Effect stacking
Champion abilities, summoner spells, items, and buffs often apply effects to characters. Different effects always stack with each other. However, if the same effect is applied several times to the same unit, sometimes it stacks and sometimes it doesn't.
When a nonstacking effect is applied to a target that already has the effect the duration of the effect is reset to the duration of the most recent application.
When a stacking effect is applied to 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. For example, stacks up to 3 times.
 The duration of all stacks is reset to the duration of the most recent application.
Example: if Jinx has 1 stack of
with 1 second left and gets another, the duration of the first stack resets to the full 2.5 seconds.Some stacking effects are just counters building up to a different effect (e.g.
). Most effects, though, have their potency multiplied by the number of stacks. Example: if Ezreal has 5 stacks of , his attack speed is increased by 50% rather than 10%..
Aura stacking
Aura effects don't stack with themselves: If champion A and champion B are generating the same aura, and champion C is affected by both auras, champion C receives the effect of these auras only once. However, for the purposes of stacking, if the aura has two different effects based on if the target is the user or somebody else, then the auras stack with each other. A unit receives the aura effect from the aura they are creating, but also the effect from the aura created by their ally.
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
 Slow resist
 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.
 ⇨ 
 
↓  ↑  
 = 

 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: