Armor

Armor reduces the amount of damage a champion receives from physical attacks by a percentage. You can gain armor from runes, masteries, some champions' abilities or from items you buy.

At level 18, armor ranges from a base of 56 armor (36% reduced damage), to  93 armor (48% reduced damage).

Damage Reduction
This percentage of reduction is determined by the formula:
 * Damage Reduction Percent = 100 * (Effective Armor) / (100 + ABS(Effective Armor))

For example, a champion with 150 points of armor would take 150 / (100 + 150) = 60% reduced damage from physical attacks, while a champion with -25 effective armor would receive -25 / (100 + 25) = 20% increased damage from physical attacks. The effective armor is determined by your personal armor and the attacker's Armor Penetration.

|x|%29+from+-500+to+500 A graph of damage reduction (y) against total armor (x).

Effective Health
Another relevant calculation is the amount of bonus effective health. This refers to the amount of health that would have to be added to be equivalent to the damage reduction. For example, if damage is reduced by 50%, essentially health has been doubled since it now takes double the damage to kill that target. The calculation for this percentage of bonus effective health is as follows (EAR = Effective Armor):
 * Bonus Effective Health Percentage = (100 / (1.0 - (EAR / (100 + EAR)))) - 100

|x|%29%29%29%29-100+from+0+to+1000 A graph of bonus effective health percent (y) against total armor (x).

Diminishing Returns
Armor does not have diminishing returns from an effective health perspective. Each point of armor increases the amount of physical damage required to kill the champion by 1% of the champion's HP. For example, 100 magic resist gives 50% damage reduction and so 100% more damage is required to kill the champion.

This table lists damage reduction and effective health bonuses granted by various armor levels. The effective health numbers are based on a champion that has 1000 HP. Note that the amount of bonus effective health gained is constant. Increasing armor from 0 to 10 gives 100 additional effective health. Increasing from 500 to 510 gives the same 100 additional effective health. No matter how much armor is present, the next 10 would provide 100 additional effective health. This is because armor does not have diminishing returns. This can also be seen in the second graph linked above which is straight line showing the constant increase.

Items

 * +18 Armor +270 HP, 24 MR, UNIQUE Aura: +15 MR, +8 Damage, and +12 Armor to nearby allied units. 1925 Gold.
 * +45 Armor +18% Crit Chance, UNIQUE Passive: Your physical damage is increased by 2% of your maximum health. 2405 Gold.
 * +45 Armor 700 Gold.
 * +18 Armor 300 Gold.
 * +9 Armor +120 HP, +10 HP Regen 475 Gold.
 * +99 Armor +500 Mana, UNIQUE Passive: 25% Cooldown Reduction and reduces the attack speed of nearby enemies by 20%. 3075Gold.
 * +45 Armor +425 Mana, UNIQUE Passive: 20% Cooldown Reduction 1675 Gold.
 * +68 Armor +40 MR, UNIQUE Passive: Revives your Champion upon death, restoring 750 Health and 375 Mana. This effect can only occur once every 5 minutes. 2600 Gold.
 * +27 Armor +200 HP, Passive: Generates 5 Gold every 10 seconds. 975 Gold.
 * +25 Armor +30 Damage, +40% Attack Speed, UNIQUE Passive: On hit, deals Magic Damage equal to 4% of the target's maximum Health. 3800 Gold.
 * +23 Armor +15 Damage, UNIQUE Passive: 15% chance on attack to deal 500 damage to a minion. 1000 Gold.
 * +25 Armor +12% Dodge, UNIQUE Passive: Enhanced Movement 2 850 Gold.
 * +80 Armor +300 HP, +25 HP Regen, UNIQUE Passive: 8% Cooldown Reduction and 20% chance on being hit to slow the attacker's movement and attack speeds by 25% for 3 seconds. UNIQUE Active: Slows movement speeds and attack speeds of surrounding units by 35% for 2 seconds + 0.5 seconds for each 100 Armor and Magic Resist. 60 second cooldown. 2925 Gold.
 * +45 Armor +450 Health, UNIQUE Passive: Deals 40 Magic Damage a second to nearby enemies. 2610 Gold.
 * +100 Armor UNIQUE Passive: On being hit by standard attacks, returns 30% of damage taken as magic damage. 2000 Gold.
 *  +50 Armor +20 HP Regen, Passive: 20% chance to reduce physical attacker's movement and attack speed by 35% for 3 seconds. 1350 Gold.
 * +30 Armor +23 Damage, +14% Life Steal, UNIQUE Passive: 20% chance on attack to deal 500 damage to a minion. UNIQUE Active: Places an invisible ward with 1100 range sight and lasts for 3 minutes. 3 minute cooldown. 1525 Gold.
 *  +50 Armor +100 Ability Power, UNIQUE Active: Places your champion into Stasis for 2 seconds, rendering you invulnerable and untargetable but unable to take any actions. 90 sec cooldown. 3300 Gold.

Champion Abilities

 * lays down a cover of smoke for 8 seconds where she gains 10/20/30/40/50 armor while in.
 * increases her armor by 10/20/30/40/50 for 15 seconds.
 * increases an allied champion's armor by 30/45/60/75/90 for 3.5 seconds.
 * increases his armor by 0.5 per kill with a maximum of 5/10/15/20/25.
 * increases his armor by 10/20/30/40/50 for 4 seconds.
 * increases his armor by 20/25/30/35/40% for 6 seconds.
 * increases his armor by 100/150/200/250/300 for 5 seconds.
 * increases an allied unit's armor by 10/15/20/25/30 for 6 seconds.
 * increases her armor by 1.5/2/2.5/3/3.5 for 5 seconds upon attacking or being hit by an enemy (stackable up to 10 times). She can also activate this ability to gain max stacks.
 * increases his armor by 50/75/100/125/150 for 6 seconds.
 * increases his armor by 35/50/65 for 25 seconds.
 * increases nearby allied champions' armor by 8/11/13/17/20 while active.
 * increases an allied unit's armor by 15/20/25/30/35 for 4 seconds.
 * increases nearby allied units' armor by 10/15/20/25/30.
 * increases his armor by 80/105/130 for 5 seconds.
 * increases his armor by 30/40/50 for 8 seconds.

Masteries

 * increases your champion's armor by 2/4/6.

Runes

 * Defense and Resilience runes may also increase armor.

Updates
Due to recent updates this article might not be actual. It was last updated during Patch 1.0.0.110. If there is a new version out, please check the data frequently.