Resistance

Resistance is a combat statistic that mitigates incoming magic damage and effects. Magic spells from the following five schools can be resisted from magical resistance: Fire, Frost, Arcane, Nature, and Shadow. There is no resistance stat for Holy, and as such no players nor NPCs can resist Holy spells. A character's current resistance score to each school is advertised on the character sheet.

In PvE, resistance is highly situational and is typically only needed for certain boss encounters. The first one of this type was Ragnaros, famous for requiring significant fire resistance on all raid members. Most recently, in Wrath of the Lich King, Sapphiron is made much easier with Frost resistance gear (although this is not necessary).

In PvP, resistance is helpful against spellcasters, although resilience is generally considered to be a more useful stat when choosing between the two.

Effective resistance and average mitigation
The effective resistance rating of the target is calculated as follows: effective resistance rating = Rb + max((Lt - Lc) * 5, 0) - min(P, Rb) Rb - target base resistance (as advertised on the character sheet) Lt - target level Lc - caster level P - caster spell penetration

In PvE boss fights, the player level is usually equal to or less than the boss level, and no spell penetration is in effect. Therefore, in this situation, it's safe to assume that the player's effective resistance is the same as the base resistance (from the character sheet).

Note that this formula renders level 83 bosses as having 15 magical resistance against level 80 player attacks. Unlike all other magical resistance in the game, this resistance from level difference cannot be overcome by spell penetration.

The percentage of magical damage that is mitigated on average is calculated by the following equation (note that this has changed in Wrath): Damage reduction percentage = 100% * effective resistance rating / ( K + effective resistance rating )

For level 80 mobs this constant K is 400. For level 83 boss mobs K is believed to be 510.

Note: Level-based resistance (not to be confused with level-based miss) can play a factor in total resists. For every level that a mob has over the player, there is 8 resist (believed; the exact number may be higher) added. For boss fights, this means there is 15-24 resistance added. This extra resistance means there will be partial resists on non-binary spells from the added resistance. However, this resistance has been shown to not apply to binary spells at all.

Binary and non-binary resist
Spell resistance is the second of two rolls to determine whether or not a spell is resisted. (Spell hit is the first roll.) The spell resistance roll will calculate in one of two different ways: one way for Binary Spells, another way for non-Binary spells. Non-binary spells are spells which only deal damage (like Fireball or Shadow Bolt). Binary spells are spells which deal a debuff (like Frost Nova or Vampiric Embrace) or, for some other reason, must take full effect or no effect at all.

Resistance reduces the chance for the binary spell to land by a certain percentage. Spell hit will not reduce this chance. It is assumed that this percentage is exactly the damage reduction percentage given above.

Resistance increases the chance to suffer only a fraction of the spell's normal damage. In Burning Crusade, the percent resisted had a chance to be 0%, 25%, 50%, 75%, or 100%. The probabilities were weighted such that the average damage mitigated worked out to the same percentage as with binary spells. In Wrath of the Lich King, the possible percentages of damage resisted are weighted in sections of 0%, 10%, 20%, ..., 100% averaging to the same overall damage mitigation. However only the closest three or four sections will ever occur at a given resistance value. This allows a sufficiently high resistance to guarantee a certain resist percentage, even in the worst case. DoT spells are now considered non-binary and the partial resistance calculation is applied on each tick.

The probability of an x% resistance is given by the following formula (AR is the average resistance calculated above):

P(x) = 50% - 250%*|x - AR|

If the probability is less than 0, it cannot occur.

Stacking
Each resistance can be a stack of the following:
 * Any number of self-only buffs, such as Mage Armor.
 * Any number of equipped items and enchants, including cooldowns and procs.
 * Only one aura, shaman totem or raid-wide buff (e.g. Gift of the Wild, Prayer of Shadow Protection), OR a.
 * Only one elixir or flask.

Example: In a group where Mark of the Wild, Aspect of the Wild, and Nature Resistance Totem is active, the group will only receive 130 Nature Resistance (either from the aspect or totem).

Effectiveness
The damage absorbed per point of damage/healing/tanking stat lost now decreases as resistance is gained, similar to armor. However guaranteeing a certain resistance percentage can be valuable, especially in fights with a large spike of elemental damage.

Magical resistances


There is a roll based on the level of the caster and the resistance stat of the target. As with armor, the level of the target is not a factor in this roll.

Average Resistance = (Target's Resistance / (Caster's Level * 5)) * 0.75

Average resistance may be no higher than 75%. Of course, what it takes to reach 75% average resistance depends on the spellcaster's level. One quirk here is that casters below level 20 are treated as if they were level 20 for purposes of calculating the resistance scaling here. So against a spellcaster of level 20 or below, it would take a resistance stat of 100 to get 75% resistance; level 30 would take 150 resistance; level 50 would take 250; level 60 would take 300; and level 63 would take 315.

Why is it called "average resistance"? For spells that have a non-damage effect&mdash;such as slow, root, stun&mdash;you'll either take the hit or avoid the hit altogether; these are examples of binary spells. The average resistance is the chance you'll totally avoid the hit. Adding the word "average" is a little redundant in this case.

However, when spells do only damage, it's not just a full hit or full miss situation. Pure damage spells can be resisted for 0%, 10%, 20%, 30%, etc. (in 10% increments up to 100%) of their regular damage. Your average resistance can still be anywhere betweeen 0% and 75%. If your average resistance is maxed out, then there's a really good chance of having 70% or 80% of the spell's damage be resisted. There's also a fairly good chance of having 100% of the spell's damage be resisted, a slightly lower chance of 50% of its damage being resisted, a small chance of only 30%, and a very small chance of even 0% of the damage being resisted. It's a weighted average. Visualize it as a bell curve around your average resistance.

Mobs known to have base Resistance
While some mobs are completely immune to effects from one spell school or another, a very few are not immune but instead have a base Resistance to one or more spell schools. Note that this is an incomplete list, and always will be, since few mobs have Resistance and tools to gather this data are either rare or nonexistent.
 * Doomwarder (level 35-36 in Mannoroc Coven): Fire Resistance
 * Heckling Fel Sprite (level 58 in Hellfire Peninsula): Fire Resistance