Armor

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

At level 18, base armor ranges from 61 to 90 ( and ).

Damage reduction
Incoming physical damage is multiplied by a factor based on the unit's armor: {C}
 * 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;1.2 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% of its maximum health in its 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.

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Armor value stacks in a linear and additive fashion. A champion who purchases a 15 armor value item and a 30 armor value item will receive a total bonus of 45 to their armor value from those two items. This linear relationship is a source of confusion among many League of Legends gamers who you might find waddling around the map swathed in armor. In order to assess the viability of excessively stacking armor as an ingame strategy, it is important to identify which qualities of armor really contribute to the "survivability" of your champion. A generally agreeable and objective determinant is the amount of raw damage that a champion can take before its health reaches zero, or effective health. Effective health differs from actual health values because it also factors in your defensive statistics.
 * Effective Health = Current Health * (1 + Damage Reduction)
 * Example: A unit with 60 armor has 37.5% physical damage resistance. If the unit has 1000 current health, it will take 1000 + 1000(.375) or 1375 physical damage to kil'l it.

You may have experienced the difference between effective health and regular health when Malphite with a Glacial Shroud manages to take more hits than Malzahar with a Rod of Ages despite having less health bars. While it might be tempting to stack armor in attempts to terrorize your enemies, closer examination of the relationship between armor values and damage reduction (armor's sole contributor to effective health) do in fact show diminishing marginal returns.


 * 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.

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 8 seconds.
 * increases an allied champion's armor by for 4 seconds.
 * increases his armor by 20%.
 * 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 his armor by for 5 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 her armor by while active.
 * 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.
 * 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 5 seconds.
 * passively increases his armor by while the ability is not on cooldown and grants himself and nearby allied units' the same bonus again as a permanent aura.
 * 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 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 25 ( for each enemy champion hit) for 8 seconds.
 * increases his armor by 25 ( for each enemy champion hit) for 8 seconds.

Masteries

 * increases your champion's armor by.

Runes

 * and runes also 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 , , , , ,  aura, Marks and Glyphs of , Seals and Quintessences of , and  enabled will give a total of approximately 1461 armor. Switching for  and having an enemy  with the same setup use  on  and the allied  use  on the enemy, will yield a total of approximately 1824 armor. This is the highest possible amount of armor, which is 94.9% reduction.