Item passive stacking describes the interactions between multiple items that have similar effects. The stacking of effects and items falls into different categories, and each category behaves differently.
Effects stack independently if they stack additively. This is the simplest and most intuitive type of stacking: multiple effects or attributes will add to each other.
- at level 25 with a has:
- Armor = base armor + bonus armor = 15.2 + 10 = 25.2 armor
Effects that stack independently
- Bonus attack damage
- Bonus attributes (strength/agility/intelligence)
- Bonus mana/health (static amount)
- Bonus health regeneration/mana regeneration
- Bonus attack speed
- Bonus armor
- Mana Break (with the exception for Anti-Mage's mana break which does not stack with that of Diffusal Blade)
- Movement Speed bonuses from different categories (some items fall in the same category and therefore only the highest value applies, for example: Boots)
- Cleave damage (with the particularity that the damage from multiple instances of cleave is not actually added: each instance deals independent damage)
Values that are stacked in a multiplicative manner have diminishing returns. The stacking effect can be calculated with the formula:
Total stack = 1 − ((1 − first instance) × (1 - second instance) × (1 - third instance) × ...) where each instance is a percentage.
- with level 4 and a has a magic resistance of:
- 1 - (1 - 0.25) × (1 - 0.45) × (1 - 0.15) = 64.9375%
Effects that stack multiplicatively
These effects stack multiplicatively, but when two instances of the same effect happen at the same time, only the highest value is kept or the duration is refreshed.
Critical strike procs are rolled from the strongest to weakest multiplier, and stop rolling once proc occurs, so that two critical strikes cannot proc on the same attack, resulting in a diminishing behavior.
For exampleholding a (230%) cannot crit when using (350%).
Only one instance of damage block can proc at a time. The highest damage block value has priority.
The strength and damage components of multiples and s will independently stack, but the bash chance will not.
The attack speed and damage components of multiples and s will independently stack, but the chain lightning chance will not.
Auras provided by items do not stack with themselves, since they place a buff/debuff on the unit. Only the aura with the highest value will work.