Spell hit

Spell hit is an attribute that increases a caster's chance to hit with spells. Spell hit is primarily obtained from gear, gems or enchants that have hit rating. The more hit rating a caster has, the lower the chance the caster's spells will miss. Spell hit is capped at 100%: with enough spell hit, it is possible to reach a state where it is impossible for spells to miss. Healing spells always hit, thus healers do not need spell hit. A spell that fails to hit is shown as a "miss", and can be corrected by acquiring more hit rating, while misses as a result of a target's resistance are shown as a "resist" and can be somewhat corrected for with spell penetration, which is a different attribute.

Both physical hit and spell hit derive from the same basic hit rating stat within the combat rating system, although the similarity largely ends there&mdash;they are effectively distinct attributes derived from the same rating. Prior to Patch 3.0, hit rating only increased a player's chance to hit with melee and ranged attacks; a separate stat, called spell hit rating, increased the chance to hit with spells. 3.0 unified both ratings together, and now hit rating increases chance to hit with both spells and melee/ranged attacks.

Calculation
The spell hit mechanic was originally confirmed by Blizzard CM Eyonix back in 2005. Base chance to hit is based on level difference between caster and target, starting at 96% for equal levels, and going up or down from there, capping at a maximum of 100%. Furthermore, the chance to hit is different for mobs (PvE) and player (PvP) targets.

For example, a level 80 caster attacking a level 83 mob will have a 83% chance to hit with spells in PvE. Players many levels below their target will always have at least a 1% chance of landing a spell.

Increasing spell hit
Spell hit chance can be increased by talents or gear up to 100%. At level 85, the highest level mob currently present is level 88 (e.g. all Cataclysm raid bosses), so assuming no other sources of spell hit, a level 85 caster has an effective hit rating cap of 1742.

In patch 4.0.1, all talents and debuffs increasing spell hit chance for casters were removed. The draenei racial trait Heroic Presence was also changed from increasing the party's hit chance to only increasing the player's hit chance. Death knights also automatically possess +9% spell hit as a result of their Runic Focus passive. Rogues can also increase the hit chance of their poisons (which are considered to be spells) through the talent Precision (rogue talent).

Balance druids, elemental shaman, and shadow priests now gain up to 100% of their spirit as spell hit rating with the talents Balance of Power, Elemental Precision, and Twisted Faith respectively.

With talents and gear combined, a player's chance to hit with spells can reach 100%. Casters should keep this cap in mind when considering their gear. For PvE raiding, bosses are considered to be 3 levels higher than the caster, meaning that +17% is the most that will be effective. For high level arena PvP sufficient +hit chance is very important as missed crowd control like Polymorph, Fear and Cyclone can easily lose matches. The base miss chance versus any player target of the same level is 4%, however certain races have an increased 2% chance to be missed by certain spell schools (nature for tauren/night elves, shadow for draenei/undead, arcane for gnomes, frost for dwarves, and all spells for blood elves). For this reason, max hit can range from 4%-6% depending on what spells you intend to use.

Spell penetration can also be used to increase the chance that spells to land or to land unresisted in PvP, but resistance is a different attribute than spell hit chance. Note that some classes can spec to gain extra chance to have spells miss them, which CANNOT be mitigated by additional chance to hit. Also, some abilities (e.g. - Cloak of Shadows) give the player using the ability extra chance to have spells miss as well. Extra spell hit does in fact mitigate these spells' extra chance to miss, but is typically unnecessary.

Effect of resistance
Spell resistance has a separate effect that is considered in addition to spell hit chance, and is based on a resistance stat for a specific school. In general a certain amount of resistance will mitigate a certain percentage of damage from a magic school. For binary spells that do not partially resist, the resistance mitigation is combined with the spell hit chance to give an overall percentage for the spell to land or be resisted. The chance to hit is multiplied by the mitigation factor from the resistance to give an overall chance to hit. For the following formula, B is base chance to hit with spells, H is increased chance to hit from gear and talents, and R is the mitigation factor given from resistance.

(B + H) * R

For example, a caster with 6% increased chance to hit, attacking a mob 3 levels higher (83% base chance) that has enough resistance to mitigate 50% of spell damage, the overall chance to hit with a binary spell is 44.5%

(83% + 6%) * 50% = 44.5%

For non-binary spells, resistance is computed as a second calculation after the spell hit chance is rolled. After a spell hit is determined, a partial resist calculation based on resistance is done, which can also result in a full resist. These two resists once appeared identically in the combat log, and there was no known way to distinguish a spell miss from a full resist. However, as of patch 3.0.2 the combat log now distinguishes between a spell miss and a resist.

The effect of Hit chance on Critical Hit chance
Unlike the melee combat system, spell crit makes absolutely no difference to hit chance. All spells, regardless of whether they are treated as binary or not, roll hit and crit separately. Conceptually, the game rolls for your hit chance first, and if the spell hits you have a separate roll for whether it crits.

Overall chance to crit over all spells cast is thus affected by hit rate. To calculate overall crit rate, multiplying the two chances together:

crit rate over all spell casts = crit * hit

For example, a caster with no spell hit rating gear or talents, against a mob 3 levels higher (83% hit chance), and 30% crit rating from gear and talents:

crit rate over all spell casts = 30% * 83% = 24.9%

In addition, direct damage spells suffer from partial resistance, but again, that has no effect on whether a spell hits or not.

Hit rating chart
The following chart shows the total hit rating required to 2 decimal places for the range of spell hit % commonly desired. It is obviously not possible to achieve a fractional hit rating, and the decision to round up or down to an integer is left to the individual. See the hit rating caps for level 80 below for the rating required to reach 100% hits with spell casting.

Hit rating caps for level 80
A common question among casters is how much hit rating is needed to hit the 100% chance to hit cap. As stated earlier, this depends on the level of the targets one expects to encounter, and any talents that add chance to hit with spells. The following table summarizes how much hit rating is needed for a certain level, after all talents and buffs are taken into consideration. To use the table, first add up how much hit you already have, then look up that row to find the hit rating cap for targets level 80 up to 83. End-game raiding casters generally use the level 83 column, as the effective level of bosses is 83 (level 80 content). However, many boss fights involve killing level 82 and below adds. An example is the Dreamwalker fight in ICC. It can be argued that a 446 hit rating against level 82 mobs is not an efficient allocation of stats.

''Note: For casters doing heroics exclusively and not planning on raiding, you may use the 82 column instead, as that is the highest level of mobs found in heroics (i.e., bosses).

The hit rating numbers in the above table are all rounded up to the next integer number. E.g. 26.23 (1% at 80) becomes 27, 393.47 (15% at 80) becomes 394. While this may seem counter-intuitive at first, the intention is to provide the number which will give 100% spell hit (i.e. the "hit cap"). If they were rounded to the nearest integer, some would still leave a small chance to miss, e.g. rounding 393.47 to 393 only provides 14.98%, leaving a 0.02% chance to miss. It is up to each individual to determine whether they are willing to tolerate that very small chance to miss, or if they wish to completely eliminate it. The assumption in the table is that it is desirable to guarantee that spells will not miss. The hit rating chart above provides the ratings to 2 decimal places for those that wish to consider the merits of rounding up or down in more detail.

As a rule of thumb, a level 80 player against a level 83 boss needs +26.232*k hit rating, to achieve +k% chance to hit with spells. That means that on 80-vs-83 fights each point of +hit rating contributes about +0.038% chance to hit with spells.