Fireball — ruling 1

For example, if X is 5 and you choose three target creatures, Fireball has a total cost of {7}{R} (even though its mana cost is just {5}{R} and its mana value is 6). If those creatures are all still legal targets as Fireball resolves, it deals 1 damage to each of them.

Fireball — ruling 2

You may cast Fireball with zero targets, regardless of the value chosen for X. If you do so, it will not be a targeted spell, and no damage will actually be dealt when it resolves.

Fireball — ruling 3

Fireball's damage is divided as Fireball resolves, not as it's cast, because there are no choices involved. The division involves only targets that are still legal as Fireball resolves.

Fireball — ruling 4

You can target more than X creatures. However, if the number of legal targets at the time Fireball resolves is greater than X, none of them will be dealt any damage.

Fireball : règles, fonctionnement et interactions (MTG)

Fireball is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell costs more to cast for each target beyond the first. Fireball deals X damage divided evenly, rounded down, among any number of targets.

How it works : Fireball

Original English text — Wizards of the Coast / Scryfall

#1November 17, 2017· Wizards of the Coast
For example, if X is 5 and you choose three target creatures, Fireball has a total cost of (even though its mana cost is just and its mana value is 6). If those creatures are all still legal targets as Fireball resolves, it deals 1 damage to each of them.
No other card is named in this ruling.
#2November 17, 2017· Wizards of the Coast
You may cast Fireball with zero targets, regardless of the value chosen for X. If you do so, it will not be a targeted spell, and no damage will actually be dealt when it resolves.
No other card is named in this ruling.
#3November 17, 2017· Wizards of the Coast
Fireball's damage is divided as Fireball resolves, not as it's cast, because there are no choices involved. The division involves only targets that are still legal as Fireball resolves.
No other card is named in this ruling.
#4November 17, 2017· Wizards of the Coast
You can target more than X creatures. However, if the number of legal targets at the time Fireball resolves is greater than X, none of them will be dealt any damage.
No other card is named in this ruling.

Popular combos

Real combos played by the community, sorted by popularity.

#1
Yavimaya Bloomsage // Channel + Fireball
Near-infinite damage
1Cast a copy of Channel by paying .
2Activate the ability granted by Channel by paying 1 life, adding .
3Repeat step 2 as desired.
4Cast Fireball by paying and all remaining available mana, dealing a large amount of damage divided among a set number of targets.

#1
Yavimaya Bloomsage // Channel + Fireball
Near-infinite damage
1Cast a copy of Channel by paying .
2Activate the ability granted by Channel by paying 1 life, adding .
3Repeat step 2 as desired.
4Cast Fireball by paying and all remaining available mana, dealing a large amount of damage divided among a set number of targets.

#2
Channel + Fireball
Near-infinite damage
1Cast Channel by paying .
2Activate the ability granted by Channel by paying 1 life, adding .
3Repeat step 2 as desired.
4Cast Fireball by paying and all remaining available mana, dealing a large amount of damage divided among a set number of targets.

Interactions with other cards

These cards' text mentions Fireball.

Delayed Blast Fireball

Delayed Blast Fireball deals 2 damage to each opponent and each creature they control. If this spell was cast from exile, it deals 5 damage to each opponent and each creature they control instead. Foretell {4}{R}{R} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

Farideh's Fireball

Farideh's Fireball deals 5 damage to target creature or planeswalker. Roll a d20. 1—9 | Farideh's Fireball deals 2 damage to each player. 10—20 | Farideh's Fireball deals 2 damage to each opponent.