Stacking

Some in-game values can be modified by multiple effects. It can be said that an effect does not stack, stacks additively, or stacks multiplicatively.

Does not stack
When effects do not stack, the most powerful effect of that type 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.

Effects that do not stack

 * Tenacity
 * Many champion abilities (such as )

Stacks additively
When effects stack additively, the cumulative effect can be calculated simply by adding together each effect.
 * Example: if your champion has one item granting +10 attack damage and another item granting +15 attack damage, your champion has 25 bonus attack damage.

Effects that stack additively

 * Ability power
 * Armor
 * Armor penetration (with caveats; see page for more information)
 * Attack damage
 * Attack speed (has a hard cap)
 * Critical hit chance (has a hard cap)
 * Health
 * Lifesteal
 * Magic penetration (with caveats; see page for more information)
 * Magic resistance
 * Movement speed (has a soft cap)
 * Slows (effects other than the strongest slow are halved)
 * Spell vamp
 * Many champion abilities (such as )

Stacks multiplicatively
When effects stack multiplicatively, the cumulative effect can be calculated by multiplying the effects together. The trick here is that the numbers you multiply aren't always the numbers the game shows.
 * Example: if your champion has an item granting 12% dodge chance and runes granting 10% dodge chance, you do not multiply 12% and 10%, but instead multiply the hit chances&mdash;0.88 and 0.9&mdash;to get 0.792, which means your champion has 20.8% dodge chance.

Effects that stack multiplicatively

 * Damage reduction (armor and magic resistance are single effects
 * Armor reduction
 * Magic resistance reduction
 * Dodge

Aura stacking
Auras are a special case when it comes to 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.