Some items have percentage life other items have vitality. What is an easy way of figuring out how much vitality the percentage equals and vice versa. Is it good to stack lots of percentage hp?
The basic equation for Character Sheet HP where V=Vitality, L=%Life (ie., 1% Life = 1):
HP = (276 + V*35) * (1 + L/100)
Taking the partial derivatives:
dHP/dV = 35+35/100*L
dHP/dL = 2.76+0.35*V
Now assuming that 200 Vit = 12% Life for most items (Diablo Inc Gamers - Item Affix Quick-look Chart), we have:
dHP/dV * 200/12 = dHP/dL
583 + 5.83*L = 2.76 + 0.35*V
L = (0.35*V-580)/5.83
Which means that given V vitality, you should optimally equip L life:
Caveats: Trading 1 stat for another is seldom simple. I used the ratio of the max possible roll on Belt,Chest,Helm,Shoulder (200/16=16.7 vit per %life). Using a comparison of different items will change the numbers (Amulet: 300/16=18.8). Another example might be gem based (34/12=2.83), changing the comparison drastically (I think this is flawed, but YMMV).
Comments: I did this analysis in a rather short period of time, its not unlikely I made a mistake. Corrections welcomed.
The fact that it is a percent means you can't compare it to a raw number, because the value it provides changes based on how much health you already have, and even simply between higher levels (as vitality starts to provide more and more health).
The easiest way to compare the two pieces of gear is by trying them both on and comparing your HP totals while wearing each piece (the change in vitality on the tooltip is misleading since it can't take the % health into account).
As to stacking them, similar to how you shouldn't forego weapon DPS for primary stat, you want a mix of both raw vitality and % health, as having some of both is going to provide more effect than stacking one to the exclusion of the other.
If you can't try the item on (by purchasing it, or if we're trying to figure out gems), your only recourse is to fall back on Math! That's right, the easiest way to solve this one is to bust out the old calculator.
%Health works exactly the way it sounds like it does. To find out your current max HP, just mouse over the gauge on the bottom left hand corner of your screen.
Vitality is a bit thornier, as the amount of additional HP you get per point of Vitality increases with level. To find out, simply mouse over the Vitality stat on the character sheet. The tooltip will tell you how much HP you gain per point of Vitality. Simply multiply this by the amount of Vitality you stand to gain, and you'll know how much health is involved.
Because the numbers involved vary with level, it's impossible to give any more hard and fast rule than that, but, for example, at level 60, with no gear equipped (or none with any +Vitality anyway), 1% Health is worth roughly 47 health. 1 Vitality is worth 35 health, making 1% health worth roughly 1.3 points of Vitality. So, for example, if you have a Radiant Amethyst, and are trying to decide whether to put it in your Helm (10% Life), or your Pants (+26 Vit), well, the gem would provide much more extra health in your pants as Vitality.
But, as you gain health, 1 Vitality will continue to always be worth 35 Health, but %Health will improve in effectiveness. If you add 1000 Vitality from gear, 1% Health is going to be worth about 82 HP, or somewhere in the neighborhood of 2.2 Vitality. If we use our same Radiant Amethyst example, we find that it still comes out behind to take the 10% health than the 26 Vitality, but it's a lot closer this time.
If add another 1000 Vitality from gear, and run the numbers again, we come out with 1% health being worth a whopping 116 HP. At this point, our Radiant Amethyst would provide 1160 health if we put it in our helm, compare to only 920 if we put it in another piece of armor.
Your best bet is always going to be to do the math, and remember, if you go this route, to unequip any items that you'll be replacing first, in order to get an effective baseline.
Here is the math:
The more Vitality you have, the more you will benefit from Life % increase affixes, and vice versa.
For example, in you have 1000 Vitality (35,276 health), each point of +Life % will give you 352 additional health. If you had only 500 Vitality (17,776 health), each point of +Life % will give you only 177 additional health.
Each point of +Life % makes Vitality better, too. If you have +10% increase, now each Vitality will give you 38.5 health (10% more), and if you have +20% increase, each point will give you 42 health.
If you want more health, always get a helmet with a socket, and add the best Amethyst gem you can attain at a reasonable cost. Even 12% more life (Flawless Square) is a large increase.
Is it good to stack lots of percentage hp? Yes. Especially at max level and with decent gear - most of your items will have +Vitality.
10% more life gives you 10% more overall hp. IE. I have a ring with +8% life and no vitality at all. It is the only piece of gear I currently have with a +life %. When I remove the ring my total hp is 64508. When I equip it, my hp goes to 69,668 which is exactly 8% more.
It varies based on your current life total. Right now for me 12 vitality = 1% more life. So if I can get more than 120 vitality I prefer that over a +10% life item. So early on % life is garbage; you could get more from vitality. But as I get more and more vitality I find that I'm using more and more % life items too. I haven't yet reached the point where it's good enough that I'm actively searching for it (120 vit or 5 other equally good stats isn't that hard to get), but I mean when comparing 2 items it's now more likely to help make one item win over another than it was before.