Armor

Armor is a stat shared by all units, including minions, monsters, and buildings. Increasing armor reduces the physical damage the unit takes. Each champion begins with some armor which increases with level ( being the only exception). You can gain additional armor from abilities, items, masteries, and runes. Armor stacks additively.

Excluding whose base armor does not scale with levels, base armor ranges from 61  to 90 ( and ) at level 18.

Damage reduction
Incoming physical damage is multiplied by a factor based on the unit's armor:

$${\rm Damage\ multiplier}=\begin{cases} {100 \over 100+{\it Armor}}, & {\rm if\ }{\it Armor} \geq 0\\ 2 - {100 \over 100 - {\it Armor}}, & {\rm otherwise} \end{cases}$$

Examples:
 * 25 armor &rarr; &times; incoming physical damage (20% reduction, +25% effective health).
 * 100 armor &rarr; &times; incoming physical damage (50% reduction, +100% effective health).
 * &minus;25 armor &rarr; &times; incoming physical damage (20% increase, effective health).

Stacking armor
Every point of armor requires a unit to take 1% more of its maximum health in physical damage to be killed. This is called effective health.
 * Example: A unit with 60 armor has 60% more of its maximum health in effective health, so if the unit has 1000 maximum health, it will take 1600 physical damage to kill it.

What this means: by definition, armor does not have diminishing returns, because each point increases the unit's effective health against physical damage by 1% of its current actual health whether the unit has 10 armor or 1000 armor.


 * Note: Unlike health, increasing armor also makes healing more effective because it takes more effort to remove the unit's health than it does to restore it.

For a more detailed explanation, see this video.

When a unit's armor is negative because of armor reduction or debuffs, armor has increasing returns, because negative armor cannot reduce effective health to less than 50% of actual health. A unit with −100 has (gains ) of its maximum health as effective health.

Armor as Scaling
These use the champion's armor to increase the magnitude of the ability. It could involve total or bonus armor. By building armor items, you can receive more benefit and power from these abilities.

Champions

 * grants bonus armor to himself equal to 20% of his bonus armor.
 * increases his armor by for 6 seconds.
 * deals magic damage equal to a base amount plus 30% of his total armor.
 * deals a flat amount plus 10% of his total armor.
 * grants him bonus attack damage equal to 25% of his total armor.
 * after using an ability, his next basic attack deals bonus magic damage equal to 20% of his armor.
 * aura grants bonus armor to himself and nearby ally champions equal to 12% of his total armor.
 * active deals a flat amount plus 20% of his total armor and reduces nearby enemy armor by 5% of his total armor for 4 seconds.
 * grants additional armor plus 20% of her bonus armor for its duration.
 * steals 20% of the target's armor. He continues to drain an additional 20% the target's armor over 4 second before decaying 4 seconds after the drain completes.

Items

 * active increases its slow duration by a base amount plus of the wearer's total armor.

Variable Availability

 * {{ile|Zhonya's Hourglass|+50 armor|, +120 ability power. Unique Active - Stasis: Places your champion into stasis for 2.5 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.|3100}
 * {{ile|Zhonya's Hourglass|+50 armor|, +120 ability power. Unique Active - Stasis: Places your champion into stasis for 2.5 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.|3100}
 * {{ile|Zhonya's Hourglass|+50 armor|, +120 ability power. Unique Active - Stasis: Places your champion into stasis for 2.5 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.|3100}
 * {{ile|Zhonya's Hourglass|+50 armor|, +120 ability power. Unique Active - Stasis: Places your champion into stasis for 2.5 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.|3100}
 * {{ile|Zhonya's Hourglass|+50 armor|, +120 ability power. Unique Active - Stasis: Places your champion into stasis for 2.5 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.|3100}
 * {{ile|Zhonya's Hourglass|+50 armor|, +120 ability power. Unique Active - Stasis: Places your champion into stasis for 2.5 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.|3100}
 * {{ile|Zhonya's Hourglass|+50 armor|, +120 ability power. Unique Active - Stasis: Places your champion into stasis for 2.5 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.|3100}
 * {{ile|Zhonya's Hourglass|+50 armor|, +120 ability power. Unique Active - Stasis: Places your champion into stasis for 2.5 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 second cooldown.|3100}

