Armor penetration
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As an example, 10 armor penetration causes a target to take damage as though they had 10 less armor, and so 20% armor penetration is equivalent to fighting a target with 20% less armor. The difference between penetration and armor reduction is that reduction can reduce armor below zero and affects any source of physical damage to a target. The effects of armor penetration, on the other hand, are ignored once the target's effective armor drops below 0, and only the source of the armor penetration reaps the benefits.
Lethality is a champion statistic introduced in Season 2017 that is converted to flat armor penetration at a rate that increases based on the enemy's level, up to 1 flat armor penetration per 1 lethality against a level 18 target.
All champions have 0 base armor penetration, but it can be increased with items, runes, masteries and some champion abilities.
Order of calculations
When attacking, armor penetration and armor reduction are considered in the following order:^{[1]}
 Armor reduction, flat
 Armor reduction, percentage
 Armor penetration, percentage
 Armor penetration, flat
Note: Base Armor and Bonus Armor are calculated separately.
Flat armor reduction
The target's armor is reduced by an amount. Flat armor reduction from different sources stacks additively and is distributed proportionally between base armor and Bonus Armor.
 Example: 30 armor reduction against a 20 Base Armor and 40 Bonus Armor target will reduce Base Armor by 10 and Bonus Armor by 20, so the target will be reduced to 30 armor ((20 − 10) + (40 − 20) = 30).
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 −15 armor.
Percentage armor reduction
The target's armor is multiplied by a percentage (100% − the listed reduction). Applies to both base and bonus armor. Percentage armor reduction stacks multiplicatively and is ignored if the target's armor is 0 or less.
 Example: 30% armor reduction multiplies the target's armor to 70%, so a target with 20 base armor and 40 bonus armor will be reduced to 42 armor ((20 × 0.7) + (40 × 0.7) = 42).
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.
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 penetration stacks multiplicatively and is ignored if the target's armor is 0 or less.
 Example: 30% armor penetration causes the target to take damage as if its armor was multiplied by 70%, so a target with 20 base armor and 40 bonus armor will be treated as though it had 42 for purposes of damage (the target's actual armor value will not change, however) ((20 × 0.7) + (40 × 0.7 = 42).
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.
Percentage bonus armor penetration
The target's bonus armor is treated as being multiplied by a percentage for purposes of damage calculation (100% − the listed penetration). Percentage bonus armor penetration stacks multiplicatively with other sources of percentage armor penetration and is ignored if the target's armor is 0 or less.
 Example: 30% bonus armor penetration multiplies the target's bonus armor to 70% without affecting base armor, so a target with 20 base armor and 40 bonus armor will be treated as though it had 48 for purposes of damage calculation (the target's actual armor value will not change, however). (20 + (40 × 0.7) = 48).
 If the player has multiple sources of percentage bonus penetration, they stack multiplicatively:
 Example: 10% armor penetration and 30% bonus armor penetration. (20 × 0.9)) + (40 × 0.7 × 0.9)) = 43.2).
Because the target's actual armor value does not change, anything else depending on that value also doesn't change (e.g.
).Percentage bonus armor penetration makes a big difference on targets who build armor items but will do nothing against base armor.
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.
).Lethality
Lethality is a new statistic introduced in Season 2017. It grants flat armor penetration using the following formula:
 Flat Armor Penetration = Lethality × (0.4 + 0.6 × Target's level ÷ 18)
Examples
Given 30 flat armor reduction and 30% armor reduction, and the target is affected by 10 flat armor penetration and 45% bonus armor penetration,
 Target A has 300 armor (100 base and 200 bonus armor).
 The 300 is reduced to 270 (90 base and 180 bonus armor) by the 30 armor reduction.
 The 270 is reduced to 189 (63 base and 126 bonus armor) by the 30% armor reduction.
 The 189 is considered to be 132.3 (63 base and 69.3 bonus armor) by the 45% bonus armor penetration.
 The 132.3 is considered to be 122.3 by the 10 armor penetration.
 Target A takes damage as if it has 122.3 armor armor.
 Target B has 18 armor.
 The 18 is reduced to −2 by the 20 armor reduction.
 The −2 is not affected by any further calculations because it is less than 0.
 Target B takes damage as if it has −2 armor.
