Mana cost

The mana cost of a card or Hero Power determines how much mana is required to play that card from the hand or to use that Hero Power. The number is displayed inside a blue crystal in the upper left corner of each card, and at the top of each Hero Power. When a card is played or a Hero Power is used, its mana cost is deducted from the player's available mana for that turn.

All cards and Hero Powers have mana costs, even those not normally played from the hand (such as minions summoned by summon effects like Murloc Tidehunter's Battlecry or Unleash the Hounds). This is relevant when cards like Evolve affect minions based on their cost, or when minions are returned to the hand.

Most cards have mana costs in the 0-10 range, while all playable Hero Powers cost 2 mana (with the exception of the Demon Hunter's Demon Claws). However, some effects can modify mana costs (even though not below 0).

Mana cost might be considered an attribute or statistic of a card, but does not affect the card in combat. The mana cost of a card determines how early or late in the game it may be played and is usually an indicator of the card's overall power. The distribution of mana costs in a deck is the mana curve, an important strategic consideration.

Modifying costs
Effects may reduce or increase the cost of cards and Hero Powers, or set them to a specific value. These effects either are ongoing effects or act by creating cost-modification enchantments (for a complete list see Cards below).
 * Cards that modify their own mana cost (such as Dread Corsair) only work while in the player's hand: they have no effect while the card is anywhere else, such as in the player's deck or in the battlefield (see in-hand effects for details). Most of them are ongoing effects, while a few (Nerubian Prophet, Knight of the Wild) are triggered effects.
 * Ongoing effects that modify the mana cost of other cards or Hero Powers (such as Mana Wraith and Maiden of the Lake) apply a modifier to all eligible cards currently in hand (but not to cards in play) and to eligible Hero Powers: removing or neutralising (for example by using Silence effects) the source of the effect will therefore immediately remove the modifier from any affected card or Hero Power.
 * Example: A Mechwarper would allow a Clockwork Gnome to be played for 0 mana, returned to the hand, and played for 0 mana again, but even with the Mechwarper in play a Recombobulator would cause the Clockwork Gnome to be transformed into a 1-Cost minion, not a 0-Cost one.


 * Cost-modification enchantments may be created by spells (like Unstable Portal), triggered effects (like Emperor Thaurissan) or Battlecries (like Millhouse Manastorm and Saboteur).
 * Cost-modification enchantments that are attached to a card in a player's deck (such as due to Renounce Darkness) are not removed when the modified card is drawn.
 * Cost-modification enchantments that are attached to a card in hand are removed once the modified card is played: effects which depend on mana cost will then use the original mana cost of the card and, if it is returned to the hand, re-playing it will cost the original amount.
 * Example: A Bloodfen Raptor drawn with Call Pet costs 0 mana when first played, but would cost 2 mana to re-play if returned to the hand by Youthful Brewmaster, and would be transformed into a 2-Cost minion if targeted by Recombobulator.
 * Some cost-modification enchantments (such as Preparation and Saboteur) are attached to a player instead of a card. Usually these enchantments have a limited duration and expire by themselves "the next time something happens" (which can be many turns later, such as for Dragon Consort and Fencing Coach) or "at the end of the turn". The only exception is Raza the Chained, whose effect lasts for the rest of the game.

When there are multiple cost-modification effects, they are applied in the order in which they have been generated (from oldest to newest; notice that cards that were in a player's deck at the beginning of the game still have that timestamp when drawn); the only exception is Summoning Portal, which is always applied first even if played last. Unlike Health/Attack enchantments and auras, cost-modification enchantments and auras have no special priority over one another.
 * Example: If Millhouse Manastorm is played followed by Loatheb, spell costs are first set to 0, then increased by 5 for a final cost of 5. In the opposite order, spell costs would first increase by 5, then be set to 0, for a final cost of 0.
 * Example: If Emperor Thaurissan's effect triggers, reducing the mana cost of an in-hand Wolfrider to 2, and Naga Sea Witch then comes into play, her effect immediately increases the Wolfrider card's cost to 5; at the end of that turn, Emperor Thaurissan triggers again, reducing the Wolfrider's mana cost to 4.
 * Example: If Emperor Thaurissan's effect triggers, reducing the mana cost of an in-hand War Golem to 6, and Aviana then comes into play, her effect immediately reduces the War Golem card's cost to 1; at the end of that turn, Emperor Thaurissan triggers again, reducing the War Golem's mana cost to 0.
 * Example: If you have a Naga Sea Witch in play and a Fiery War Axe equipped and draw a Dread Corsair from your deck, the Dread Corsair's final mana cost will be 5: the Dread Corsair card, being in your deck since the beginning of the game, is older than the Naga Sea Witch minion in play, and thus its effect is applied first, and then overwritten by the newer one by Naga Sea Witch. However, if you later return the Dread Corsair minion from the battlefield to your hand, it becomes a new Dread Corsair card, which is then newer than the Naga Sea Witch minion: thus, the Dread Corsair card's cost is first set to 5 by Naga Sea Witch and then, if you are still equipping the Fiery War Axe, it is reduced by 3, to a final mana cost of 2.
 * Example: If Knight of the Wild triggered twice before Aviana enters play, early cost-modification effects are overridden by the "newer" Aviana effect, for a final mana cost of 1; if you later summon another Beast, Knight of the Wild triggers again, reducing its mana cost to 0.
 * Example: If you play Naga Sea Witch and then Aviana, minion cards in your hand will cost 1; if you play Aviana and then Naga Sea Witch, minion cards in your hand will cost 5.
 * Example: Suppose you have Wilfred Fizzlebang on the battlefield. If you play Aviana and then use Life Tap, the minion card you draw will cost 0; if you use Life Tap and then play Aviana, the minion card you draw will cost 1.
 * Example: If Bright-Eyed Scout's Battlecry draws Mountain Giant, its cost will be set to 5 regardless of the number of cards in hand, and the cost will not change as the hand gains or loses cards: since the Mountain Giant card was created at the beginning of the game, its effect is always considered older than the Bright-Eyed Scout's Battlecry. 
 * Example: If Bright-Eyed Scout's Battlecry draws Nerubian Prophet, its cost will be set to 5, and at the start of each subsequent turn its cost will drop to 4, 3, 2, and so on.

