Devil Room chance calculation

The Devil Room has a chance to appear after defeating a boss in the floors between and including Basement II - Womb II. It usually contains a different set of items each time in which Isaac can do a 'deal with the devil', by trading health containers for collectable items. By taking a devil deal, this also makes it impossible to find Angel Rooms later in the run.

The chance for a Devil Room door appearing on a floor can be manipulated by meeting a set of criteria. Your percentage chance of seeing the devil room goes down if you take red heart damage on the floor, and can increase by having certain items. By default your lowest possible chance is 1%, then additionally the following conditions can lower/increase that chance:

Condition	Rebirth Percentage (%)	Afterbirth Percentage (%)
Not taking red heart damage on the entire floor	+99%	+99%
Not taking red heart damage on the boss fight	+35%	+35%
Holding the Book of Belial	+2500%	+12.5%
Holding Goat Head	Guaranteed	+6660%
Holding Book of Revelations	+25%	+17.5%
Killing a regular coin beggar	+35%	+35%
Killing a dead shopkeeper on the floor	+10%	+10%
Holding Pentagram	+20%	+10%
Holding a second Pentagram	+10%	+5%
Holding Black Candle	+30%	+15%
Saw a Devil/Angel room on the previous floor?	Percentage * 0.25 (75% reduction)	Percentage * 0.25 (75% reduction)
Saw a Devil/Angel room 2 floors ago?	Percentage * 0.5 (50% reduction)	Percentage * 0.5 (50% reduction)

Combining the above conditions will give you your final devil/angel door percentage chance. By default, after meeting the conditions for an angel room to appear, you have a 50/50 chance of the door that spawns being either a devil or angel room. If you have not yet met the angel room conditions or have locked in your devil deals by taking one, it will always be a devil room door.

Notes:

The devil room can also contain 3 red chests or the Krampus boss fight (10% chance of Krampus, jumping up to 40% if you have already taken a devil deal)
It is recommended that you enter the Devil Room as soon as the door appears, due to the fact that your percentage chance is re-evaluated again the next time you exit and re-enter the boss room. The 0.25 multiplier for seeing a devil room on the previous floor is also applied at this moment, meaning your chance is much lower than the first time it was checked.
The Goat Head will always guarantee a devil deal and force the door to stay open.
Exploding a normal beggar gives an extra +35% chance for the door to open, however all other beggars only count towards increasing the chance of Angel Rooms in favour of devil rooms.
There are other methods of entering the devil room, such as a Joker card or being teleported by a Red Chest.
Soul hearts are extremely useful for getting devil rooms as they provide a buffer that can stop you taking red heart damage, which would reduce your percentage significantly.
Demon Beggars, Blood Donation Machines, IV Bag, Blood Rights, Razor Blade do not count as taking red heart damage.
You are guaranteed to see a devil room on Basement II if you take no red heart damage on the entire floor. This means Blue Baby will always see a devil room on Basement II.