Stacking

Stacking refers to two different things, depending on the context. Effect stacking refers to a single effect being applied more than once on a target. Champion statistic stacking refers to how champion statistics behave when modified by multiple effects.

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 below).

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. stacks up to 10 times).
 * The duration of the effect is reset to whatever the duration is of the most recent application.
 * This happens regardless of whether the maximum number of stacks has been reached.

Note that multiple stacks of an effect are not the same as multiple effects. Even if an effect has multiple stacks, the entire effect has a single duration; each stack is not kept track of separately.
 * Example: if a character has 1 stack of with 3 seconds left and has it applied again, the result is &minus;16 magic resistance for the full 8 seconds; the result isn't reduced in 3 seconds as if the first effect expired.

When an effect has more than one stack, the effect is applied that many times.
 * Example: if a target has 10 stacks of, the target's magic resistance is decreased by 8 a total of 10 times, resulting in &minus;80 magic resistance.

Aura stacks
Auras are a special case when it comes to effect stacking. You can get two stacks from the same aura, but only when one comes from your champion and one comes from an allied champion; duplicates of an aura item on your champion do not stack, and multiple allied champions with the same aura do not stack.

Champion statistic stacking
Some in-game values can be modified by multiple effects. It can be said that a champion statistic does not stack, stacks additively, or stacks multiplicatively.

Does not stack
When a champion statistic does not stack, the most powerful relevant effect applying to that statistic is the one that takes effect.
 * Example: if your champion has one item granting +25 tenacity and another item granting +35 tenacity, your champion has 35 tenacity.

Champion statistics that do not stack

 * Tenacity

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
 * Attack damage
 * Attack speed (has a hard cap)
 * Critical strike chance (has a hard cap)
 * Health
 * Life steal
 * Magic resistance
 * 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 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% dodge should instead be understood as 90% hit chance, and 25% damage reduction should instead be understood as 75% damage.


 * Example: If your champion has two items granting +10% dodge chance each (90% hit chance each), you multiply the hit chances&mdash;0.9 and 0.9&mdash;to get 0.81, which means your champion has a 19% dodge chance.

Effects that stack multiplicatively

 * Damage reduction1
 * Dodge

1 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.

Effects with special stacking type
Please visit the respective articles to understand how the stacking of those effects work:
 * Armor penetration
 * Armor reduction
 * Magic penetration
 * Magic resistance reduction
 * Movement speed