What this calculator does
Armour ratings are not percentages, and the gap between the two is where most gearing mistakes happen. Most games turn a rating into reduction with a curve of the form armour ÷ (armour + K), which has diminishing returns in percentage terms: going from 0 to 100 armour might buy 50% reduction, while the next 100 buys only 17 more.
The surprise is that effective health does not have diminishing returns at all. Each point of armour adds the same amount to your effective pool, forever. Both statements are true at once, and which one matters depends on whether you are reading a tooltip or deciding what to wear.
The formula
Reduction is armour ÷ (armour + K), where K is the game constant that sets how much armour a 50% reduction costs. Effective HP is the health you would need with no armour to survive the same damage, which is health ÷ (1 − reduction). Those two combine and simplify to health × (1 + armour ÷ K), which is a straight line in armour. That is why the last point of armour adds exactly as much effective health as the first.
| Term | Meaning |
|---|---|
| Armour rating | The raw number on your gear, not a percentage. |
| K | The constant in the reduction curve. Armour equal to K gives exactly 50% reduction. |
| Damage reduction | The share of incoming damage removed. |
| Effective HP | The unarmoured health pool that would survive the same amount of raw damage. |
The inputs explained
| Field | What to enter |
|---|---|
| Health | Your health pool before armour is considered. |
| Armour rating | The armour rating from your character sheet. |
| Armour constant K | The game constant. Find it by looking up what armour value gives 50% reduction in your game and entering that. 100 is a common value and the default here. |
When to use it
Deciding between a health item and an armour item
Enter your current numbers, then enter them again with each item added, and compare the effective HP line. This is the comparison that matters, and it frequently disagrees with the one the tooltips invite you to make.
Reading a tooltip that says diminishing returns
The reduction percentage does have diminishing returns, and the table below shows it clearly. Effective HP does not. If what you care about is surviving longer, read the effective HP column and ignore the percentage.
Working out post-mitigation damage
Take the reduction figure and apply it to an attack to get the damage you actually take, then feed that into time to kill to see how long you last.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Does armour have diminishing returns?
Armour doubles each step. Watch the two columns behave completely differently.
| Armour | Effective HP | Damage reduction | Extra HP the armour buys |
|---|---|---|---|
| 0 | 1,000 | 0.000% | 0 |
| 25 | 1,250 | 20.0% | 250 |
| 50 | 1,500 | 33.3% | 500 |
| 100 | 2,000 | 50.0% | 1,000 |
| 200 | 3,000 | 66.7% | 2,000 |
| 400 | 5,000 | 80.0% | 4,000 |
| 800 | 9,000 | 88.9% | 8,000 |
How much does the armour constant matter?
The same armour rating in games that tune K differently.
Questions
What value of K should I use?
Look up what armour rating gives 50% reduction in your game and enter that number. If your game lists a reduction percentage for a known armour value instead, K is armour × (1 − reduction) ÷ reduction.
Why do people say armour has diminishing returns?
Because the reduction percentage does. Each point adds less reduction than the last. But effective health is linear in armour, so each point adds the same survivability. Both are true, and the second is usually the one you want.
Does this work for games with flat damage reduction?
No. If your game subtracts a flat amount per hit rather than a percentage, the result depends on the size of the hit and this formula does not apply.
What about resistances to specific damage types?
Run the calculation once per damage type with that type's rating. Effective HP against magic and against physical are separate numbers, and the lower one is usually what kills you.
The health figure here feeds straight into time to kill from the attacker's side, and DPS covers the damage going the other way.