Armor

Armor is an attribute shared by all units. It reduces the amount of physical damage a unit takes, but can also increase the amount of damage if it's negative. Higher armor values reduce more damage. Armor is increased by Agility, certain abilities, and some items. Most items in the armor shop will provide armor bonuses.

Damage reduction/increase
Damage reduction is derivative of a unit's armor value. All units, including buildings, have an inherent base armor value and armor type. Any physical damage dealt to a unit will suffer either damage reduction or a damage increase depending on the target's armor.


 * Damage reduction = ((1) (1 + 0.06  armor))  100

Units with negative armor values increase any physical damage they receive.


 * Damage increase = 1 &minus; 0.94&minus;armor

Armor cannot be reduced below &minus;20 (71% damage increase). Further reducing the armor will not change the damage increase.

Unit armor
Unit armor refers to the four basic armor types of most non-Hero units. Light armor is the default armor type of units. Unarmored and light units take 100% damage from all sources except piercing damage, which deals 200% to light armor and 150% to unarmored units. Medium armor takes reduced damage from piercing and siege damage, but increased normal damage. Heavy armor takes reduced damage from piercing damage, but increased damage from siege and normal damage.

Fortified armor
Fortified armor is the type used by buildings and some other units. It takes reduced damage from most sources and is one of the sturdiest types of armor, but takes increased damage from siege. Fortified armor has an innate 50% damage resistance to Heroes.

Hero armor
Hero armor is used by all Heroes in Dota 2. It provides increased resistance to most damage types, except other Heroes and Chaos. Hero armor has an innate 25% magic resistance, though this is calculated separately from armor.

Armor stacking
Armor value stacks with multiple instances of items and abilities. Every point of armor requires a Hero to take 6% more of their maximum hit points in damage to kill. This is called "effective hit points", or EHP. A Hero with 10 armor has 160% of their maximum hit points as EHP, meaning that if a Hero had 1000 hit points, it would take 1600 points of physical damage to kill them. If that Hero had 20 armor, he or she would take 2200 points of physical damage to kill, doubling their protection.

Note, damage reduction does not stack directly.