This has been around for a few days and I've been meaning to post stuff about this as I think it will lead to other games being revealed including Destiny.
The Chinese Government’s legislation on online games containing loot boxes rules that gamers who purchase these titles know exactly what they’ll be getting. In turn, publishers must comply with the laws and reveal the drop rates for said loot boxes. Blizzard has been the first to comply with the Chinese laws, turning over the Overwatch loot box drop rates.
You can see the probabilities revealed for the loot boxes below:
For every loot box awarded to players, they will obtain at least one blue (rare) item. The average drop rate for purple (epic) items is one for every 5.5 loot boxes. The average drop rate for orange (legendary) items is one for every 13.5 loot boxes.
There are some really insightful mathematic geeks on reddit that have done some work into how likely getting 4 epics are. When I get chance I'll post some more info later on it.
It's good to understand this for timed events so you know you'll need approx 5 to 6 boxes to get an epic drop and that favourite skin you're hoping for and whether or not you need to buy with your own credits as you won't have the time left.