From Diablo Wiki
All Resistance (AKA All Res, Res All, AR or RA) is an item affix in Diablo 3. One of the most valuable, useful, and popular affixes in the game, this one increases the resistance to all types of damage; physical, cold, fire, poison, lightning, and arcane.
Resistances in Diablo 3 are not expressed as percentages, as they were in Diablo 1 and Diablo 2. They are instead cumulative values, much like Armor, where more of the stat provides damage mitigation of some percentage that varies depending on the level of the character, the enemy, and other factors.
The Toughness stat, added in D3v2, incorporates All Res and other defensive properties (such as armor and hit points) into one number, for easier reference and comprehension. Many players feel that All Res is undervalued by Toughness, and that it provides much more benefit than bonuses to Armor, or to a single resistance. It's recommended that
The most dangerous sources of elemental damage in D3 and RoS are Elite Affixes, such as Molten, Arcane Enchanted, Desecrator, Poison Enchanted, and so on. While some resistances are more valuable than others, and in Loot 2.0 individual resistances roll higher values than All Res, it's better to have them all, and All Res makes that possible.
- The maximum All Res per item affix in Diablo 3 vanilla was 80 points.
- The maximum All Res per item affix rises to 100 on legendary items in Loot 2.0 and Reaper of Souls.
- All Res can also be added via Paragon Points spent in the Defense Tab, where each point gains 5, up to a maximum of 250 from 50 Paragon Points.
- 10 points of Intelligence adds 1 point of All Res. Thus an item that adds 500 INT is worth 50 All Res. This provides a huge bonus to Wizards and Witch Doctors, as they get hundreds of points of "free" All Res as a side effect of their mainstat increasing.
- A Wizard or Witch Doctor at lvl 70 often has 8000 Mainstat, worth 800 All Res. Thus Wizards and Witch Doctors can easily exceed 1500 All Res with decent gear, while other classes must struggle to reach 1000.
Obtaining All Res
All Res can spawn as a random affix on every type of armor: helms, shoulders, bracers, chest armor, pants, belts, and boots, along with rings, amulets, and shields. (It is an inherent property on many of the best legendary armor pieces in the game.)
All Res can not be found as a random modifier on weapons or non-shield off-hand items quivers, sources, or mojos, though it appears as an inherent property on a few legendary items in those slots, such as the Empyrean Messenger.
Items in Diablo 3 Vanilla could spawn with All Res and a bonus to a single resistance. This is not possible on Loot 2.0 items in Diablo 3 or Reaper of Souls, aside from a few legendary items with inherent bonuses to resistance. Other than those few exceptions, items in the game today spawn with All Res (as a primary affix) or a single resistance (as a secondary affix), but never both on the same item.
All Res Strategy
There is no minimum or maximum recommended value for All Res. More points in resistance grant more damage mitigation to that type of elemental attack, and while some elements are more valuable than others, Diablo 3 varies the attack types enough that it avoids the system seen in Diablo 2, where resistance to Lightning was by far the most important and Cold and Poison resistance hardly mattered at all.
All Res shows diminishing returns in the higher values, with more and more points required to achieve substantial returns. A character who raises their resistance from 200 to 400 will see a much larger absolute benefit than a character who raises their resistance from 800 to 1000. At the higher end though, every point matters since enemies on the higher difficulty levels do such huge amounts of damage.
As a general guideline, melee characters need more resistance than ranged attackers, since melee fighters can not move out of the path of Molten, Arcane, Plagued, etc, so easily. Hardcore characters will generally want more resistance than Normal characters.
Resistances required for survival increase with the difficulty level. On Hard difficulty, 500 All Res is plenty, but to survive in Torment most players want considerably more.
All Res Development
During the days of Diablo 3 vanilla (D3v2 appeared in February 2014) All Res was virtually a mandatory stat on any high end armor, and this did much to reduce gear variety. The developers discussed various ways to reduce the necessity of this stat, and ways to stop resistance stacking, when All Res + a single res appeared on the same item. This was especially useful to Monks thanks to the One With Everything passive.
Wyatt Cheng spoke on this in May 2013:
- Wyatt Cheng: Let me comment on One With Everything the Monk Passive. I think the most likely — and I’m definitely not committing to this as a final plan — the most likely solution is to try and get a new set of resistance packages into the game in the future. That’s pretty far down the line probably. It’s part of the itemization changes.
- Those will make the individual resistances roll higher than All Resistance can roll. People think it’s weird that resistance to everything is a bigger number than individual resistances, but let me explain how we got here. History lesson.
- When single resist was way higher during testing, you were incentivized to keep a bunch of varied resistance gear in your inventory and swap it in for specific circumstances. So when you saw a Molten or Plagued or something, you’d change to your item with really big resistance to one modifier. So to change that we made All Resistance with the highest value, and that was kind of better than gear swapping, but I think ultimately we just traded one problem for another.
- When the higher single res values roll out with the itemization changes, you will no longer see single resistances on the same item with all res. And OWE will still be a big benefit, but it won’t be as huge as it can be now. So to use some numbers, say that All Res goes up to 80 on an item, in the future single res will go up to 100 or 115. But you’ll never get both on the same item.
All Res Hard Cap?
Another topic of debate during Diablo 3 vanilla was a hard cap for All Res. Diablo 1 and Diablo 2 had these, with 75% resistance the most possible, though some unique items could raise the potential max resistance higher than that. This limited the utility of All Res gear, since there was only so much that was useful, and beyond that the benefit was wasted. (Though some Diablo II bosses and PvP skills could lower resistance, making it useful to stack higher than 75%.)
Though the developers discussed hardcaps on some affixes during Reaper of Soul's development,  All Res was not amongst them and the developers ultimately ruled out hard caps for almost all affixes, including All Res.
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