Dota 2 Armor

In Dota 2, armor mitigates only physical damage, though hero armor does have additional resistances towards specific damage types. Each point of armor increases your Effective Hit Points by 5%. It’s important to note that this 5% refers to your max HP and not your current EHP. The calculation for EHP can be condensed to this:
EHP = HP + 0.05*Armor*Health
Taking a hero with 1000 HP and 0 Armor, adding 10 armor results in an EHP of 1500, an increase of 50%. Adding 20 armor instead of 10 results in an EHP of 2000, an increase of 100%. If you double the gain in armor, you double the gain in effective hit points. This is a slight misinterpretation and oversimplification however. This only holds true if we compare going from 0 to 10 armor alongside going from 0 to 20 armor. Assuming we more realistically gain 10 armor for a 50% improvement, then acquire another 10 armor afterward, we see that our HP increased from 1500 to 2000, an improvement of only 33.3%, even though we doubled our armor.
This may be why many players incorrectly think armor is a rapidly diminishing attribute. In actuality, armor retains its worth a lot longer than you’d expect. Rearranging our old formula for EHP, we see that the amount of health needed to be gained from a purchase to be more worthwhile than just one more point of armor, looks like this.
Health needed = 0.05/(1 + 0.05*Armor)
As your total armor increases, strength and HP items become worth more and more EHP per gold than armor alone. Armor and HP work hand in hand in this regard.
When deciding between more armor or hit points, you should almost always favor hit points. The amount of favoritism is based almost entirely on the enemy team. If you’re playing against a magic heavy line up such as Lina, Lich, Lion or Dark Willow, you’re going to lean more towards hit points. Items like Black King Bar and Hood of Defiance are also going to be on your shopping list if you aim to improve your durability.
If you’re against a physical heavy line up such as Sniper, Drow or Phantom Assassin, you’re going to want a balance between HP and armor. The only time you’ll want to actually FAVOR armor over hit points is the rare occasion where the enemy team is stacking armor reduction, such as Desolator, Medallion of Courage, Assault Cuirass or spells such as Gush and Corrosive Haze. In this unlikely scenario, you’ll see greater returns keeping your armor up against these effects than just stacking more hit points.
Stacking armor reduction: so what about using this offensively? Contrary to common belief, the amount of armor your enemy has doesn’t actually impact whether or not you want armor reduction. Since armor can be reduced below zero, whether your enemy has high or low armor, you will still see damage returns. Instead, this decision is based entirely on your primary damage source being physical, and your current total damage per hit. Because all of your armor reduction effects will apply as a sum total, you can simply multiply for the total impact.
1000 HP and 10 armor = 1500 EHP 	(-5 armor = 1250 EHP, 	- 16.67% EHP)
1000 HP and 5 armor = 1250 EHP	(-5 armor = 1000 EHP,	-20% EHP)
Each additional instance does contribute more than its predecessor. This can be considered a bonus damage percentage buff as it has the same overall effect. It is superior than typical hero auras as it increases based on your total physical damage, not just base plus attribute. Additionally, many items and spells that reduce armor do so to the target for a duration, meaning you increase the damage of your allies as well.
MINUS 10 ARMOR GRANTS + 33.33% DAMAGE
To conclude, remember that you always want items that complement your hero. This can be because of how they interact with your hero’s spells, or compensate their natural deficiencies. There should always be room for 1 to 2 items based on the enemy team, or the current state of the game. Hopefully, now you understand armor works as a mechanic, and how you can take advantage of armor reduction.
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