Because I am very, very tired of people who link to this page and totally misunderstand it and misinterpret it to their guilds, this next piece has been added in big letters.

There are no tiers. There are no “break points.” Blizzard’s “good/very good/etc” means nothing, it’s a thumbnail commentary, not of any real value… it’s like if I said, well, 0% dodge is lousy, 10 is ok, 20 is pretty decent, 30 is great. Every single point of resistance from 1 to 300 (315 on +3 mobs) contributes towards your resistance. Each additional point is more powerful than the point before it. The last 15 points provide life expectancy extension equivalent to about the first 140 points.
We now return you to our usual webpage.

Spell Resistance is important. Spell Resistance is what makes you not become dead when the angry dragon looks at you. Spell Resistance is a pain in the ass. It makes you fill up your bags, and prevents you from using all the shiny epics you’ve spent so much time getting. But how does it work?

First off, plain and simple. Resistance numbers calculate your *average* resistance. It won’t be the same every time, but over time, it will average to that amount. There is a 75% cap on resistance averages. Anything beyond 315 of a given resist is currently useless (unless the resistance is debuffed somehow) until mobs higher than level 63 enter the game.

Here is the basic formula used to calculate your average resist percentage:

75*MIN(resistYouHave,5*mobLevel)/(5*mobLevel) = Average Resist Percentage

So for example, a MT with 315 resist fighting a level 63 mob (such as Ragnaros) — 75*MIN(315,5*63)/(5*63) = Average Resist Percentage — 75 * 315/315 = 75 Average Resist Percentage

Say you’re a rogue, with 200 fire resist — 75*(200)/315 = 47.62% Average Resist Percentage

Great. So what on earth does all this mean?

Well, the point of resistances is to decrease the damage you take from magical attacks (and of course, to resist other harmful effects). Because of how resistances work, the improvement ramps up as you get more. For example, at 105 resistance, someone with 4000 HP can take 5334 HP worth of pre-resist damage. So the first 105 resist is worth 1334 HP. But at 210 resist, someone with 4000 HP can take 8000 HP worth of pre-resist damage. So the second 105 resist is worth 2666 HP. And at 315, the maximum, someone with 4000 HP can take 16,000 HP worth of pre-resist damage. So the last 105 resist is worth 8000 HP worth of pre-resist damage. The last 15 alone is worth 2000 HP. By comparison, the first 15 is worth 149 HP. You can see the chart below for more information; and click it to download a spreadsheet.

So… how do you apply this knowledge? It’s simple. If your resistance is going to be negligible, don’t gimp your other stats, particularly stamina, to attain it. But once you’re past 210, resist-only items with no stats become significantly more viable.

Of course, you’ll want to take into account things like totems and auras. A tank shouldn’t need more than 255 unbuffed resistance; since a shammy totem or pally/hunter aura will add 60. But if you’re a rogue, you may not always have that option available to you, and you should have gear to compensate. Should you happen to get the aura, you can switch some of that gear out for higher damage equipment without losing any resistances. You should have Mark of the Wild, though, and that provides 20-27 resist (which does not stack with the auras)
As for “How much do I need?”, it depends on the encounter and the role and how good your stats are with the gear.

For example, on Ragnaros, our tanks go for 315 buffed, our rogues try for 150-200, and the rest of the players put on whatever fire resist they have which does not gimp their mana pool for healers and their damage output for DPSers. For Vaelstraz, the healers put on every bit of fire resist they can, to get as close to 315 as possible (no auras because the pallies will have concentration aura on to prevent interrupts); the tanks have 315; the rogues actually we’re trying with their normal DPS gear because the priests have infinite mana and holy nova is instant cast; and the ranged are much the same but they put on any bits that don’t gimp their damage.