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 bought.

At level 18, armor ranges from a base of 56 armor (36% reduced damage), to  and  90 armor (47% reduced damage).

Damage Reductions
Incoming damage is multiplied by a specific factor based on your armor:
 * Damage Multiplier = 100 / (100 + Armor) when Armor ≥ 0

Example: 25 Armor &rarr; Damage Multiplier = 100 / (100 + 25) = 100 / 125 = 0.8, so incoming damage is reduced by 20 percent.

It is important to note that the amount of health you have is irrelevant to the effectiveness of armor. For example: 25 armor always reduces damage by 20 percent.

A graph showing the damage multiplier decrease as armor increases.

Effective Health
You can also view armor as a sort of "extra health," where each point of armor gives your champion 1 percent extra health. For example: if your champion has 100 armor, you could calculate the damage multiplier as being 0.5, or imagine that your champion has 100 percent more health. No matter how you view it, the end result is that physical attacks are half as effective against your champion.

This extra health, combined with your normal health, is called effective health. A champion's effective health can be calculated as:
 * Effective Health = Health &times; (100 + Armor) / 100 when Armor ≥ 0

Example: 500 Health and 25 Armor &rarr; Effective Health = 500 &times; (100 + 25) / 100 = 500 &times; 125 / 100 = 500 &times; 1.25 = 625.

A graph showing effective health increase as armor increases. (Effective health represented as a multiplier of base health.)

Negative Armor
Effects that reduce armor (not to be confused with armor penetration) can bring armor into negative values, causing physical damage to be more effective. Negative armor does not follow the same formula as positive armor, however (see below for why). Each point of negative armor increases incoming physical damage by 1 percent, as shown in the following formula:
 * Damage Multiplier = 1 &minus; Armor / 100 when Armor ≤ 0

Example: &minus;15 Armor &rarr; Damage Multiplier = 1 &minus (&minus;15) / 100 = 1 + 15 / 100 = 1.15, so incoming physical damage is increased by 10 percent.

As a result, the formula for effective health is different as well:
 * Effective Health = 100 / (100 &minus; Armor) when Armor ≤ 0

Reasoning
If the original damage multiplier formula is followed back into negative numbers, you can see that the multiplier begins to increase dramatically, literally approaching infinity when armor reaches &minus;100 and being negative when armor is less than that. In order to keep this from happening, a different formula is used that does not have such an asymptote; with this new formula having &minus;100 armor doubles physical damage instead of making it infinite, and the damage increase is linear beyond that.

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 percent of the champion's health. For example: 100 armor reduces physical damage by 50 percent, so 100 percent 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 health. Note that the effective health increase is constant as armor increases; increasing armor from 0 to 10 gives 100 additional effective health, and 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 can also be seen in the graph linked above (see section: Effective Health, showing a straight line that represents the constant increase.

Items

 * +10 Armor +120 HP, +8 Hp5. 475 Gold.
 * +18 Armor 300 Gold.
 * +18 Armor +270 HP, 24 MR, UNIQUE Aura: +15 MR, +8 Damage, and +12 Armor to nearby allied units. 1925 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.
 * +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.
 * +30 Armor +23 Damage, +18% Life Steal, UNIQUE Passive: 20% chance on attack to deal 500 magic damage to a minion. UNIQUE Active: Places an invisible ward with 1100 range sight and lasts for 3 minutes. 3 minute cooldown. 1525 Gold.
 * +45 Armor 700 Gold.
 * +45 Armor +425 Mana, UNIQUE Passive: 15% Cooldown Reduction 1525 Gold.
 * +45 Armor +18% Crit Chance, UNIQUE Passive: Your physical damage is increased by 2% of your maximum health. 2405 Gold.
 * +45 Armor +450 Health, UNIQUE Passive: Deals 35 Magic Damage a second to nearby enemies. 2610 Gold.
 * +50 Armor +20 Hp5, Passive: 20% chance to reduce physical attacker's movement and attack speed by 35% for 3 seconds. 1350 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.
 * +68 Armor +38 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.
 * +75 Armor +350 HP, +25 Hp5, UNIQUE Passive: 5% Cooldown Reduction and 20% chance on being hit to slow the attacker's movement and attack speeds by 35% 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. 3075 Gold.
 * +99 Armor +500 Mana, UNIQUE Passive: 20% Cooldown Reduction and reduces the attack speed of nearby enemies by 20%. 2775 Gold.
 * +100 Armor UNIQUE Passive: On being hit by standard attacks, returns 30% of damage taken as magic damage. 2000 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.
 * passively increases his armor by 10/14/18/22/26. When activated grants the passive benefit to nearby allies and to himself again.
 * 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 (stacks 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 8/11/14/17/20 while active and persists for two seconds after deactivation.
 * increases an allied unit's armor by 20/35/50/65/80 for 4 seconds.
 * increases nearby allied units' armor by 10/15/20/25/30.
 * increases his armor by 15/20/25% of enemy's armor, doubles the amount over the next 6 seconds.
 * increases his armor by 80/105/130 for 5 seconds.
 * increases his armor by 25 (+7/10/13 for each enemy champion hit) for 8 seconds.

Masteries

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

Runes

 * Defense and Resilience runes may also increase armor.

Note
Due to recent updates this article might not be accurate.