RPG Damage Reduction from Armor Calculator

Understanding the RPG armor damage reduction calculator — concept and principle

Many RPGs and MOBAs, League of Legends among them, reduce incoming physical damage with a formula of the form armor ÷ (armor + K). This calculator applies percentage and flat armor penetration, then shows the effective armor, the damage reduction, the damage actually taken from a hit and the effective HP multiplier, so you can compare defensive and offensive item choices.

Percent penetration first, then flat; the remaining 60 armor blocks 37.5% of each hit.

How it is calculated

Effective armor = max(0, armor × (1 − % pen) − flat pen). Percentage penetration is applied first, then flat penetration, as in League of Legends.
Damage reduction = effective armor ÷ (effective armor + K).
Damage taken = raw damage × K ÷ (K + effective armor).
Effective HP multiplier = 1 + effective armor ÷ K.

K is the game's constant (100 in League of Legends). Negative armor is clamped to zero here.

The principle

The formula is designed so that each point of armor adds the same amount of effective health. Damage taken is multiplied by K ÷ (K + armor), so the hits needed to kill you grow by the reciprocal, (K + armor) ÷ K = 1 + armor ÷ K. With K = 100, every 100 armor lets you survive 100% more physical damage. In percentage terms the reduction shows diminishing returns (50% at 100 armor, 66.7% at 200), but in survivability the value is linear, which keeps armor from ever reaching 100% reduction.

Worked example

Inputs: armor 100, K = 100, flat pen 10, 30% pen, raw damage 500.
Effective armor = 100 × (1 − 0.30) − 10 = 70 − 10 = 60.
Damage reduction = 60 ÷ (60 + 100) = 37.5%.
Damage taken = 500 × 100 ÷ 160 = 312.5.
EHP multiplier = 1 + 60 ÷ 100 = 1.6.

Each point is the calculator re-run with only “Armor” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Armor	Effective armor
25	7.5
50	25
75	42.5
100	60
125	77.5
150	95
200	130

How to read the result

Compare the multiplier with the HP you could buy instead: if 1.6 × your HP is more than what extra health would give, armor is the better defense against physical damage. For attackers, percentage penetration is strongest against high-armor targets, while flat penetration matters most against low-armor targets, because subtracting 10 from 20 armor removes half of it.

Common mistakes

Adding reductions as percentages; two sources of 30% reduction are not 60%.
Applying flat penetration before percentage penetration in a game that uses the opposite order.
Using K = 100 for games with a different constant or a level-scaled formula.

Keep in mind: The order of penetration, the constant and the handling of negative armor differ between games and patches. In League of Legends negative armor increases damage taken, which this calculator does not model, so check the current game rules.

Frequently asked questions

Why does armor never reach 100% reduction?

Reduction = armor ÷ (armor + K) approaches 1 only as armor grows without limit.

Is armor better than health?

It depends on your current values; armor scales with your HP pool and HP scales with your armor, so a balanced mix gives the most effective HP per gold.

Does magic resist use the same formula?

In League of Legends it does, with magic penetration applied in the same way.

Sources: League of Legends Wiki, "Armor" and "Armor penetration" articles

Checked 8 Nov 2026. Official league / federation formulas where they exist; widely used community formulas otherwise. For fun and analysis — not betting advice.

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