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. 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. (It is yet uncomfirmed as to whether additional hit rating will overcome this difference for binary spells and whether this applies to PvP level difference as well.)

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

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

The result of this second formula is capped at 75%. Hence, the effective resistance value is capped at caster level * 5. For example, against a level 83 boss, 83 * 5 = 415 resistance has the maximum effect.

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, indicated by the tables below. This chance to miss is combined with the spell hit roll and thus additional spell hit will help mitigate this lost chance to hit (for PvP, mostly).

For non-binary spells

Resistance increases the chance to suffer only a fraction of the spell's normal damage. In The Burning Crusade, the percent resisted had a chance to be 0%, 25%, 50%, 75%, or 100%.^[1] The probabilities are weighted such that the average damage mitigated works out to the same percentage as with binary spells, indicated by the tables below. In Wrath of the Lich King, however, the possible percentages of damage resisted are weighted in sections of 0%, 10%, 20%, ..., 100% averaging to the same overall damage mitigation. DoT spells are now considered non-binary and the partial resistance calculation is applied on each tick.

Mitigation Percentage Tables

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

Damage Reduction	Resistance Needed					Partial Resist Amount
Damage Reduction	Lv. 60	Lv. 70	Lv. 73	Lv. 80	Lv. 83	100%	75%	50%	25%	0%
75%	300	350	365	400	415	25%	55%	16%	3%	1%
60%	240	280	292	320	332	11%	34%	40%	14%	1%
45%	180	210	219	240	249	1%	18%	48%	26%	7%
30%	120	140	146	160	166	1%	6%	24%	49%	20%
15%	60	70	73	80	83	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.

Example: With 300 effective resistance vs. a level 83 caster, this formula yields 75 * 300 / (83 * 5) = 54.2%. This means that an average of 54.2% damage will be mitigated and binary spells have 54.2% less chance to hit.

Sources of Resistance

	School	Value
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
Undead	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 and binary spell roll, and will not contribute to the non-binary damage mitigation.

Stacking

The following list contains sources of resistance and stacking information. Each line will stack with other lines.

Self-only buffs, such as [Demon Armor] or [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 [Magic Resistance Potion] do not stack with themselves - only the highest single effect of these will apply;
Elixirs and Flasks providing resistance do not stack with themselves.

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 of Resistance

The damage absorbed per point of damage/healing/tanking stat lost scales up signifiantly. So, although it might be tempting to wear as little resistance gear in favor of boosting class stats, it is typically much more beneficial to the raid as a whole to wear as much resistance up to the cap as possible for these fights.

For example, DPS wearing a full 415 frost resistance set for Sapphiron would be mitigating 75% of incoming damage and require 75% less healing. However, the stats lost in doing so would not come close to removing a full 75% of their DPS. This means that the healers will be able to maintain their mana long enough for the DPS to do more damage than if no resistance gear was used.