Armor penetration

Armor penetration is a champion statistic that allows the champion to ignore some or all of a target's armor. All champions have 0 base armor penetration but can increase it with items, runes, masteries and some champion abilities. This cannot affect enemy armor beyond 0 to deal bonus damage.

Armor reduction is the effect of some items and champion abilities that temporarily reduces the armor of one or more targets. This allows allies to benefit from the reduction of armor. This can reduce armor below 0, resulting in bonus damage (see Armor).

Order of calculations
When attacking, armor penetration and armor reduction are considered on the target champion in the following order:
 * 1) Flat armor reduction
 * 2) Percentage armor reduction
 * 3) Flat armor penetration
 * 4) Percentage armor penetration

Flat armor reduction
The target's armor is reduced by an amount. Flat armor reduction stacks additively.
 * Example: 20 armor reduction reduces the target's armor by 20, so a target with 30 armor will be reduced to 10 armor. (302010) 

Flat armor reduction can reduce a target's armor below zero. For example, if an enemy with 10 armor has their armor reduced by 25, the enemy will have &minus;15 armor (causing him or her to take extra damage; see Armor).

Percentage armor reduction
The target's armor is multiplied by a percentage (100%the listed reduction). Percentage armor reduction stacks multiplicatively. Percentage armor reduction is ignored if the target's armor is 0 or less.
 * Example: 20% armor reduction multiplies the target's armor by 80%, so a target with 30 armor will be reduced to 24 armor. (300.824) 

Percentage armor reduction makes a bigger difference on targets with higher armor. For instance, with 40% armor reduction, a target with 200 armor will lose 80 while a target with only 50 armor will lose 20.

Flat armor penetration
The target's armor is treated as being reduced by an amount for purposes of damage calculation, but cannot be reduced below 0. Flat armor penetration stacks additively.
 * Example: 20 armor penetration causes the target to take damage as if its armor was 20 less, so a target with 30 armor will be treated as though it had 10 for purposes of damage calculation (the target's actual armor value will not change, however).

Because the target's actual armor value does not change, anything else depending on that value also doesn't change (e.g. ).

Percentage armor penetration
The target's armor is treated as being multiplied by a percentage for purposes of damage calculation (100%the listed penetration). Percentage armor reduction stacks multiplicatively. Percentage armor penetration is ignored if the target's armor is less than 0.
 * Example: 20% armor penetration causes the target to take damage as if its armor was multiplied by 80%, so a target with 30 armor will be treated as though it had 24 for purposes of damage calculation (the target's actual armor value will not change, however).

Because the target's actual armor value does not change, anything else depending on that value also doesn't change (e.g. ).

Percentage armor penetration makes a bigger difference on targets with higher armor. For instance, with 40% armor penetration, a target with 200 armor will be considered as having 80 less while a target with only 50 armor will be considered as having 20 less.

Examples
Given 20 flat armor reduction and 30% armor reduction, and the target is affected by 10 flat armor penetration and 40% armor penetration,
 * Target A has 80 armor.
 * The 80 is reduced to 60 by the 20 armor reduction.
 * The 60 is reduced to 42 by the 30% armor reduction.
 * The 42 is considered to be 32 by your 10 armor penetration.
 * The 32 is considered to be 19.2 by your 40% armor penetration.
 * Target A takes damage as if it has 19.2 armor.
 * Target B has 18 armor.
 * The 18 is reduced to &minus;2 by your 20 armor reduction.
 * The &minus;2 is not affected by any further calculations because it is less than 0.
 * Target B takes damage as if it has &minus;2 armor.

Champion abilities
Note: Only the armor penetration effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
 * passive grants him armor penetration.

Masteries

 * : Gives 10% armor penetration.
 * : Gives armor penetration.

Runes

 * runes provide flat armor penetration. These are commonly purchased because, especially along with armor penetration quintessences and against an enemy with low starting armor, armor can be brought very low, if not to zero, from the very beginning of the game and armor penetration is hard to acquire otherwise from items or abilities. runes provide flat armor penetration and magic penetration.

Neutral buffs

 * Crystal Scar's Aura grants every champion 12% armor penetration while playing on the Dominion game mode.

Champion abilities
Note: Only the armor reduction effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
 * fires automatically every half second, reducing enemies armor with each hit by for 2 seconds.
 * reduces the targets armor and magic resistance by for 5 seconds.
 * , while in ranks 2, 3, 4 and 5, the turrets' attacks reduce armor and magic resist by 1 per hit stacking up to 50 times.
 * reduces the armor of all enemies hit by for 3 seconds.
 * reduces the armor and magic resistance of all enemies that walk through the wall by for 5 seconds.
 * reduces the armor and magic resistance of enemies hit by her autoattacks by 3% for 5 seconds. This debuff stacks up to 5 times.
 * reduces the target's armor and magic resistance by for 4 seconds.
 * reduces the armor of all enemies in the area by for the time they are on it plus 1 second.
 * reduces the armor and magic resistance of enemies that step on the trap by for 12 seconds.
 * reduces the target's armor by for  seconds.
 * , when empowered by having more than 50 Fury, Dice will reduce the armor of all enemies hit by for 4 seconds.
 * reduces the armor of all targets hit by 15% for 4 seconds.
 * reduces the armor of all enemies hit by its activation damage by for 4 seconds.
 * targets an enemy to steal their armor and magic resistance by, this amount is doubled over the next 6 seconds.
 * reduced the armor of all enemies hit by for 5 seconds.
 * reduces the armor of the target hit by 30% for 3 seconds.