Mana

Mana is a magical power resource available to casters which they draw upon when casting spells. It has some parallels with the warrior's rage, the rogue's energy, the hunter's focus and the death knight's runic power. The amount of mana currently available to the caster is denoted with a special blue mana bar below their health bar. By default, casters start with a full bar of mana which is then depleted as they cast spells.

Characters have an innate replenishment of both health and mana. The rate at which mana replenishes (mana regeneration) is based on the Spirit attribute. Some passive abilities can raise the replenishment rate for these stats independently. During combat mana regeneration is reduced, but some mana can be replenished instantly during combat using mana potions from alchemists or from various magical items. Out of combat, mana regeneration can be increased significantly by using various drinks. Mana regeneration can also be increased through the use of certain buffs.

Since patch 5.0.4, the maximum amount of mana available to a caster is determined by their spec and level, although certain modifiers such as  [Ember Shadowspirit Diamond] can increase it by a small amount.

Prior to Mists of Pandaria the maximum amount of mana available to a caster was determined by a combination of the character's class, level and their intellect.

Terms

Base mana is a term used to refer how much mana a character has before it is modified by other effects. Many spells require a percentage of your base mana to be cast. Mana pool refers to the total amount of mana a player possesses, disregarding any increasing or decreasing effects (though it usually refers to the sum of increasing).

When out of mana, /oom issues a verbal comment to others around stating that you are out of mana or low on mana. You can also just use "OOM", "oom", "LOM", or "lom" in chat.

Base mana and Mana pool

In general, a character's Mana Pool is either equal to the Base mana or 5 times (400% greater than) the Base mana.

• Melee/Caster hybrid classes in a melee specialization have the same Base mana and Mana pool. At level 90, this is 60K mana. These specializations include Feral and Guardian Druids, Protection and Retribution Paladins, and Enhancement Shaman.

• Melee/Caster hybrid classes in a caster specialization have a Mana pool that's 5 times greater than their base mana. At level 90 this is 5 X 60K = 300K mana. These specializations include Balance and Restoration Druids, Holy Paladins, and Elemental and Restoration Shaman. These specialization have passive abilities like [Natural Insight], [Holy Insight], [Spiritual Insight](for restoration shaman) and [Spiritual Insight](for elemental shaman) that "multiply" their mana pools.

• Pure Caster classes also have the same Base mana and Mana pool. However their Base mana is inherently 5 times greater than that of a Melee/Caster hybrid, giving them a Base mana and Mana pool of 300K mana. These classes include Mages, Monks, Priests and Warlocks. Although Monks are an example of Melee/Caster hybrid classes, they only have access to mana in their spell-casting specialization i.e. Mistweaver. So they have no need of "mana multipliers" like the other hybrid classes and thus have the same Base mana and Mana pool.

Note that there are minor deviations to this rule, such as the aforementioned  [Ember Shadowspirit Diamond] and the Gnome racial [Expansive Mind] that increase the Mana pool in relation to Base mana regardless of class or specialization.

Old Mana Formula

The subject of this section has been removed from World of Warcraft in patch 5.0.4.

A player's Mana pool is mainly determined by their Base Mana and their intelligence (not counting any mana increasing effects or items):

MaxMana = BaseMana + ManaFromIntellect
        = BaseMana + ( min(20, Int) + 15*(Int - min(20, Int) )
        = BaseMana + (20 + (15*(Int – 20))       //for Intellect 20 or over
        = BaseMana + (Int)                       //for Intellect under 20

Example: Level 48 Human Mage

MaxMana = BaseMana + ManaFromIntellect
        = 1021     + 20 + (15 * (Int-20))
        = 1021     + 20 + (15 * (220-20))
        = 1021     + 20 + (15 * (200))
        = 1021     + 20 + (3000)
        = 1021     + 3020
        = 4041

Patch changes

• Patch 5.0.4 (2012-08-28): Mana is no longer affected by Intellect.
  
• Patch 3.0.2 (2008-10-14): Mana and no longer uses the "tick" system and instead regenerates in real time.