Blizzard’s announcement was dug up by the site metabomb. Due to a recently enforced Chinese law, any company that sells randomized loot boxes must disclose the probability of its items.
As a result, we now have the probability of item drops from Overwatch’s loot boxes! At least, the probability of item drops in China. Blizzard could have different probability for its items in North America.
This is quoted directly from metabomb:
Each Loot Box will contain four items. If an item is already owned, it will be converted into currency. Within each Loot Box there will be at least one item of Rare, Epic or Legendary quality. You need to open, on average, 5.5 Loot Boxes in order to gain an Epic item. You’ll have to open an average of 13.5 Loot Boxes if you want a Legendary.
It is unlikely we will see the North America numbers any time soon as it is unnecessary for Blizzard to disclose this information. Whether the chances match up with North America or not, it’s nice to see some transparency on how Blizzard designed their loot boxes.