Board Thread:Questions and Answers/@comment-27703542-20160205131840/@comment-10955477-20160205211536

There isn't an exact percentage on the rarity of obtaining an Epic or Rare Card, except the guarenteed no. of cards shown in an in-game chest's information.

e.g. An Arena 6 Magical Chest contains x66 Cards, with x2 guarenteed Epic cards, and 13 guarenteed Rare cards.