Casting speed
From WoWWiki
Casting speed is the time it takes for a spell to take effect. Some spells are "instant" and take effect immediately. Some spells are "channeled" and have their effects occur over the duration of the cast. Others take effect after a specific "cast time" with typically more powerful spells having longer cast times. Regardless of the type of spell, there is a global cooldown which prevents rapid spellcasting even with "instant" spells.
Casting speed affects the cast time of spells. It is determined by several factors. The base "cast time" is specified in the spell. Most classes have talents which can reduce the cast time of a spell (eg. Light's Grace and its effect on Holy Light), a class of spell (eg. Divine Fury), a school of spell (eg. all holy spells), or all spells in general (eg. Martyrdom and the Focused Casting effect). Additionally, some classes have spells which temporarily reduce the cast time of subsequent spells (eg. Nature's Swiftness). Spell haste is calculated based on modified casting speed, i.e. with the Bane talent for warlocks, the spell haste would be calculated with 2.5 second Shadowbolts rather than 3 seconds.
Finally, certain items provide a "spell haste" bonus or a "spell haste rating" bonus. Spell haste rating is used to determine the characters spell haste bonus. At level 70, 15.7 (as of 2.2) points of spell haste rating grants 1% spell haste bonus. As of 2.4.0 spell haste also influences spellcasting global cooldown.
Certain debuffs (eg. Curse of Tongues, Mind-numbing Poison) can decrease your casting speed.
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Notes
Because of the general global cooldown on spells of 1.5 secs, spell haste effectively had a cap on it for casters. This has changed with patch 2.4 -- spell haste also reduces the global cooldown for spells, to a minimum of 1 second. However, spell haste still reduces the actual time spent casting which can be advantageous when dealing with spell push back or trying to avoid interrupts. [1]
Spell haste also reduces the amount of time before the first cast takes effect, which is advantageous in the opening of duels or in situations calling for sporadic, but quick, healing.
Channeled spells are affected by haste and will see a reduction in their time spent channeling the spell but no change in overall damage, resulting in a DPS boost. Additionally, haste scales at the same rate for channeled spells and spells with a standard cast time.
Casting speed calculation
Speed = 1 + (Spell Haste Rating/1570)
New Casting Time = 1 / (Speed * (1 / Base Casting Time))
or
New casting Time = Base Casting Time / Speed
or
New Casting Time = Base Casting Time / (1 + (Spell Haste Rating/1570))
or
New Casting Time = (Base Casting Time * 1570) / (1570 + Spell Haste Rating)
Note that "base casting time" is after talents. For example, a warlock's shadow bolt is 3.0 "base cast time" under normal usage of the term "base cast time". However, for the purpose of this formula, you should use the improved cast time from your talents (2.5 cast time, in this example) if you have that talent, or the formula will not give the correct result.
This may seem counter intuitive at first. Speed is not the same thing as casting time. Speed is a measurement of casts per second, while casting time is seconds per cast. Haste is a multiplier applied to speed, in exactly the same way mounted speed is calculated. That means if one has 10% haste, cast speed is 110%. In order to correctly calculate the new casting time, it must be inverted (giving you the spell's speed), multiplied by the speed percentage, and then inverted again. This figure is the spell's new casting time.
The formula is (very slightly) not linear. The less haste you have, the more a single point of haste is worth. For a 2.5 cast speed (sorry, I play a warlock) 1 point of spell haste = 0.00159 seconds removed from the cast time. 200 points of spell haste makes each point worth 0.00141 seconds per point. This might seem like it has diminishing returns, however, this is not the case. If we divide DPS increase by haste points, the single point is a 0.063694267% increase in DPS per haste point, and the 200 points are also 0.063694268% per point. Practically speaking it will give you a constant increase in DPS per point, something to keep in mind when selecting your gear!
Essentially, 1% haste means 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:
Spell Haste Rating = 1570 * ((Base Cast Time / New Cast Time) - 1)
Example: I want my Fireball (3s), Fireball (3s), Scorch (1.5s) cast chain to take 6.5 seconds. Therefore, a [1570 * ((7.5 / 6.5) - 1) = ~242 Spell Haste Rating] is required.
Note: Blizzard rounds to 4 significant digits.
Improving casting speed
Various sources that improve casting speed are listed on the Casting Speed improving items page.
Note: Haste rating stacks additively with itself but haste stacks multiplicatively. That means that if you have 157 haste rating, you will have +10% haste, no matter how many sources and items that haste rating comes from. If you then use troll Berserking for +30% haste, you would have 110% * 130% = 143% haste.
Losing casting speed
Chance of Ignoring Spell Interruption When Damaged = 100% - Chance to Be Interrupted By Damage
The chance to have your spells delayed by damage is additive. If you have a talent for 70% chance to avoid spell interrupts, and a paladin with concentration aura (another 35%) is in your party, you will not receive any spell interrupts from damage.
Global Cooldown
Using the formulas above, it would require 785 Spell Haste to bring the Global Cooldown down to 1 second.
- 1.5 / (1 + (785 / 1 570)) = 1
