Effective HP & Damage Reduction Calculator
Turn armour or resistance into damage reduction with the common game formulas — armour ÷ (armour + K), hit-size armour (PoE-style), capped linear resistance or flat reduction — then effective HP from health, shields, extra reduction layers, dodge and block, penetration, healing and incoming DPS: hits to die, the biggest hit you survive, time to die and whether armour or HP is the better buy.
How it works
Ratio: DR = A ÷ (A + K) · hit-size: DR = A ÷ (A + 5 × hit) · linear: DR = min(resistance, cap) · flat: DR = flat ÷ hit · armour after penetration = (armour − flat pen) × (1 − % pen) · share taken = (1 − DR) × (1 − other layers) × (1 − block chance × block amount) · EHP = (HP ÷ share taken + shield ÷ shield share) ÷ (1 − avoidance) · hits to die = EHP without avoidance ÷ hit · armour for a target DR = K × DR ÷ (1 − DR)
Defensive stats are only comparable once they are converted into effective health — the raw damage an enemy must deal to kill you. Many games use armour ÷ (armour + K), whose percentage looks like it tails off but which actually adds a constant amount of effective HP per point, so armour never stops paying off; some, such as Path of Exile, put the hit size in place of K, which makes armour strong against small hits and weak against big ones. Games with linear resistances cap them, making each point near the cap very valuable and every point past it worthless, and flat reduction favours tanks facing swarms of weak hits. Other layers — buffs, stances, “less damage taken” — usually multiply after armour, partial blocks shave the average hit, and shields or barriers add a separate pool in front of health. Dodge and full blocks multiply average EHP but do nothing for the hit that connects, so the calculator also finds the biggest single hit you can survive. Add healing and the attacker’s hit rate to see how long you last, and compare the EHP from one more armour point with one more health point to choose your next upgrade.
Worked example
2,500 HP with 150 armour under a K = 100 formula: 60% reduction, so each hit does 40% and effective HP is 6,250 — 6,944 with 10% dodge. A 300-damage hit lands 20.8 times before death, and the biggest single hit you survive is 6,250. Here +1 armour is worth 27.8 EHP against 2.8 for +1 HP, so armour is the far better point-for-point buy. Taking one 300 hit per second with 10% dodge, you last about 23 seconds. Reaching 75% reduction would need 300 armour.
Frequently asked questions
Why does 60% → 70% reduction matter more than 10% → 20%?
Because damage taken drops from 40% to 30% — a 25% cut in damage — versus 90% to 80%, an 11% cut. Think in “damage taken” or effective HP, not in reduction percentage.
How does armour penetration interact?
Flat penetration is subtracted from armour before the formula; percentage penetration then removes a share of what is left. Order differs by game — League of Legends applies percentage penetration before flat lethality — and below zero armour many games deal bonus damage instead.
How do I stack several damage reduction buffs?
Most games multiply them: 10% and 20% reduction leave 0.9 × 0.8 = 72% of the damage, a combined 28%, not 30%. Enter the combined figure in “Other damage reduction layers” under More options.
Why does my armour feel weaker against bosses?
With the hit-size formula used by games like Path of Exile, armour’s reduction depends on the hit: 25,000 armour stops 50% of a 5,000 hit but only 20% of a 20,000 slam. The damage-by-hit-size table shows this for your numbers.
What is effective HP (EHP)?
The raw damage you can absorb before dying once armour, resistances and avoidance are counted. With 1,000 HP and 50% reduction, your EHP is 2,000.
Related calculators
Generic game-mechanics maths; individual games apply their own rounding, caps, diminishing returns and hidden modifiers, so use this to compare builds rather than as an exact in-game value.