When calculating the mana value of an object with an {X} in its mana cost, X is treated as 0 while the object is not on the stack, and X is treated as the number chosen for it while the object is on the stack.
October 5, 2018 · Guilds of Ravnica
When calculating the converted mana cost of an object with an {X} in its mana cost, X is treated as 0 while the object is not on the stack, and X is treated as the number chosen for it while the object is on the stack.
July 22, 2016 · Eldritch Moon
When calculating the converted mana cost of an object with a hybrid mana symbol in its mana cost, use the largest component of each hybrid symbol.
April 8, 2016 · Shadows over Innistrad
Each Phyrexian mana symbol in a card’s mana cost contributes 1 to its converted mana cost.