feature lethality, a scaling variant of flat penetration that becomes stronger as the attacker reaches higher character levels. Understanding how these three types interact — and in what order they are applied — is critical to optimizing your itemization in any game that uses them.

This armor penetration calculator implements the widely-used MOBA application order: percent penetration is applied first to the raw armor value, then flat penetration and scaled lethality are subtracted from the result. The final effective armor is clamped at zero so negative armor scenarios (if the game supports them) are handled separately. Once effective armor is determined, the standard damage multiplier formula produces the actual damage dealt.

The calculator uses the following sequence of formulas to compute every output value. Each step builds on the previous, so the order of operations matters significantly.

Step 1 — Scale Lethality: Lethality is a level-gated form of flat penetration. At level 1 it provides 60% of its face value; at the max level of 18 it provides the full 100%. The scaling factor is 0.6 + 0.4 × (level / 18).

Step 2 — Apply Percent Penetration: Percent penetration is always applied to the raw armor value before any flat subtraction. This makes it stronger against heavily-armored targets than flat pen.

Step 3 — Subtract Flat Pen and Lethality: After percent pen reduces the armor, flat penetration and effective lethality are subtracted. The result is clamped at zero.

Step 4 — Calculate Damage Multiplier: The standard formula 100 / (100 + effectiveArmor) converts effective armor into the fraction of base damage that actually lands.

The choice between flat and percent armor penetration depends almost entirely on the amount of armor your target has stacked. Understanding the crossover point helps you make smarter itemization decisions during any match.

Flat penetration subtracts a fixed number from the target's effective armor after percent pen has already been applied. Because the armor-to-mitigation curve is nonlinear, each point of armor reduction is worth more when the target has low armor. Reducing a target from 50 armor to 0 (removing 33% mitigation) is far more impactful than reducing them from 400 armor to 350 (barely moving the needle). Flat pen items are therefore most efficient against targets with 100–200 armor.

Percent penetration shines against high-armor targets. Because it acts multiplicatively on the raw armor value before flat reductions, a 30% pen source always removes 30% of whatever armor the target has, regardless of how high it is. Against a 600-armor tank, 30% pen removes 180 armor — far more than any flat pen item. However, percent pen has diminishing returns when stacked; two 20% sources don't combine to 40% because each applies to the post-previous-pen armor value.

In practice, the optimal strategy is usually to take one percent pen source (often a keystone ability or one item) and then fill remaining item slots with flat pen or lethality to compound the penetration. This multiplicative-then-subtractive layering is exactly what this calculator models.

Lethality occupies a middle ground. At maximum level it equals its face value in flat pen, but at early levels it underperforms. Champions who reach high levels quickly or who deal the majority of their damage in late-game team fights benefit more from lethality items than those who rely on early-game burst.

The table below shows how different armor values translate into damage mitigation percentages, and how much damage a 1000 base-damage ability deals both without penetration and with a typical 30% percent pen + 50 flat pen setup (using the calculator's formula at level 18).

Notice how the damage dealt with penetration remains substantially higher at every armor tier. Against a 50-armor squishy target, 30% percent pen + 50 flat pen completely nullifies their armor, dealing full base damage. Against a 500-armor tank the same loadout still increases damage output by 50% compared to zero pen.

This tool is designed to help you evaluate penetration builds quickly before or during a match. Here is how to get the most accurate results:

1. Target Armor: Enter the enemy's current armor value. Check the item shop's target information panel or look up the champion's base armor + item totals. Remember that some targets may have armor buffs from their own items or abilities that temporarily inflate this number.
2. Base Damage: Enter the raw pre-mitigation physical damage of the ability or auto-attack you are evaluating. You can find this in the champion's ability tooltip or spell data sheets.
3. Flat Penetration: Total flat armor penetration from all your items and runes combined. Do not include lethality here — use the dedicated Lethality field instead.
4. Percent Penetration: Enter your total percent armor pen as a number (e.g., type "30" for 30%). If you have two percent pen sources, apply them sequentially in your head: 1 − (1 − 0.20) × (1 − 0.15) ≈ 32%, then enter 32.
5. Lethality: Enter the total lethality stat shown on your champion's stat sheet. The calculator automatically scales it by your level.
6. Player Level: Your current level. This only affects lethality scaling; if you have no lethality items, level has no impact on the output.

The results panel shows effective armor after all reductions, the final damage dealt, and a side-by-side comparison against what you would deal with zero penetration. Use the Damage Increase and Percent Increase values to quickly compare two different build paths.

A useful workflow is to save the default values for your current build, then swap individual items and re-check the output. If switching from a 50-flat-pen item to a 30%-pen item raises your damage dealt against the specific target you are fighting, the item swap is worthwhile.

The penetration mechanics modeled by this calculator are based on common MOBA conventions, but similar systems appear across a wide range of titles. Understanding the shared design patterns makes it easier to adapt the calculator to whichever game you are playing.

League of Legends uses exactly the formula in this calculator. Lethality scales with attacker level using the coefficient 0.6 + 0.4 × level / 18. Void Staff and similar items provide percent armor penetration (also called "magic penetration" for ability power builds). The application order — percent first, flat second — is official Riot game design.

Dota 2 uses a conceptually similar but mechanically distinct system. Physical resistance (analogous to armor) is computed from an armor value using a different coefficient, and armor reduction (negative armor application) is handled differently. If you are using this calculator for Dota 2, the results are approximate rather than exact.

Path of Exile uses flat armor that reduces physical hits according to a formula heavily weighted toward large hits, making it less comparable. Penetration in PoE applies as a separate reduction layer for elemental damage.

Diablo series games use armor as a flat damage reduction percentage capped at a maximum value, which differs significantly from the unbounded formula used here.

For the most accurate results, verify your target game's exact armor formula and application order, then compare it against the formulas described in the Penetration Formulas section of this page. For League of Legends the results are exact; for other games, treat the output as a strong approximation.