[Terms & Definitions:]

Armor: Armor gives you % damage reduction against all types of damage, not just physical. It is increased by armor value on items, strength (1:1) and certain skills.

Diminishing Returns: The idea that an extra point in something (Armor) yields less return than the points before it. Ex. the 100 point in armor is less effective than the 99th point.

Effective Health: Effective health is the amount of damage it takes to kill you (not to be confused with max health). If you have 1000 max health and 25% damage reduction, you have an effective health of 1333. You can determine your effective health by the following formula: MaxHealth/(1-DamageReduction)

This is where a lot of people get confused. Having 50% damage reduction does not allow you to take 50% more damage, it's a lot more than that. Consider this scenario: You have 1000 max health and 50% damage reduction. A monster is hitting you for 100 unmitigated damage, which only does 50 damage to you because you have 50% DR. It will take that monster 20 hits to kill you (1000 HP / 50 dmg) which means you can actually take 2,000 unmitigated damage (20 hits * 100 dmg). So with 50% DR you have 200% effective health, not 150%.

mLvl: Stands for monster level. The level of the monster you're fighting.

DR: Damage reduced (from armor).

Explanation:

DR from Armor = Armor / (Armor+(50*mLvl))

As you can see, there is a constant in the bottom (50*mLvl). Armor is being divided by (Armor + 3000). It will always approach but never reach 1 because Armor can never be greater than or equal to (Armor + positive integer).

Graph illustrating damage reduced as armor increases:

http://i50.tinypic.com/1pgtaf.jpg

The graph shows that as armor increases, additional points in armor yield less and less DR. Most people call this diminishing returns and explain that the more armor you have the less additional armor is worth. That is wrong. The reason it is wrong is because the effect of each additional point of DR is more powerful than the last. So in order to keep the effectiveness of armor linear, each point must must yield less damage reduction because each point of damage reduction is more effective than the last.

Graph illustrating your effective health as DR increases:

http://i47.tinypic.com/21d1u87.jpg

To further illustrate my last point lets look at an example:

At 70% DR you have an effective health of 333.3%, increase over last = 10.8%

At 71% DR you have an effective health of 344.8%, increase over last = 11.5%

At 72% DR you have an effective health of 357.1%, increase over last = 12.3%

...

At 95% DR you have an effective health of 2000%, increase over last = 333.35%

At 96% DR you have an effective health of 2500%, increase over last = 500%

If every point in armor gave you the same amount of DR, each point of armor would be more effective than the least. Meaning that the best way to play the game would be to stack as much armor as possible. This is why Blizzard has balanced armor so that its relationship with effective health is linear. Most of you who have played WoW are probably familiar with this concept because they do it with almost all damage reduction stats. As a little side note, you may remember that resilience in WoW was slightly unbalanced so that each additional point in resilience was better than the last (but not by much).

Without further ado, here is the most important graph in the series. This graph shows your effective health as armor increases:

http://i49.tinypic.com/20ge53r.jpg

People should also note that the same thing holds true for resistances (and resistances via int). The formulas are basically the same.

DR from resistance = Resistance / (Resistance + (5 * mLvl))

Resistance from int = int * 0.1

Which boils down to, DR from int = int / (int + (50 * mLvL))

Which is the same formula as armor. So yes for int also the effect on survivability is linear.

******************TLDR***************

Every 1000 points of Armor increases your effective health by 1/3 of your max health (with no other DR sources), regardless of how many points you do or do not have in Armor already. The same goes for resistances. Here is a link to a google doc with graphs showing effective health and DR as armor increases:

https://docs.google.com/spreadsheet/...0E&output=html

How this effects actual game play

We all have the same question: Which is better for me? It depends on your current stats! (surprise, right?). For example, I play a barb with about 42k HP and 7000 armor. Currently 1 point of vitality is worth a little more than 8 points in armor.

If Item One has 10 extra vitality and Item Two has 70 extra armor, I will go with Item One.

If Item One has 10 extra vitality and Item Two has 90 extra armor, I will go with Item Two.

The same probably doesn't hold true for you. So I made it easy. Here is a dashboard you can use to input your current stats and have it tell you how good vitality is vs armor for you. You can also select "Compare Items" and compare up to 5 items to each other based on their Vitality and Armor stats:

http://www.swfcabin.com/open/1337704597

If there's any real use out of the thing I can incorporate +%life and resistances and stuff. But for now it just tells you Armor vs Vitality.

***************** END TLDR *****************

##Final Note on Resistance/Armor: There's a bit more to the whole thing that just that.

First, 1 resistance is basically always better than 1 armor, obviously. And 1 int is worth 0.1 resistance, so 10int > 1 armor.

Second, how good resistance/int is versus armor depends on your current armor and resistance. So don't necessarily stack one to the detriment of the other.

The more Armor you have the better resistance becomes.

The more resistance you have the better Armor becomes, relative to each other.

Now comes the question: which is better for me specifically? Well, it should be possible to tell how much armor is worth 1 resistance point. I have put together a matrix with Armor running across the top and Resistance running down the side, find where your armor and resistance intersect and it should tell you how many points of armor equal 1 point of resistance.

Resistance vs Armor Matrix:

https://docs.google.com/spreadsheet/...EE&output=html

For me personally, my character's res/armor ratio is about 20. So if I have to choose between 500 armor and 20 resistance, I will choose the armor.

And to the people who are like "omg what about int?" Well, I don't know how you got this far without realizing you can just take those values and divide them by 10, but here is a matrix showing the value of 1 additional point of int versus 1 additional point of armor:

https://docs.google.com/spreadsheet/...3c&output=html

Credit goes to: TBD

From: US Diablo 3 Official Forums