Effective HP Calculator (Armor and Damage Reduction)
Convert health plus armor or resistance into effective HP, the raw damage it takes to kill you, using either a percentage damage reduction or an armor rating with a constant (the League-style 100 ÷ (100 + armor) curve), with an extra multiplicative reduction on top.
Last updated: October 2026
Effective HP
1,500 HP
Formula below · 2 sources (wiki.leagueoflegends.com, maxroll.gg) · Updated Oct 2026
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About this calculator
Effective HP (EHP) is the amount of raw, pre-mitigation damage it takes to kill a character: health divided by the fraction of damage that gets through. It is the number that lets you compare a health item with an armor item, or a tank with a lot of health against one with a lot of resistance, because both end up in the same unit.
Percentage reduction
If a defence blocks a fixed share of damage, DR, then every raw hit does (1 − DR) of its damage, and EHP = HP ÷ (1 − DR). 30% reduction on 1,000 HP gives 1,429 EHP; 50% doubles it to 2,000; 75% quadruples it. Each extra percentage point of reduction is worth more than the last, which is why games cap percentage reductions or hand them out through a diminishing curve.
Armor with a constant
Most RPGs and MOBAs turn an armor rating into a reduction with DR = armor ÷ (armor + K). In League of Legends K is 100, so damage taken is multiplied by 100 ÷ (100 + armor) and EHP = HP × (1 + armor ÷ 100): every point of armor adds 1% of health as effective health, with no diminishing returns in EHP terms even though the percentage reduction flattens out. Other games use a different K (and some scale K with the attacker's level or hit size); enter the constant your game uses and the same formula applies.
Stacking layers
Separate reductions multiply: a 20% damage-reduction buff on top of 50 armor leaves 0.667 × 0.8 = 53.3% of damage, not 100% − 33% − 20%. The calculator applies the extra layer multiplicatively, which is how almost every game stacks a buff, a shield or a second resistance on top of armor. Flat shields and bonus health are simpler: add them to health. Keep in mind that EHP is per damage type: physical armor does nothing against magic or true damage, and in a mixed-damage fight the right comparison is a weighted EHP across the damage you expect to take.
How to use
Example: a champion with 1,000 health and 50 armor in a game with K = 100. Damage reduction = 50 ÷ (50 + 100) = 33.3%, so each hit does two thirds of its damage and EHP = 1,000 ÷ 0.667 = 1,000 × (1 + 50 ÷ 100) = 1,500 against physical damage. Those are the defaults. With 120 armor it is 1,000 × 2.2 = 2,200. In the percentage model, 1,000 HP with 30% damage reduction is 1,000 ÷ 0.7 = 1,428.6 EHP. Add a 20% multiplicative reduction buff to the 50-armor champion: damage taken is 0.667 × 0.8 = 0.533, EHP = 1,000 ÷ 0.533 = 1,875, and the total reduction is 46.7%, not 53.3%.
Frequently asked questions
What is effective HP?
The raw damage needed to kill you: health ÷ (1 − damage reduction). It puts health and armor in one unit so items and builds can be compared. 1,000 HP with 50% reduction is 2,000 EHP, the same as 2,000 HP with no armor against that damage type.
How does armor convert to damage reduction?
In the common model, reduction = armor ÷ (armor + K). With K = 100 (League of Legends), 50 armor is 33.3% reduction, 100 armor 50%, 300 armor 75%. The percentage has diminishing returns but EHP does not: EHP = HP × (1 + armor ÷ K), so every armor point adds HP ÷ K of effective health, 10 EHP per point at 1,000 HP.
Is health or armor better?
EHP from health is HP × (1 + armor ÷ K), and from armor it is the same product, so the better buy is the one you have less of relative to the other: with 1,000 HP and 50 armor, 100 HP adds 150 EHP (1 + 0.5) and 10 armor adds 100 EHP (1,000 ÷ 100 × 10). Health also works against every damage type, armor only against the type it resists. Compare the EHP of each option with the calculator.
How do two reductions stack?
Multiplicatively in most games: the damage that gets through is (1 − DR1) × (1 − DR2). 33.3% from armor and 20% from a buff leave 0.667 × 0.8 = 53.3% of the damage, a 46.7% total reduction. Adding the percentages (53.3%) overstates the defence; the extra reduction box in the calculator applies the correct product.
What about shields and flat damage reduction?
A shield or a bonus-health effect is extra health: add it to the health box. Flat reduction per hit (minus 10 damage per attack) depends on hit size and cannot be expressed as a single EHP; against hits of size H it is a reduction of 10 ÷ H, which you can enter in the percentage model for the hit size you expect.
Does armor penetration change the result?
Yes, from the attacker's side: flat penetration subtracts from the armor rating and percentage penetration multiplies it, before the armor ÷ (armor + K) formula is applied. To see your EHP against a specific attacker, enter your armor after their penetration.
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