Mana value of X in Magic: stack vs elsewhere
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To calculate the mana value of a card with {X} in Magic, first look at where the object is. While it is a spell in the stack, each {X} in the mana cost equals the number chosen when the spell is cast. Outside the stack, {X} is worth zero for that calculation, even though the card retains an X in its text. Thus, a spell with a cost of {X}{R} cast with X equal to four has a mana value of five while it is resolving; the same card in hand, library, graveyard, or on the battlefield has a mana value of one. Current rule 202.3e establishes this difference. The mana you actually paid may be changed by other effects, but it does not substitute for the mana cost when calculating the value.
Mana value is not the same as mana paid
Mana value is a number derived from the mana cost printed on the card or from rules applicable to the object. The rules give the example {3}{U}{U}: its mana value is five, regardless of color. If a card costs {X}{R}, its fixed portion is one red mana and the variable portion depends on X while in the stack. Do not calculate the value by asking how many lands you played: a discount might reduce what you pay, an additional cost might increase it, and an alternative casting method might change the payment without necessarily altering the mana value.
This distinction appears in cards that counter "a spell with mana value 4 or less" or that search for "a card with mana value 3" in the library. The first typically looks at an object in the stack and uses the chosen X. The second looks at a card outside the stack and treats X as zero. If you don't separate zones, the same printed card appears to change its number without explanation. In reality, the printed text does not change; instead, the applicable evaluation rules for the object differ depending on its zone.
How X is chosen when casting a spell
When casting a spell whose cost contains {X}, you choose a legal value for X during the casting process, unless the text specifies otherwise. That value is used for the cost and, while the spell is in the stack, for its mana value. If you choose X equal to zero for {X}{R}, the value will be one; if you choose X equal to three, it will be four. Two printed copies of the same card can be in different zones with different mana values calculated at that moment. The choice belongs to that casting and is not permanently written on the card for future turns.
When a spell with {X} resolves and enters as a permanent, it is no longer in the stack. To determine its mana value outside the stack, X is treated as zero again. Some effects or counters created during casting may preserve information about the chosen value according to their own text, but they do not automatically change the general rule for a permanent's mana value. If an effect asks, "What is the mana value of this permanent?", do not respond with the value you paid when casting it last turn. Instead, look at its current mana cost and apply X equal to zero in that zone.
Examples with one and two X's
For {X}{G}, the fixed portion contributes one. In the library, the card has a mana value of one. In the stack, if you choose X equal to six, the spell has a value of seven. When it goes to the graveyard after resolving, it returns to a value of one for effects that check its value there. For {X}{X}{U}, there are two X symbols. Normally, all Xs on the same object have the same chosen value: if X is three, both contribute three and the blue symbol adds one, so the value in the stack is seven. Outside the stack, both Xs count as zero and the value is one.
Do not confuse "X equal to three" with "I paid three in total" in a cost of {X}{X}{U}. The variable cost represents twice the chosen number, plus the fixed portion. An effect that reduces the cost might lower what you actually pay, but the spell's mana value in the stack would still be calculated using the two chosen Xs. When explaining a play, state first "I choose X = 3," then show the resulting cost, and finally indicate discounts or additional costs. This order helps everyone verify the mana value without confusing it with actually spent resources.
What happens in hand, library, and graveyard
Rule 107.3g requires that if a card with {X} in its mana cost is in any zone other than the stack, {X} is treated as zero, even if X is defined elsewhere in its text. Thus, a card {X}{B}{B} in your library has a mana value of two for a search that requires "mana value 2." You cannot announce "I want X to be five" so that that card meets a search for mana value seven: you are not casting the spell, and there is no choice of X in the stack applicable to that inquiry.
The same applies in the graveyard. If the spell was previously cast with X equal to eight, a card now examined for its mana value in the graveyard treats X as zero. The payment history is not preserved as a permanent characteristic. When the card is returned to hand or library, it follows the same evaluation outside the stack. A card with X can therefore be easily searched for by effects limited to small values and become very powerful when cast later with a large X, provided the effect's text and other rules allow it.