Champion abilities
Note: Only the armor buff effect of these abilities is shown here, to read more information on each of these abilities, follow the link on each of them.
 * lays down a cloud of smoke for 8 seconds. While inside of the cloud, she gains armor.
 * allows her to enter an egg-state for up to 6 seconds upon reaching 0 health. While in this state, she will receive an armor modifier of.
 * increases her armor by for 5 seconds.
 * increases an allied champion's armor by for 4 seconds.
 * increases his armor by 20% of his bonus armor.
 * increases his armor by every second he remains in combat. This bonus stacks up to 10 times. Graves is considered in combat if he has dealt or received damage in the last 3 seconds.
 * passively increases his armor by.
 * increases his armor by  for 8 seconds.
 * increases his armor by.
 * increases his armor by for 4 seconds.
 * increases her armor by  for 3 seconds, deals damage after that time to units around her, and retains the defensive buff for an additional 3 seconds if any enemy is struck by the blast.
 * increases his armor by for 6 seconds.
 * increases an allied unit's armor by for 6 seconds.
 * passively increases his armor by.
 * increases an allied champion's armor by for as long as the ball is attached to them.
 * increases her armor by for 5 seconds upon attacking or being hit by an enemy, stacks up to 10 times. She can also activate this ability to gain max stacks.
 * increases his armor by for 6 seconds.
 * deals damage to units around him and increases his Armor by for 3 seconds if an enemy is damaged by the ability.
 * grants her bonus armor each times she damages an enemy with an ability or attack for 2 seconds. Subsequent damage will increase the duration by 2 seconds up to a cap of 8 seconds, after where damage will refresh the duration.
 * passively increases her armor by . This bonus is doubled while she is in Dragon Form.
 * increases his armor by for 25 seconds.
 * Aura: increases nearby allied champions' armor by while active and persists for two seconds after deactivation.
 * Activation: increases hers and one nearby allied champion's armor by an extra for 1 second.
 * increases an allied unit's armor by for 2 seconds.
 * passively increases his armor by while the ability is not on cooldown and grants himself and nearby allied units' 12% of his armor.
 * passively generates souls for him to collect, with each soul collected granting armor permanently.
 * takes 40% of the target's armor and gains an equal amount of armor. Over 4 seconds, the bonus armor is doubled. The armor bonus lasts for another 4 seconds after the drain completes.
 * increases his armor by for 5 seconds upon activation.
 * passively grants him armor for each nearby enemy champion.
 * increases his armor by for each enemy champion hit for 8 seconds.
 * increases his armor by for each enemy champion hit for 8 seconds.

Masteries

 * increases bonus armor and magic resistance by.
 * increases your champion's armor by.
 * grants your champion armor and  magic resistance for each nearby enemy champion.

Runes

 * runes increase armor.

Ways to reduce armor
See armor penetration. Note that armor penetration and armor reduction are different.

List of champions' armor
1 does not gain armor per level, instead gaining armor through.

Trivia
Before the 3.14 patch a level 18 with 6, 3 points in , 3 points in , 1 point in , , , ,  aura,  aura, Glyphs and Marks of , Seals and Quintessences of , and  enabled will give a total of approximately 1624 armor.

Having an enemy with the same setup use  on  will yield a total of roughly 1770 armor. This is the highest possible finite amount of armor (without using metagame effects—see below), and reduces physical damage by.

It is still possible to go higher through gimmicky means. If a player disconnects from the server and is in the fountain, he will gain 1000 armor. With the same setup as above, a disconnected  with a disconnected  will have approximately 2779 armor. cannot exceed this. This reduces physical damage by.

If disconnecting grants armor quickly enough that  remains active, can reach 3315 armor. This reduces physical damage by.

, with his effectively infinite stacking, can obtain a maximum of  armor off his passive alone. With the same set-up as above, he can obtain a total of about armor, reducing physical damage by. If he disconnects, he can obtain up to armor, reducing roughly the same amount.

pre-patch 3.10 with 1, 3 points in , 3 points in , 1 point in , Glyphs, Marks, Seals and Quintessences of , , , , from  with the same runes, masteries, and , with  enabled, with all champions at level 1 will give a total of approximately 273 armor. This was the highest amount of armor any champion could have at the very beginning of the game.

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