If a card's or Hero Power's modified cost would be negative, 0 is displayed and used instead (players can never gain mana from paying the cost of a card or Hero Power).
 * This minimum of 0 is only enforced after all other cost modifications have been taken into account: this allows reductions like Far Sight to counteract subsequent increases such as Loatheb's.
 * One may think of cards and Hero Powers as having two mana costs, an "actual" cost and a "hidden" cost. The hidden cost is simply the original mana cost with all subsequent modifications, and has no lower limit. The actual cost, which is displayed on the card or Hero Power and charged when the card is played or the Hero Power is used, is the greater of the hidden cost and 0.
 * Example: If Far Sight draws an Earth Shock, the Earth Shock has a hidden cost of -2 and an actual cost of 0; if the opponent then plays Loatheb the hidden cost increases by 5 and becomes -2 + 5 = 3, and the actual cost rises to 3 too. Thus, since only actual costs are displayed, apparently Far Sight reduced the cost of Earth Shock only by 1 (instead of 3) and Loatheb increased it only by 3 (instead of 5): however, even though Far Sight's reduction of -3 was temporarily, partially nullified by the lower limit of 0, it still remained in effect for later recalculation of the mana cost.

If a card's modified cost is different from its base cost, its mana cost will be displayed in a different colour:
 * if the modified cost is lower than the base cost, it will be displayed in green;
 * if the modified cost is higher than the base cost, it will be displayed in red;
 * if a card's modified cost is equal to its base cost (for example because multiple cost modifying effects cancel each other out), the colour of its mana cost will not be altered.

Cho'gall's and Seadevil Stinger's Battlecry causes the next spell or Murloc the player plays that turn to deal to their hero an amount of damage equal to the card's mana cost, instead of expending mana.
 * The mana costs of those cards are affected as usual by cost-modification effects, and thus their "Health cost" is modified accordingly.
 * Despite the card's ambiguous wording, Cho'gall's and Seadevil Stinger's effect deal damage to the controlling hero, thus affecting Armor (if any) before reducing the hero's Health.
 * Playing an affected card will cost no mana. However, card text effects which consume mana (such as Forbidden Ritual) are not altered.
 * The player must be Immune or have a combined Health and Armor total greater than the final mana cost of an affected card in order to be allowed to play it (regardless of their currently available mana). Trying to play an affected card will otherwise result in an error message.
 * Cards in hand that can be played using Cho'gall's or Seadevil Stinger's effect will have their mana cost highlighted by red bubbles.

Strategy

 * Main article: Mana curve

The mana cost of a card determines which round it will become available to play, as well as which cards it can be used in combination with. The mana curve of a deck describes how many cards are present at each mana cost, and is an important factor determining options available to a player based on the amount of mana available on each turn. When building a deck, it is wise to consider whether mana costs of cards and combinations follow the desired curve.

Expert players can also use this knowledge to predict which cards may be played on certain rounds. Mind Control, for instance, generally only becomes playable from round 10 onwards, while a Leeroy Jenkins + Power Overwhelming combo is available once the warlock has 6 mana.

Cards that modify mana cost
Cost-modifying effects can be categorized by which entities they affect.

Cards
The following cards have effects that modify the cost of other cards.

Self
The following cards modify their own mana cost while in hand. Most act on an ongoing basis based on current game conditions or history, but Nerubian Prophet and Knight of the Wild accumulate enchantments on themselves in response to game events that occur while in hand.

Cost-affected cards
The following cards have effects that scale based on the mana cost of other existing cards.

Cost-specifying cards
The following cards only affect those existing cards which meet a specified cost-based criteria.

Cost-triggered cards
The following cards are triggered by cards of a specific cost.

Cost-listing cards
The following cards generate their random outcomes by considering all collectible cards that meet a specified cost-based criteria.