Increasing Lethality
Items
 Unique: +15 Lethality (6.5  15 (based on target's level) armor penetration). : Passive:
 Unique: +15 Lethality (6.5  15 (based on target's level) armor penetration). : Passive:
 Unique: +10 Lethality (4.33  10 (based on target's level) armor penetration). : Passive:
 Unique: +20 Lethality (8.67  20 (based on target's level) armor penetration). : Passive:
Masteries
 magic penetration : Grants 1.7 / 3.4 / 5.1 / 6.8 / 8.5 Lethality and 0.6 + (0.06 × level) / 1.2 + (0.12 × level) / 1.8 + (0.18 × level) / 2.4 + (0.24 × level) / 3 + (0.3 × level)
 Rank 1: 0.66  1.68 (based on level)
 Rank 2: 1.32  3.36 (based on level)
 Rank 3: 1.98  5.04 (based on level)
 Rank 4: 2.64  6.72 (based on level)
 Rank 5: 3.3  8.4 (based on level)
Runes
Increasing armor penetration
Items
 Unique – Last Whisper: +30% bonus armor penetration. : Passive:
 Unique – Last Whisper: +45% bonus armor penetration. : Passive:
 Unique – Last Whisper: +45% bonus armor penetration. : Passive:
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.
 passively grants him 5 / 10 / 15 / 20 / 25% armor penetration.
 ignore 40% of targets' armor.
 critical strikes gain 50% penetration to bonus armor for 15 seconds. upon activation his
Masteries
 : Grants 1.4 / 2.8 / 4.2 / 5.6 / 7% armor penetration.
Reducing armor
Items
 Unique: Dealing physical damage to an enemy champion Cleaves them, reducing their armor by 5% for 6 seconds, stacking up to 6 times for a maximum of 30%. : Passive:
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.
Percentage Reduction:
 reduces the armor of all enemy champions hit by 4 spins by 25% for 6 seconds.
 reduces the armor of all enemies hit by 10 / 14 / 18 / 22 / 26% for 3 seconds.
 reduces the armor and magic resistance of the enemy hit by his first attack after cast, by 10 / 15 / 20 / 25% for 5 seconds
 reduces the armor 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 12 / 16 / 20 / 24 / 28% for 4 seconds.
 , when empowered by having more than 50 Fury, Dice will reduce the armor of all enemies hit by 15 / 20 / 25 / 30 / 35% for 4 seconds.
 reduces the armor of the first target hit by 20% for 2.5 seconds.
 reduces the armor of his target by 20% and gains for the same amount. This amount is doubled over the next 4 seconds. The armor reduction lasts for another 4 seconds after the drain completes.
 reduces the armor of all enemies hit by 12 / 14 / 16 / 18 / 20% for 5 seconds.
 reduces the target's armor by 20% for 4 seconds after every third consecutive hit.
 reduces the armor of the target hit by 10 / 15 / 20 / 25 / 30% for 3 seconds.
 reduces the armor of targets hit by his basic attacks or by 15% for 3 seconds. Only one target may be challenged at a given time.
Flat Reduction:
 fires automatically every half second, reducing enemies armor with each hit by 1 / 2 / 3 / 4 / 5 for 2 seconds up to a maximum of 8 stacks (8 / 16 / 24 / 32 / 40).
 reduces the armor of all enemies in the area by 20 / 25 / 30 / 35 / 40 for the time they are on it plus 1 second.
 reduces the target's armor by 10 / 15 / 20 / 25 / 30 for 1.25 / 1.5 / 1.75 / 2 / 2.25 seconds.
Notes
 Armor penetration is a very valuable stat for physical damage dealing champions. There are few sources for it, so armor penetration Marks and Quintessences are very popular runes.
 Armor penetration benefits all sources of physical damage including all champion abilities that deal physical damage.
 For example, will benefit from armor penetration as it deals physical damage, however his will not benefit from armor penetration as it deals magical damage.
 The items that deal onhit physical damage: , , , , and , will also benefit from armor penetration.
 Turrets are also subjected to armor penetration effects.
Trivia
 The biggest amount of obtainable lethality is 92.5, with any champion:
 Runes: +24 lethality
 9 × Marks of Lethality (+9 × 1.6 lethality)
 3 × Quintessences of Lethality (+3 × 3.2 lethality)
 Masteries:
 (+8.5 lethality)
 Items: +60 lethality
 1 (+15 lethality)
 1 (+15 lethality)
 1 (+10 lethality)
 1 (+20 lethality)
 Relevant mathematics:
 Lethality = 60 + 24 + 8.5 = 92.5 lethality
References
 ↑ If I technically had 100% armor reduction on an enemy... at League of Legend
Champion statistics  

Offensive 

Defensive  
Magical  
Movement 