Resistance

Resistance is a combat statistic that mitigates incoming magic damage and effects. Magic spells from the following five schools can be resisted: Fire, Frost, Arcane, Nature, and Shadow (this means Holy spells cannot normally be resisted). A characters current resistance score to each school is advertised on the character sheet.

Resistance is a highly situational stat, which is needed only very rarely (but when it's needed there is no way around it). Players usually store several sets of gear, one for each school of magic. It's most frequently needed in PvE boss encounters, the first one of this type was Ragnaros, famous for the need for significant fire resistance on all raid members. Some amount of resistance can also help in PvP if expecting to face spellcasters (although Resilience is generally considered to be a more useful stat in PvP).

Spell resist mechanic

Magic spells can be resisted in two different ways. The first chance for a spell to fail uses the level difference between the caster and the target, and also incorporates the spell hit rating stat. This first check is discussed in more detail in the article for spell hit.

The second chance to resist takes into account the amount of resistance to the school of the spell. Each point of resistance effectively mitigates a certain percentage of magical damage, or reduces the chance for a magical debuff to take effect.^[1]

Formulas

The effective resistance value of the target is calculated as follows:

effective resistance value = 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 target level is usually equal to or less than the caster level (when the boss attacks a player), and no spell penetration is in effect. Therefore in this situation it's safe to assume that the effective resistance is the same as the base resistance.

The percentage of magical damage that is effectively mitigated is calculated by the following equation:

Damage reduction percentage = (effective resistance value / (caster level * 5)) * 75

The result of this second formula is capped at 75%, thus an effective resistance value greater than caster level * 5 is pointless.

Example: With 300 effective resistance vs. a level 70 caster, this formula yields 300 / 350 * 75 = 64%. This means that 64% of all effects are resisted in the average.

Binary spells and partial resists

There are two different mechanics how spells can be resisted. Spells which deal damage only (like [Fireball] or [Shadow Bolt]) can be resisted partially. Spells which deal a debuff (like [Frost Nova] or [Vampiric Embrace]) or some other effect can either be resisted fully or not at all (these are binary spells).

In the binary case, it's rather straightforward - in the average, a target with 300 arcane resistance facing a level 70 mage is completely unaffected by 64 of 100 Slows, while 36 take full effect.

For pure damage spells the mechanic is more complicated. A Fireblast which nominally would hit for 1000 damage can in fact do exactly one of the following amounts of damage: 0, 250, 500, 750 or 1000. Over many spells cast, the percentage of damage avoided matches the result of the above formula, this means if 100 fireblasts for 1000 damage each are thrown by a lvl 70 mage at a target with 300 fire resistance, the target takes 36,000 damage overall while resisting 64,000. For DoT spells, the partial resistance calculation is applied on each tick.

The following table gives the probabilities for various partial resist outcomes depending on chosen resistance percentage and caster level.^[2]

Damage Reduction	Resistance Needed			Partial Resist Amount
Damage Reduction	Lv. 60	Lv. 70	Lv. 73	100%	75%	50%	25%	0%
75%	300	350	365	25%	55%	16%	3%	1%
60%	240	280	292	11%	34%	40%	14%	1%
45%	180	210	219	1%	18%	48%	26%	7%
30%	120	140	146	1%	6%	24%	49%	20%
15%	60	70	73	0%	2%	11%	33%	54%

Since 75% is the maximum effect which can be gained from resistance, the first line of this table shows the effective resistance caps for various caster levels.

Sources

	School	Value
Blood Elf	All	+ 5
Draenei	Shadow	+ 10
Dwarf	Frost	+ 10
Gnome	Arcane	+ 10
Night Elf	Nature	+ 10
Tauren	Nature	+ 10
Undead	Shadow	+ 10

Resistance can be obtained from buffs, potions, racial abilities, certain class talents, and most frequently from gear. As detailed in the resistance equipment article, gear (and enchants) should be chosen carefully. When increasing their resistance scores, players need to be aware that they lose other desirable stats, and that by wearing low-quality resistance gear they can actually be worse off than wearing none at all.

Stacking

The following list contains sources of resistance and stacking information:

Racial bonuses always count
Self buffs, such as [Demon Armor] or [Mage Armor] do stack independently of other bonuses
Equipped items and enchants stack freely
Only one Elixir or Flask providing resistance can be in effect at a time
Auras, totems, and buffs do not stack - the highest single effect of this type applies

Example: A hunter using [Aspect of the Wild] with +70 nature resist makes any other auras in the group useless (unless the hunter dies). It however fully stacks with equipment bonuses as well as a racial bonus and a guardian elixir.