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 = ((0.06 armor)  (1 + 0.06  armor))  100

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


 * Damage increase = ((0.06 (-armor))  (1 + 0.06  (-armor)))  100

In Dota 2, the hard cap on this number is 100% damage amp, but it requires an infinite amount of negative armor to truly reach this cap. Currently -armor is the same as +armor as listed below (The numbers in the table below do not match Dota 2 tooltip values; Perhaps the formula has been altered for Dota 2).

In Dota 1, 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 with the exception of Visage, whose magic resist starts at 10% and is increased by levels in Gravekeeper's Cloak, and Meepo, with a base of 35%. This is calculated separately from armor.

Armor and Damage block

 * Actual damage = (Damage from attacker - ¹Damage block) * (1 - ²physical resistance) * (³Armor type %)

¹Only when damage block occurs. ²Based on armor value and its formula to yield resistance. ³According to 'Table of resistances' above.

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. One has to consider that EHP does not take magic damage in consideration (therefore cannot be measured as a real EHP). This becomes more evident with burst magical damage (such as Lina's or Lion's ultimate): the armor value in those cases count for nothing.

A hero with 0 armor and a base HP of 1000 will increase his EHP by 60% if he increases his armor to 10.


 * 0 → 10 armor: 1000 → 1600 EHP.


 * 0 → 20 armor: 1000 → 2200 EHP.

As your armor value increases though, strength or HP items gradually become more important to increase your EHP than armor alone.


 * Optimal HP to your armor: (50 Armor) + 900


 * Optimal armor to your HP: (HP - 900)/50

Rough estimates: 11 armor/500 HP over 1000.

Example:


 * 12 Armor at 1500 HP


 * 22 Armor at 2000 HP

Calculating EHP
To find a hero's Effective HP versus physical damage multiply their total HP with their armor and 6 %, then add their total HP.


 * Effective HP = Total HP (Armor points  0.06 + 1)

Example:


 * A level 25 Sven has a total HP of 2183 and 11.46 points of armor. What is his EHP vs physical damage?


 * 2183 HP (11.46  0.06 + 1) = 2183 HP  1.6876 = 3684 EHP


 * Sven should take 3684 of physical damage before he dies.