Spell critical strike

Spell critical strike (often abbreviated as "spell crit") is when a spell or magical effect (most damaging spells, healing spells, some damage over time effects) has an increased effect. To see your chance to critically strike with spells, open your character pane and view your spell stats. Mobs are unable to get critical strikes with spells.

Spell critical strike chance is gained from both critical strike rating and intellect, as well as various talents and abilities.

Spell crit damage
At base effect -- without talents, talent-tree primary skills, class skills or abilities, meta gems, item bonuses, or other enhancements -- spell critical strikes deal either 150% (druids, priests, shaman, and all pets and minions) or 200% (mages and warlocks) of normal damage or healing. Hybrid DPS casters (Shadow/Balance/Elemental) have passive specialization bonuses that increase their critical strike multiplier to 200%.

With damage over time, heal over time and area of effect spells, the critical strike chance applies independently to each tick and each target.

Certain metagems, such as the Chaotic Shadowspirit Diamond, increase spell critical strike damage beyond 200%.

Critical heals only heal for 150% of their normal amount.

Base spell crit chance
The base chance to critically strike with spells is based on the Intellect attribute and a class specific constant.

Increasing spell crit chance
Spell critical strikes have a generally low chance of occurring, however, you can increase your crit chance in certain ways:


 * Attributes
 * Talents (some classes)
 * Equipment
 * Gems
 * Enchantments
 * Glyphs
 * Potions (temporary)
 * Buffs (temporary)

Critical strike rating
The easiest method to increase the chance of a critical strike is to equip gear with the attribute "Increases your critical strike rating by 'x'." For some classes, critical strike rating is of less value. Unlike Intellect, critical strike rating increases the chance to get a critical strike equally for all classes.

Formula
For a level 85 character: crit chance % = (intellect / 648.91) + class constant + (crit rating / 179.28)

Note:
 * Talents that affect the crit chance of "all spells" (e.g.: Piercing Ice), or talents that specifically affect spells of a certain school, directly add to the calculated "Crit Chance %" on the character pane.
 * Talents that only affect the spell crit chance of a specific spell are NOT included in the spell "Crit Chance %" on the character pane.
 * Mousing over the "Intellect" value in your character pane will show you the Intellect portion of your spell "Crit Chance" (eg: "Increases Spell Critical Hit by x.xx%"). This works out to the "(INT/648.91)+ClassConstant" portion of the formula.

Using spell crit effectively
When evaluating the tradeoff between critical strike rating and other secondary stats, there are some things to consider:
 * Some classes have abilities or talents which proc off of spell critical strikes, as do some items.
 * Spell crit can lead to more unpredictable damage. As such, it can be powerful, but unreliable.
 * In a group, a string of "lucky" spell crits can pull aggro off the tank, possibly resulting in a dead caster.

Spell hit or spell crit?
All other things being equal and without considering talents, an 1% increase in spell crit chance will result in a gain of 0.5% of a spell's base damage, whereas a 1% increase in spell hit chance will give a 1% increase of a spell's base damage. As long as no other effects come into play (like Ignite, which would make +crit slightly more important), +hit rating is more effective than +crit rating. But although increasing +hit over 99% is not a complete waste if using binary against a resist chance (since in that case the resist chance works as a reduction of the hit chance), in general a 99% spell hit chance will be sufficient for most casters.

There is still debate whether spell casting is based on a one-roll system (like "normal" melee) or not, although there is currently overwhelming evidence that it is a "two-roll" system - hit rolled for first, followed by a roll for crit.

Another important factor to consider are the many effects which can proc from spell crits, e.g., Mages saving mana on spell crits with Master of Elements.