In all zones except the stack, is equal to 0. While on the stack, a spell's CMC counts as the announced value. For example, Apocalypse Hydra has CMC 2 while in your library. If you cast it for =5, then it has CMC 7 while on the stack and CMC 2 again when it enters the battlefield.
Note, however, that when you cast a spell without paying its mana cost, defaults to 0. You can't choose a value for it unless you cast it for its mana cost. Green Sun's Zenith into Apocalypse Hydra would result in a dead Apocalypse Hydra.
January 16, 2015
11:55 p.m.
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Epochalyptik says... Accepted answer #1
In all zones except the stack,
is equal to 0. While on the stack, a spell's CMC counts 
as the announced value. For example, Apocalypse Hydra has CMC 2 while in your library. If you cast it for 
=5, then it has CMC 7 while on the stack and CMC 2 again when it enters the battlefield.
Note, however, that when you cast a spell without paying its mana cost,
defaults to 0. You can't choose a value for it unless you cast it for its mana cost. Green Sun's Zenith into Apocalypse Hydra would result in a dead Apocalypse Hydra.
January 16, 2015 11:55 p.m.