Subtractive ( DMG − DEF , clamped to ≥ 1) — the simplest. Used in classic Dragon Quest / Final Fantasy. Problem: at high DEF the damage hits the clamp and combat becomes binary (you either can damage or you can't). Easy to break with armor scaling. Divisive ( DMG × 100 / (100 + DEF) ) — the default in modern action RPGs (League of Legends, Path of Exile uses a variant). Damage smoothly decreases as DEF rises, never reaches zero, scales predictably. DEF = 100 means 50% damage taken; DEF = 200 means 33%. Percent resist ( DMG × (1 − resist%) ) — common for elemental damage. Resist 30% = take 70% damage. Cap resist at 75% or 80% to avoid invincibility. Custom JS expression — for game-specific formulas. Variables in scope: atk , wpn , def (effective, post-pen), res , pen , armor , plus Math . Expression is evaluated with new Function , sandboxed to those names.

Variance is on weapon damage only. ATK and DEF don't roll. This matches most games — weapon damage is the "physical roll", stats are the static modifiers. Armor pen reduces effective DEF, not damage. The order is: effective DEF = max(0, DEF + flat armor − pen), then the formula. This is the standard ordering — applying pen after the formula breaks most balance. Resistance is multiplicative. Two 30% resists on the same damage type don't stack to 60%; they stack to (1 − 0.7×0.7) = 51%. For the tool, enter your total effective resist, not the sum of sources. Crit multiplier is total, not bonus. A 2× crit means 200% of normal damage, not 100% + 100%. If your game uses bonus stacking, convert before entering. DPS is a paper number. Real combat has cooldowns, dodges, positioning, animation lock, and resource costs. DPS is a starting point for balance, not the destination. Custom expressions evaluate live. Don't reference DOM, globals, or storage. The sandbox is just Math + the named variables. Errors show inline.

Damage Calculator

Combat damage formula tester for RPGs. Attack vs defense, crit chance, resistances, multipliers, armor pen. See expected damage range + crit-adjusted average + damage-per-second. For game balance tuning.

Formula

⚔️ Attacker

ATK stat

Weapon damage

Variance %

Crit chance %

Crit multiplier

Armor pen (flat)

🛡 Defender

DEF stat

Resistance %

Flat armor

Combat speed

Attacks per second

Compare with config B

Result

What is this for?

Combat in an RPG is a formula that takes attacker stats and defender stats and produces a number — but which formula? Subtractive, divisive, percent-resist, and custom hybrids all behave differently as numbers scale. A formula that feels fine at level 1 with ATK 10 vs DEF 5 can produce one-shot kills at level 50 with ATK 500 vs DEF 50. This tool lets you plug in the formula and the stats and immediately see expected damage, min/max range, the crit-adjusted average, and DPS at a given attack speed — so you can sanity-check that your level-50 fight isn't accidentally a one-shot.

The formulas

Subtractive (DMG − DEF, clamped to ≥ 1) — the simplest. Used in classic Dragon Quest / Final Fantasy. Problem: at high DEF the damage hits the clamp and combat becomes binary (you either can damage or you can't). Easy to break with armor scaling.
Divisive (DMG × 100 / (100 + DEF)) — the default in modern action RPGs (League of Legends, Path of Exile uses a variant). Damage smoothly decreases as DEF rises, never reaches zero, scales predictably. DEF = 100 means 50% damage taken; DEF = 200 means 33%.
Percent resist (DMG × (1 − resist%)) — common for elemental damage. Resist 30% = take 70% damage. Cap resist at 75% or 80% to avoid invincibility.
Custom JS expression — for game-specific formulas. Variables in scope: atk, wpn, def (effective, post-pen), res, pen, armor, plus Math. Expression is evaluated with new Function, sandboxed to those names.

What the numbers mean

Expected damage — the formula output at neutral variance. The single-number summary.
Range — applies the variance % to weapon damage. ±15% is a common default.
Crit-adjusted average — expected damage × (1 + critChance × (critMult − 1)). The number that matters for sustained DPS, not just a single hit.
DPS — crit-adjusted average × attacks per second. Compare two builds at this number to decide which is "stronger" on paper.

Common gotchas

Variance is on weapon damage only. ATK and DEF don't roll. This matches most games — weapon damage is the "physical roll", stats are the static modifiers.
Armor pen reduces effective DEF, not damage. The order is: effective DEF = max(0, DEF + flat armor − pen), then the formula. This is the standard ordering — applying pen after the formula breaks most balance.
Resistance is multiplicative. Two 30% resists on the same damage type don't stack to 60%; they stack to (1 − 0.7×0.7) = 51%. For the tool, enter your total effective resist, not the sum of sources.
Crit multiplier is total, not bonus. A 2× crit means 200% of normal damage, not 100% + 100%. If your game uses bonus stacking, convert before entering.
DPS is a paper number. Real combat has cooldowns, dodges, positioning, animation lock, and resource costs. DPS is a starting point for balance, not the destination.
Custom expressions evaluate live. Don't reference DOM, globals, or storage. The sandbox is just Math + the named variables. Errors show inline.