Armor is a stat that modifies the amount of physical damage taken by a unit. Armor values can be modified by items, hero abilities and Alliance bonuses. A positive value reduces the amount of physical damage received by a unit, while a negative value increases the damage received.
Armor has no effect on Magical damage or Pure damage.
Physical damage dealt to a unit with armor will be multiplied by the damage multiplier. For positive armor values, the multiplier will be less than 1. For negative armor values, the multiplier will be greater than 1. It is calculated by:
damage multiplier = 1 - ((0.052 × armor) ÷ (0.9 + 0.048 × |armor|))
The damage multiplier has a maximum of 25/12 and a minimum of zero. Due to the essence of the calculation, every additional point of armor increases the effective HP (see below) of a unit more than the previous point.
The extra damage a unit can take due to armor is known as its extra effective HP (or extra EHP) for physical damage. At 0 armor, 1 armor grants roughly 5.8% effective HP. This means a unit with 1,000 health and 1 armor can take 1058 physical damage. At 30 armor, this figure scales upwards to 7.7%, meaning armor is more effective at higher values.
Effective HP is calculated by:
effective HP = current HP ÷ damage multiplier
For a sample creature with 1000 HP, here's how its survivability is affected by armor.
|Armor||Damage multiplier||Effective HP||Increase in effective HP
From adding 5 armor
Because armor is more effective at higher values, a drop in armor is more effective at high values, too. Desolator reduces armor by 5, which means that if our sample creature has 15 armor, its EHP go down by 324; but if has 0 armor, its EHP only go down by 186.
List of armor modifiers
|Jull||Jull passive||+5 per adjacent unit|
|Treant Protector||Treant Protector 3 Star Effect||+5 armor|
|Armlet of Mordiggian||+10 armor|
|Blade Mail||+5 armor|
|Craggy Coat||+15 armor|
|Shiva's Guard||+10 armor|
|Vladmir's Offering||+5 armor|
|Warrior||Alliance bonus||+[10/25] armor|
|Alchemist||Acid Spray||-[3/6/9] armor|
|Slardar||Corrosive Haze||-[10/20/30] armor|
|Snapfire||Snapfire 3 Star Effect||-5 armor|
|Templar Assassin||Meld||-[2/4/6] armor|
|Vengeful Spirit||Wave of Terror||-[3/5/7] armor|
|Void Spirit||Dissimilate||-[10/15/20] armor|
|Heartless||Alliance bonus||-[4/9/15] armor|
If the formula for armor listed above is correct, then at 225 armor, a unit would take 0 damage. Any higher and the unit would start to be healed.