Casting speed or Spell haste is a secondary attribute that reduces the casting time for spells and cast-time abilities, and reduces the related global cooldown. It may also reduce casting time for the player's pets (such as for the warlock's Imp).
Spell haste can be found on the character sheet under the Spell heading. It is separate to melee and ranged haste. Spell haste can be acquired from a number of sources; gear, passive bonuses, talent procs, active abilities, buffs and items.
For a more detailed exploration of haste, its sources and effects, see Haste.
Spell haste directly reduces the time it take to cast spells. The cast time of all eligible abilities is reduced by the player's spell haste percentage, and this change is reflected in abilities' tooltips. 50% haste therefore allows a player to deal 50% more damage or healing in the same time, by casting spells at 150% of the speed (base 100% speed plus 50% haste). See section below for calculations.
Haste works just as well for cast-time and channeled spells. In both cases players will see a reduction in their time spent casting/channeling the spell, resulting in a DPS/HPS boost. Haste scales at the same rate for channeled spells and spells with a standard cast time.
By reducing the global cooldown (as well its effects on DoT and HoT effects, see section below) spell haste can be valuable for players who use instant abilities.
Spell haste will only reduce the global cooldown down to a minimum of 1 second. This minimum can reduce the benefit of additional spell haste for casters, by partially preventing them from benefiting from the reduced cast times of their spells. However additional spell haste still has its advantages, reducing likelihood of interruption and benefiting the use of slower (and often more efficient) spells. It also reduces the amount of time before the first cast takes effect, which is advantageous in the opening of duels, split-second make-or-break casts, and in situations calling for sporadic, but quick, healing.
Attack speed is the melee and ranged equivalent of casting speed, but works very differently. Attack speed measures the time between attacks, while casting speed measure's spell haste, with additional haste lowering attack speed but increasing casting speed. A high attack speed therefore means the player will attack very slowly, while a high casting speed means the player will cast very quickly.
Casting speed calculation
Cast Time = Base Cast Time / ((Spell Haste Percentage / 100) + 1)
Note that "base cast time" is after talents. For example, a warlock's has a base cast time of 3.0 seconds. However, if the warlock has spent 3 points in , Shadow Bolt's base cast time will be 2.5 seconds, and this is the number that should be used in any calculations.
Essentially, 1% haste allows a player to cast 1 additional spell in the time it would normally take to cast 100 spells. If the spell is a 3 second cast, that means one can theoretically cast 101 spells in 300 seconds, or 2.97 seconds per spell.
To determine the amount of time taken to cast a given sequence of spells, the reverse formula can be used, where Base Casting Time is the sum of all the spells in the given sequence:
Haste required = ((Base Cast Time / Desired Cast Time) - 1) * 100
2.5 + 2.5 + 1.5 = 6.5 ((6.5 / 5.5) - 1) * 100 = 18.18% haste required
Note: Blizzard rounds to 4 significant digits.
Global cooldown calculation
Haste reduces the global cooldown according to the general formula for spell haste. The base global cooldown for casters is 1.5 seconds.
Global Cooldown = Base Global Cooldown / ((Spell Haste Percentage / 100) + 1)
However, the minimum possible global cooldown is 1 second and haste will not reduce it below this.
The amount of haste required to reduce the global cooldown to a desired value can also be calculated exactly as for spells:
Haste required = ((Base Global Cooldown / Desired Global Cooldown) - 1) * 100
(( 1.5 / 1 ) - 1) * 100 = 50 = 50% haste
50% is the amount of haste required to reduce the global cooldown to its 1-second minimum.
Spell haste can be obtained from a number of sources. Some haste comes from Haste rating, which increases all types of haste, while other sources specifically benefit spell haste without increasing other forms.
Haste stacks in a multiplicative manner. Haste rating is considered a single source, and all haste rating is combined additively with itself before being multiplied with other sources. For example:
1.07 * 1.14 * 1.2 = 1.46 = 46% spell haste
Haste rating can be converted to haste using the appropriate conversion factor:
|Haste Rating Required Per 1%|
|Level 60||Level 70||Level 80||Level 85||Level 90|
For example, for a level 85 priest, with 500 haste rating:
500 / 128.125 = 3.9% haste
The amount of haste rating required to attain a percentage of haste can therefore be calculated:
Haste rating required = (Haste Percentage Required * Conversion Factor)
Without any other sources of spell haste, the following amounts of haste rating are therefore required to reduce the global cooldown to its 1-second minimum:
|Level 60||Level 70||Level 80||Level 85||Level 90|
However, in practice almost all casters will benefit from other sources of spell haste. These may be from passive bonuses (), talent procs () or outside sources, such as party and raid members' buffs (, ).
DoTs / HoTs
Since Patch 4.0 all DoTs and HoTs can benefit from haste. The period of time the DoT / HoT is running remains relatively similar, but additional ticks are added if the haste is high enough. Because only whole ticks can be added, rounding is used to determine the number of ticks. This means that when haste has reduced a spell's duration to a 1/2 tick shorter than its original duration, an extra tick will be added. So it is sufficient to have haste for a half additional tick to get the whole tick.
The haste required to obtain a given number of additional ticks can be calculated as follows:
Haste required = ( ( Base Number of Ticks + Number of Desired Additional Ticks - 0.5 ) / Base Number of Ticks ) - 1 ) * 100
((4 + 1 - 0.5) / 4) - 1) * 100 = 12.5 = 12.5% Haste required
Because this is the necessary haste for 4.5 ticks (rounded up to 5 - so one tick additional to the 4 base ticks), two additional ticks will not require 25% haste but
((4 + 2 - 0.5) / 4) - 1) * 100 = 37.5 %
Essentially, each additional tick after the first can be seen to require ((1 / Base Number of Ticks) * 100) % haste. The first additional tick always requires half this amount. For Rejuvenation:
(1 / 4) = 25% First additional tick = 25 / 2 = 12.5% Second additional tick = 25% additional haste = 37.5% total Third additional tick = 25% additional haste = 62.5% total Fourth additional tick = 25% additional haste = 87.5% total
Many periodic effects have a maximum tick rate of 1 tick per second. This limits spell haste's benefit to DoT and HoT effects at just under 200% haste. The exact cap can be found with (200 - ((1 / Base Number of Ticks) * 50)). For (with a base number of 4 ticks):
200 - ((1 / 4) * 50) = 187.5% spell haste
Haste Rating used to be split into "Haste Rating" (which only affected attack speed) and "Spell Haste Rating" (which only affected spell casting speed). The two were unified in patch 3.0.2 into one universal Haste Rating stat. All gear which used to have Spell Haste Rating now has an equal amount of Haste Rating.