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 ( Heimerdinger) to 90 ( Maokai and Poppy).

Damage reduction

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

Damage multiplier = 100 / (100 + Armor) if Armor ≥ 0

Damage multiplier = 2 − 100 / (100 − Armor) if Armor ≤ 0

Examples:

• 25 armor → ×0.8 incoming physical damage (20% reduction).
• 100 armor → ×0.5 incoming physical damage (50% reduction).
• −25 armor → ×1.25 incoming physical damage (25% 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

How to reduce enemy armor?

Did you know?

A level 18 Malphite with 6 Thornmails, 3 points in Hardiness, Astral Blessing, Bulwark, Aria of Perseverance, Command: Protect, Aegis of the Legion aura, Marks and Glyphs of Resilience, Seals and Quintessences of Defense, and Brutal Strikes enabled will give a total of approximately 1461 armor. Switching Orianna for Trundle and having an enemy Trundle with the same setup use Agony on Malphite and the allied Trundle use Agony on the enemy Trundle, will yield a total of approximately 1824 armor. This is the highest possible amount of armor, which is 94.9% reduction.