Stacking
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Effect stacking
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 nonstacking 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. Kayle's Holy Fervor stacks up to 5 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 Kayle's Holy Fervor with 3 seconds left and has it applied again, the first stack resets to the full 5 seconds.
Some stacking effects are just counters building up to a different effect (e.g. Caitlyn's Headshot). Most effects, though, have their potency multiplied by the number of stacks.
 Example: if a unit has 5 stacks of Kayle's Holy Fervor, the unit's armor and magic resistance is decreased by 15% instead of just 3%.
Aura stacking
Auras are a special case when it comes to effect stacking. An item that produces a friendly aura effect produces two separate nonstacking effects: one on your champion (named, e.g., "Zeke's Herald Aura"), and one on allies an area around your champion (named, e.g., "Zeke's Herald"). 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 ingame values can be modified by multiple effects. It can be said that a champion statistic stacks additively or stacks multiplicatively.
Stacks additively
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
 Armor penetration, flat
 Armor reduction, flat
 Attack damage
 Attack speed (has a hard cap)
 Cooldown reduction (has a hard cap)
 Critical strike chance
 Critical strike damage
 Damage amplification
 Health
 Health regeneration
 Life steal
 Magic penetration, flat
 Magic resistance
 Magic resistance reduction, flat
 Mana
 Mana regeneration
 Spell vamp
Stacks multiplicatively
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 ingame 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.
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↓  ↑  
 = 

 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
 Attack speed reduction
 Armor penetration, percent
 Armor reduction, percent
 Damage reduction^{†}
 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.
Special stacking
Please visit the respective articles for details on these statistics stack: