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==
Prior to Cataclysm, a target of higher level than its attacker would gain a kind of hidden, intrinsic spell resistance to all schools of magic.
Miss chance
Level 	PvE 	PvP
Vs. +0 level (same level)	4% 	4%
Vs. +1 level 	5% 	5%
Vs. +2 level 	6% 	6%
Vs. +3 level 	17% 	13%
Vs. +4 level 	28% 	20%
Each additional level 	+11%	+7%
The effective resistance rating of the target was 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 was the same as the base resistance (from the character sheet).
Note that this formula renders level 83 bosses as having had 15 magical resistance against level 80 player attacks. Unlike all other magical resistance in the game, this resistance from level difference could not 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 was believed to be 510.
Note: Level-based resistance (not to be confused with level-based miss) could play a factor in total resists. For every level that a mob had over the player, there was 8 resist (believed; the exact number may have been higher) added. For boss fights, this means there was 15-24 resistance added. This extra resistance meant there would be partial resists on non-binary spells from the added resistance. However, this resistance had 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.
==Sources==
Prior to patch 5.0.4, Resistance could be obtained from buffs, potions, certain class talents, racials and gear.
As of patch 5.0.4, player resistances have been removed from all but certain legacy armor items. 
===Stacking===
Each resistance can be a stack of the following:
* Any number of self-only buffs, such as Shadow Resistance (racial) and Mage Armor.
* Any number of equipped items and enchants, including cooldowns and procs.
* Only one aura, or raid-wide buff (e.g. .
* Only one elixir or flask.
==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—such as slow, root, stun—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 between 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.
==Patch changes==
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::*The combat log now differentiates between a spell failure due to resistance and spell failure due to missing the target. Where once both events reported as a resist; a spell missing the target is now reported as a miss.
::*When a source of damage is entirely prevented (by a shield block, a full resist, or a damage shield like Power Word: Shield), the prevented amount will now be displayed.}}
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==References==
Category:Attributes
Category:Combat
Category:Game terms
Category:Gameplay
 
 
Category:Tanking
Category:Formulas and game mechanics