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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, 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.
In Mists of PandariaThis section concerns content exclusive to Mists of Pandaria.
Spell resistance (elemental and otherwise) is being removed as a stat and mechanic in Mists of Pandaria according to Ghostcrawler (Greg Street). Zarhym later explains that stats being sacrificed to add resistance which was only meaningful in narrow situations was a big part in removing this mechanic from the game.
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. 
Binary Resist 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.
For binary spells
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.
For non-binary spells
Resistance increases the chance to suffer only a fraction of the spell's normal damage. As of 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.
In CataclysmThis section concerns content exclusive to Cataclysm.
Partial resists no longer occur against bosses.
In Burning CrusadeThis section concerns content exclusive to The Burning Crusade.
In The 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.
Sources of Resistance
|Blood Elf||All||+ 2% miss|
|Draenei||Shadow||+ 2% miss|
|Dwarf||Frost||+ 2% miss|
|Gnome||Arcane||+ 2% miss|
|Night Elf||Nature||+ 2% miss|
|Tauren||Nature||+ 2% miss|
|Forsaken||Shadow||+ 2% miss|
Resistance can be obtained from buffs, potions, certain class talents, and gear. Racial abilities no longer grant magical resistance and instead grant a percentage chance to miss, as indicated by the table to the right. This chance to miss is added to the spell hit roll, and will not contribute to the non-binary damage mitigation.
The following list contains sources of resistance and stacking information. Each line will stack with other lines.
- Self-only buffs, such as Mage Armor stack freely;
- Equipped items and enchants stack freely;
- Auras, Shaman totems, raid buffs (e.g. Gift of the Wild, Prayer of Shadow Protection), and do not stack with themselves - only the highest single effect of these will apply;
- Elixirs and Flasks providing resistance do not stack with themselves.
Effectiveness of Resistance
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.
- ^ Ghostcrawler 2012-03-02. Dev Watercooler – Mists of Pandaria Stat Changes. Official World of Warcraft Community site (US).
Ghostcrawler 2012-03-02. Coffee With the Devs — Mists of Pandaria Stat Changes. Official World of Warcraft Community site (EU).
- ^ Zarhym 2012-03-06. #154 - Elemental Damage Should Matter. Official Mists of Pandaria forum (US).
- ^ a b Elitist Jerks. Resistance Mechanics in WotLK - Elitist Jerks. Retrieved on 2009-12-15.
- ^ ieatpaperbag 2011-12-05. Combat Guide for Cata [12/01/2011]. ElitistJerks.com Rogues forum.
- ^ Blizzard Official US site. Resistances. Retrieved on 2007-09-05.