This is a pretty important concept. Here we show how armour and hitpoints are dependent upon each other in the grand scheme of things, and how they interact with each other. Always remember that you need to balance what you do with your armour and hitpoints against avoidance. And, of course, this entire thing is meaningless against spells. This is directly related to the theory of Effective Health, which examines this relationship in a slightly different way.
We can equate 1 point of stamina to a number of points of armour that will give you equivalence. That is, X points of armour will increase your time to live by the same amount as 1 point of stamina. The caveat here is that this applies only for melee damage; as soon as someone starts slinging spells it all means nothing, making this a mostly academic exercise. First, we note that the armour dynamic is a simple asymptotic function. Mitigation from armour is capped at 75%, and you can prove to yourself that it is still there by going and fighting much lower level opponents (or by looking at the armour formula in the game's UI files). Here's the analysis using 20000 armour (A) and 20000 health (H), facing a level 80 opponent.
For the formula to determine mitigation (M) for a given armour rating (A), we begin with:
Where L is the level of the opponent being fought, then:
K = 467.5L - 22167.5
We get 56.8% mitigation against a level 80 opponent with 20000 armour. Since we'll need it later, solving for A in terms
of mitigation gives:
Let's define survivability in terms of how long a given opponent takes to kill you. Assume we have 500 damage per second incoming, unmitigated, melee only.
Therefore, at 20000A and 20000H, 47.4 stamina is equivalent to 834A, or 1:17.6 stamina-armour equivalence, in terms of survivability under the given damage output.
- Taking into account 56.8% mitigation, that's 216.2DPS.
- Solving for time, the time to live under that damage rate is 92.5 seconds.
- Now to increase mitigation by 1%...
- Using the above formula, increasing mitigation from 56.8% to 57.8% means we need 20834 (+834) armour
- At 57.8% mitigation, incoming DPS is reduced to 211.2DPS.
- The time to live at that damage rate is 94.7 seconds.
- The change in time to live between 56.8% and 57.8% mitigation is 2.19 seconds, or 474 damage at 216.2DPS.
Solving this for several other armour values while keeping health constant at 20000 and 25000:
Changing health total while keeping armour constant at 10000 and 15000:
18000A 1 stamina to 17A 1 stamina to 13A
20000A 1 stamina to 18A 1 stamina to 14A
22000A 1 stamina to 19A 1 stamina to 15A
25000A 1 stamina to 20A 1 stamina to 16A
30000A 1 stamina to 23A 1 stamina to 18A
35000A 1 stamina to 25A 1 stamina to 20A
40000A 1 stamina to 28A 1 stamina to 22A
We see that this equivalence is variable on both health and armour, but also that increasing health increases the value of armour very quickly, while increasing armour increases the value of health at a slower rate. Either way, overloading on one over the other drastically increases the value of the lower one.
15000H 1 stamina to 23A 1 stamina to 27A
18000H 1 stamina to 20A 1 stamina to 22A
20000H 1 stamina to 18A 1 stamina to 20A
22000H 1 stamina to 16A 1 stamina to 18A
24000H 1 stamina to 15A 1 stamina to 17A
26000H 1 stamina to 14A 1 stamina to 15A
28000H 1 stamina to 13A 1 stamina to 14A
30000H 1 stamina to 12A 1 stamina to 13A
One of the fundamentals of the calculation above is 56.8% mitigation against a level 80 opponent. Against raid level targets, which will be higher than level 80, your mitigation starts to go down. The effects are shown here:
Finally, we can do some solving to get a general formula for stamina-armour equivalence against a given level opponent:
Level 20000A/20000H 25000A/20000H 20000A/25000H
Level 80 1 stamina to 17.6A 1 stamina to 20.1A 1 stamina to 14.1A
Level 81 1 stamina to 17.9A 1 stamina to 20.4A 1 stamina to 14.3A
Level 82 1 stamina to 18.1A 1 stamina to 20.6A 1 stamina to 14.5A
Level 83 1 stamina to 18.6A 1 stamina to 20.8A 1 stamina to 14.7A
Level 84 1 stamina to 18.6A 1 stamina to 21.1A 1 stamina to 14.8A
Level 25000A/25000H 30000A/30000H
Level 80 1 stamina to 16.1A 1 stamina to 15.1A
Level 81 1 stamina to 16.3A 1 stamina to 15.2A
Level 82 1 stamina to 16.5A 1 stamina to 15.3A
Level 83 1 stamina to 16.7A 1 stamina to 15.5A
Level 84 1 stamina to 16.8A 1 stamina to 15.7A
Where A is your current armour, H is your current health, and K is as defined previously.
1 stamina: ------- points of armour
Recall that there are buffs (Kings) and talents that will change the value of one stamina for you:
- If you have kings, change 10 to 11
- If you have stamina talents, change 10 to 10.6 for a warrior or 11.4 for a paladin
- If you have multiple, then they are multiplicative, for example 11.66 for a warrior with Vitality and Kings
- And so on
Also, if you have armour increasing talents, modify your total armour by the value of the multiplier you get (e.g. 1.1 for paladins and warriors)
A final note is that this formula is not affected by the actual rate of incoming damage per second. As such, it is not affected by avoidance. An avoidance total of 20% from the combined dodge and parry of an individual character does nothing more than reduce the rate of incoming DPS by 20%, on average. While a 20% reduction of incoming DPS is significant (as is the reduction in critical hits that comes with it), it has no bearing on the stamina-armour equivalence.
So, what should you aim for when choosing gear? What we see here is that balance is pretty key to the whole thing. If you stack one at the expense of the other, you are doing yourself more harm than good overall. Having a good balance between armour and stamina tends to focus both for you - when you are taking physical damage. On the other hand, it's pretty hard to actively gear for armour. In general, you're going to gem and enchant for stamina. When you find items with bonus armour, they tend to be the best choice.
Magical damage is a factor that helps to balance the difference between higher health and higher armour. Having very high armour at the expense of health might seem to be of benefit against pure melee opponents, but is a definite disadvantage to opponents using magic, and the amount of buffer you have against hard hits and mishap is probably going to be too small. Your mileage will vary.