When a permanent is on the battlefield face down, it has no mana cost and therefore its mana value (formerly called converted mana cost) is 0. This is explicitly stated in CR 202.3b: if an object has no mana cost, its mana value is 0. Face-down permanents are also treated as having no name, no subtypes, and no abilities other than those granted by the effect that turned them face down (typically the Morph or Disguise rules).
This applies universally — even if the face-down creature is normally a very expensive spell like Emrakul, the Aeons Torn (which has a mana value of 15 face up), it has a mana value of 0 while face down on the battlefield. Effects that care about mana value, such as Engineered Explosives or a spell that says 'destroy target creature with mana value 3 or less,' will see the creature as having mana value 0.
It is important to note this rule applies specifically on the battlefield. In other zones, the card's actual mana cost is used. For example, if a face-down creature card is somehow referenced in the graveyard, its real mana value applies there.
Example: You control a face-down creature (secretly Sphinx of the Steel Wind, mana value 8). Your opponent casts Engineered Explosives with X=0. When they activate it, it will destroy your face-down creature because its mana value is 0 on the battlefield, not 8.
Unofficial fan resource — not affiliated with or endorsed by Wizards of the Coast. Answers are AI-generated estimates grounded in the Comprehensive Rules and are not a substitute for an official judge. Verify anything match-critical.