Champion abilities, summoner spells, items, and buffs often apply effects to characters. Simply put, an effect either stacks or doesn't stack, though aura effects are an exception (see § Aura stacking).
When a non-stacking effect is applied on a target that already has the effect:
- There is still only one effect.
- The duration of the effect is reset to whatever the duration is of the most recent application.
When a stacking effect is applied on a target that already has the effect:
- The number of stacks of that effect increases by one (unless the maximum number of stacks has been reached; e.g. Soraka's Starcall stacks up to 10 times).
- The duration of each stack is reset to the duration of the most recent application.
- This happens regardless of whether the maximum number of stacks has been reached.
Specifically note that each stack does not have a separate duration; each application of an effect resets the duration of all stacks.
- Example: if a unit has 1 stack of Soraka's Starcall with 3 seconds left and has it applied again, the first stack resets to the full 5 seconds.
- Example: if a unit has 10 stacks of max-level Starcall, the unit's magic resistance is decreased by 120 instead of just 12.
Auras are a special case when it comes to effect stacking. An item that produces a friendly aura effect produces two separate non-stacking effects: one on your champion (named, e.g., "Zeke's Herald"[confirm?]), and one on allies an area around your champion (named, e.g., "Zeke's Herald Aura"[confirm?]). These are two separate effects despite doing the same thing, and can therefore stack with each other. Thus, you can double the "aura" effect listed for the item by getting the item and also being affected by an ally's aura; duplicates of an aura item on your champion do not stack, and duplicates of an aura from multiple allied champions do not stack.
Champion statistic stacking
Some in-game values can be modified by multiple effects. It can be said that a champion statistic stacks additively or stacks multiplicatively.
When effects stack additively, the cumulative effect can be calculated simply by adding together each effect.
- Example: if your champion has two items granting +100 health each, your champion has 200 bonus health.
Effects that stack additively
- Ability power
- Armor penetration, flat
- Armor reduction, flat
- Attack damage
- Attack speed (has a hard cap)
- Cooldown reduction (has a hard cap)
- Critical strike chance (has a hard cap)
- Critical strike damage
- Health regeneration
- Life steal
- Magic penetration, flat
- Magic resistance
- Magic resistance reduction, flat
- Mana regeneration
- Spell vamp
- Damage amplification
When effects stack multiplicatively, the cumulative effect can be calculated by multiplying the effects together. Note that the game interface rounds champion statistics, so what is shown in-game may be slightly different than what is calculated.
It is important to understand that most champion statistics that stack multiplicatively are displayed as the opposite of the actual numbers that are multiplied. For example, 10% damage reduction should instead be understood as 90% damage taken, and 25% damage reduction should instead be understood as 75% damage.
- Example: If your champion has armor granting 10% damage reduction and an item granting 10% damage reduction, you multiply the damage factors—0.9 and 0.9—to get 0.81, which means your champion has 19% damage reduction (against physical damage).
Effects that stack multiplicatively
- Damage reduction†
- Attack speed reduction
- Armor penetration, percent
- Armor reduction, percent
- Magic penetration, percent
- Magic resistance reduction, percent
- Crowd Control Reduction
- Note that all sourced from items will not stack as they share the same named item effect.
† Armor and magic resistance each contribute a single percentage value to the larger category of "damage reduction." Calculate the final percentage of each before combining with other sources of damage reduction.
Please visit the respective articles for details on these statistics stack: