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:


 * Damage multiplier100(100Armor)&emsp;if Armor ≥ 0
 * Damage multiplier2100(100Armor)&emsp;if Armor ≤ 0

Examples:
 * 25 armor &rarr; &times;0.8 incoming physical damage (20% reduction).
 * 100 armor &rarr; &times;0.5 incoming physical damage (50% reduction).
 * &minus;25 armor &rarr; &times;1.20 incoming physical damage (20% increase).

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.

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.
 * 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.
 * aura grants bonus armor to himself and nearby ally champions equal to 12% of his total armor.
 * active deals magic damage equal to a base amount plus 30% of his total armor.
 * takes of the target's armor and gains an equal amount of armor. Over 6 seconds, the bonus armor is doubled.

Items

 * active increases its slow duration by a base amount plus 0.5% of the wearer's total armor.

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 her armor by.
 * 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 . When activated it throws a flag to a target location that grants the passive benefit to nearby allies and to himself again.
 * 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.
 * causes him to channel for up to 5 seconds, increasing his armor by for the duration.
 * increases an allied unit's armor by for 6 seconds.
 * increases his armor by for 6 seconds.
 * 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.
 * Rengar lets out a battle roar, damaging enemies increases his Armor by for 3 seconds.
 * 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 3 seconds.
 * passively increases his armor by while the ability is not on cooldown and grants himself and nearby allied units' gains 12% of his armor.
 * passively generates souls for him to collect, with each soul collected granting armor permanently, with the first soul being worth 1 armor and subsequent souls being worth .01 less down to .5 armor per soul collected.
 * targets an enemy to steal their armor by . This amount is doubled over the next 6 seconds. Trundle increases his own armor by the same flat amount as the armor they enemy lost.
 * increases his armor and magic resist by 4% per stack.
 * increases his armor by for 5 seconds upon activation.
 * passively grants him armor for each nearby enemy champion.
 * passively grants 15% of his ability power as bonus armor.
 * 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 your champion's armor by 1 for each nearby enemy champion.
 * increases your champion's armor by.
 * increases bonus armor by.

Runes

 * runes increase armor.

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

Trivia
A level 18 with 6, 3 points in , 3 points in , 1 point in , , , , ,  aura, Glyphs and Marks of , Seals and Quintessences of, and  enabled will give a total of approximately 1531 armor. Switching for  and having an enemy  with the same setup use  on  and the allied  uses  on the enemy, will yield a total of roughly 1870 armor. This is the highest possible finite amount of armor (without using metagame effects—see below), and reduces physical damage by 94.9%.

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  will have approximately 2550 armor. cannot exceed this. This reduces physical damage by 96.2%.

If disconnecting grants armor quickly enough that  remains active, can reach 3031 armor. This reduces physical damage by 96.8%.

, with his effectively infinite stacking, can obtain a maximum of 500,024.75 armor off his passive alone. With the same set-up as above, he can obtain a total of about 526,014.3 armor, reducing physical damage by 99.981%. If he disconnects, he can obtain up to 527,064.3 armor, reducing roughly the same amount.

this need to be changued due patch 3.03 taric increases allis armor by his armor so he can grant more than 30 armor(with 6 thonrmail and armor ruens masteries he could grant more tahn 70 armor Armor