Traditionally, you're supposed to have 60 cards in your main deck, and 15 cards in your sideboard. But let's say that I had 64 cards in my main deck and 15 cards in my sideboard. For game 2 & 3, could I reduce my main deck to 60 cards and effectively have 4 more cards in my sideboard, or do I have to keep my main deck at 64 cards?


You can in fact change your deck size after sideboarding, with restrictions. The Magic Tournament Rules, section 6.1 says

Constructed decks must contain a minimum of sixty cards. There is no maximum deck size. If a player chooses to use a sideboard, it may not contain more than fifteen cards.

And section 6.2 says

Players may exchange any number of cards between their deck and sideboard, provided that the resulting deck and sideboard are legal. There are no restrictions on the number of cards a player may exchange this way. Cards do not need to be exchanged on a one-for-one basis.

So, you can move a different number of cards into your sideboard and into your deck, but when you're done, you still have to have at least 60 cards in your deck and at most 15 cards in your sideboard.

In your specific case, you cannot decrease your deck size because your sideboard is already at the maximum size.

  • It is worth noting that although there is no maximum deck size you need to be able to shuffle your deck in a timely manner. That is why modern Battle of Wits decks are virtually impossible to play in real life.
    – Neil Meyer
    Jan 26 '16 at 16:04

No you can not.

100.4a In constructed play, a sideboard may contain no more than fifteen cards

If you brought your deck down to 60 cards, at that point your sideboard would be 19 cards, violating this rule.

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