League of Legends Damage Calculator

Two players can build the exact same amount of raw damage and still hit a target for wildly different numbers, because League of Legends never applies raw damage directly. Every physical and magic hit gets run through the target's armor or magic resist first, and penetration stats change that math in ways that are easy to misjudge by eye. This calculator plugs in Riot's own resistance formula, along with the real lethality-to-armor-penetration scaling curve, so you can see the mitigated number before you commit to a build.

The Actual Formula

League's damage mitigation is a single, well-documented formula, and it is the same shape for armor against physical damage and magic resist against magic damage. When effective resistance is zero or positive:

Mitigated Damage = Raw Damage × 100 / (100 + Effective Resistance)

At 100 armor, that formula reduces incoming physical damage by exactly 50%. At 200 armor, it is 66.7%, not 100%, because the curve has diminishing returns built into it on purpose, doubling a stat never doubles its effect. If a target's effective resistance is pushed below zero (which does happen against heavy penetration or resistance-shred effects), the formula flips to Mitigated Damage = Raw Damage × (2 − 100 / (100 − Effective Resistance)), which is how negative armor starts amplifying damage past 100% instead of just failing to reduce it. This calculator floors effective resistance at zero rather than modeling the amplification branch, since pushing a target's resistance negative from penetration alone is rare outside a handful of specific shred effects.

Why Lethality Is the Only Flat Pen Stat

This calculator has a single flat-penetration input for physical damage, Lethality, and that is not a simplification, it matches how the game actually works today. Riot retired old-style flat armor penetration back in the 2017 armor pen rework and replaced it outright with lethality on every item and rune that used to grant it. There is no separate flat armor pen stat left to enter, lethality is the modern name for it. Where lethality differs from the old flat stat is the level scaling attached to it, using the formula Flat Armor Pen from Lethality = Lethality × (0.6 + 0.4 × Level / 18). At level 1, a rune or item granting 18 lethality only converts to about 11 flat armor penetration. At level 18, that same 18 lethality converts to the full 18. Riot built that curve in specifically so early-game burst didn't scale as hard against squishy targets who have little armor to begin with, this calculator's level field exists because ignoring that scaling badly overstates early-game damage and badly understates it if you are theorycrafting a level 18 teamfight.

Percent armor penetration (from items like Last Whisper-line components) works differently again, it is a multiplier applied to the target's current armor rather than a flat subtraction, which is why it scales better against high-armor tanks than lethality does. Stacking both types is common and intentional in real builds, an assassin might run lethality for the early-game burst window and pick up a percent-pen item later for scaling into tankier targets.

Why Order of Operations Matters

The client resolves resistance-modifying effects in a fixed order: percent-based resistance reduction first (rare shred effects, not modeled in this simple version), then percent penetration, then flat penetration last, with the result floored at zero before the final mitigation formula runs. That order is not cosmetic, it changes the answer. Applying a flat penetration value before a percent penetration value against the same target produces a different effective resistance than applying them in the reverse order, because percent penetration multiplies whatever armor is still present, and flat penetration only has as much armor left to subtract from as the percent step left behind. This calculator applies percent penetration to the target's base resistance first, then subtracts flat penetration (including the lethality conversion) from what remains, matching how the game actually computes it rather than averaging the two operations together.

A Worked Example

Say you are dealing 300 raw physical damage into a target sitting at 100 armor, with no penetration at all. Effective armor stays at 100, so mitigated damage is 300 × 100 / (100 + 100) = 150. Now add 30% armor penetration: base armor of 100 drops to 70 after the percent step. Add 18 lethality at level 18 on top of that (a full 18 flat armor pen at max level), and effective armor falls to 52. Mitigated damage becomes 300 × 100 / (100 + 52) = 197.4, a jump of nearly 47 damage over the no-penetration case, purely from stats that never touched your raw damage number at all. That gap is exactly what this calculator's comparison row is built to show, since it is easy to undervalue penetration stats when you are only looking at an item's raw AD or AP line.

How to Use This Calculator

Pick Physical or Magic depending on the hit you are checking, enter the raw damage number your ability or auto-attack deals before mitigation (this is the number you would read off an ability tooltip at your current rank and stats), then fill in whichever penetration stats you actually have. For physical damage, set your champion level too, since it directly changes how much of your lethality converts. Leave a field at its default of zero if you are not running that stat, the calculator treats zero penetration as a clean baseline so you can compare builds by changing one field at a time and watching the mitigated number move.

Frequently Asked Questions