The battlefield is also outside the stack
If a permanent with {X} in its mana cost is on the battlefield, its ordinary mana value also treats X as zero. This is surprising when the permanent entered due to a large payment and received many counters related to X. Those counters may remain and affect its size or abilities, but they do not add to the card's mana value. If a spell destroys "a permanent with mana value 3 or less," it checks the permanent's mana cost on the battlefield with X equal to zero. Do not use the number of counters as a shortcut.
Hypothetical example: A creature with cost {X}{G} was cast with X equal to five and entered with five counters from its own text. In the stack, it had a mana value of six. On the battlefield, if no one changed its mana cost, it has a value of one even though it has five counters. The counters may make it large in combat; the mana value still answers a different question. This separation explains why certain effects that check mana value can target permanents that appear to have cost a large amount to cast.
Additional costs, reductions, and alternatives
An additional cost is paid when casting the spell, but it is not part of its mana cost and does not increase its mana value. If the printed cost is {X}{R} and you choose X equal to four, its value in the stack is five even though you also have to pay another cost or sacrifice something due to the card's text. Similarly, a reduction that allows you to pay less mana does not lower the mana value. If a card lets you cast it at an alternative cost, that alternative payment should not be confused with the mana value calculated from the object's mana cost; read the specific rules of that method to determine how X is chosen.
There is one especially useful case: if an effect lets you cast a spell "without paying its mana cost" and you do not pay an alternative cost that includes X or the card's text defines X, rule 107.3b requires choosing X equal to zero. You cannot take advantage of not paying to declare a massive X. This limitation does not apply simply because a normal reduction leaves the final cost at zero; in that case, there is still a casting with X chosen according to its rules. The difference between "without paying its mana cost" and "mana cost reduced" matters both for the spell's effect and for its value in the stack.
An X in the text does not always change the value
Some cards use the letter X in their rules text, in an activated ability, or in an additional cost, without having {X} in the mana cost in the corner. The mana value is calculated from the mana cost, not by adding each X that appears in the card. If a permanent costs {2}{U} and has an ability that says "pay {X}" to produce an effect, the permanent's mana value remains three. The X in the ability may determine how much that activation produces, but it does not rewrite the printed mana cost of the card.
Even when the same letter X appears both in the mana cost and in the spell's text, it's best to separate functions. The choice of X during casting can determine the intensity of the effect and the mana value while in the stack. After leaving the stack, the value evaluation treats the cost symbol as zero, even if some effect created by the spell uses the previously chosen value. If a specific interaction asks about X within a text effect, consult the rules for that text; do not automatically respond with the mana value.
How to resolve a card that "looks at mana value"
Read the object being examined: is it a spell in the stack, a card in the graveyard, a revealed card from the library, or a permanent on the battlefield? If it's a spell in the stack, identify how much X is worth in that casting. If it's in any other zone, treat X as zero for this calculation. Then add the fixed portion of the mana cost and any applicable symbols. Only then compare it to the number required by the effect. This sequence of three questions—zone, X, fixed portion—works better than remembering a loose phrase like "X is always zero."
Exercise: you're searching the library for a card with mana value two. A card {X}{W}{W} meets that requirement outside the stack, since X is zero and the two white symbols total two. Later, you cast it choosing X equal to four: while in the stack, it has a value of six. If your opponent has a response that only affects spells with mana value five or less, they must use the six calculated there, not the two the card had in the library. After resolving or going to the graveyard, it recalculates in that new zone.
Common mistakes
The first is using the actual mana paid as the mana value, ignoring discounts or additional costs. The second is retaining the X value from a previous casting in the graveyard. The third is counting an X written only in an ability as if it appeared in the mana cost. The fourth is adding the cost {X}{X} only once. All are corrected by looking at the cost in the corner, the object, and its zone. If a card modifies characteristics or has an alternate face with special rules, consult the text and full rules; this procedure covers the common case of {X} in a normal mana cost.
Frequently Asked Questions
How much is X on a card in my library?
Zero for calculating its mana value. Add the fixed portion of the mana cost.
How much is X while the spell is in the stack?
The number chosen or determined for that casting; each X symbol in the cost contributes that value.
Does a cost reduction also reduce the mana value?
No on its own. It changes what you pay, but the mana value is still calculated from the mana cost and the X chosen in the stack.