Item Pool

An item pool is the set of items that can be generated under a given circumstance. There are many item pools that exist within The Binding of Isaac: Rebirth.

Item Weight

Each item that is in an item pool has a weight associated with it. The weight represents how likely it is for the game to spawn the item. Nearly every item in the game has a weight of 1.
- For example, there are 339 items in the Treasure Room pool. Each has a weight of 1, which means that there is a 1 in 339 (0.29%) chance to get any particular item.
Only a few items in the game do not have a weight of 1:
- Bomber Boy has a weight of 0.2 in the Bomb Bum pool, making it 80% less likely to encounter.
- Delirious has a weight of 0.4 in Angel Room pool, making it 60% less likely to encounter.
- Mega Blast has a weight of 0.2 in the Devil Room and Greed Mode Devil Room pools, making it 80% less likely to encounter.
- Plan C has a weight of 0.2 in the Curse Room, Devil Beggar, Devil Room, Greed Mode Shop, Red Chest and Shop pools, making it 80% less likely to encounter.

Weight Decrementation

When you see an item spawn, the weight of that item is automatically decremented inside of the respective item pool. However, the amount of the decrementation varies depending on the specific item pool.
If an item happens to be in more than one item pool, the weight decrementation will also apply to other pools.
Touching an item will remove it from all item pools.

The following item pools are fully decremented on sight:

Angel Room and Greed Mode Angel Room
Boss Room and Greed Mode Boss Room
Devil Room and Greed Mode Devil Room
Secret Room
Treasure Room and Greed Mode Treasure Room

The following item pools are decremented by 50% (0.5) on sight:

The following item pools are decremented by 40% (0.4) on sight:

The following item pools are decremented by 10% (0.1) on sight:

Library and Greed Mode Library

Examples

All items in the Treasure Room pool are completely decremented on sight. Thus, seeing a Notched Axe in the Basement 1 Treasure Room will make it impossible to see another Notched Axe in a future Treasure Room. (In this case, players do not have to touch the Notched Axe in order to prevent the game from ever spawning another one.)
All items in the shop pool are decremented by 0.5 on sight. Thus, if you use a Restock Machine to reroll a Treasure Map, you have a chance for another Treasure Map to appear. However, after the Treasure Map has already appeared twice on a run, it is completely decremented from the shop pool and it will never appear again.
If the player sees Telepathy for Dummies in the Basement 1 Treasure Room, it is completely removed from all other pools for the rest of the run even without touching it. Seeing it in a Library and not touching it, however, allows it still to be encountered as a Treasure item, with slightly lowered weight.

Unused Item Pools

In the itempools.xml file, we can see that there are some unused item pools:

bossrush - Likely intended to be used for the Boss Rush. However, in the actual game, the Boss Rush draws from the Treasure Room pool.
challenge - Likely intended to be used for Challenge Rooms. However, in the actual game, Challenge Rooms draw from the Treasure Room pool.
dungeon - Likely intended to be used for the Crawl Spaces. However, in the actual game, Crawl Spaces draw from the Treasure Room pool.

Set Drops

Some specific collectible items are hard-coded to have a chance to drop from certain special events. These are referred to as "set drops".
Set drops are not affected by item pools. For example, if you touch The Virus in the Treasure Room on Basement 1, and then you fight Lust in the Mini-Boss Room of Basement 2, Lust will still have the normal chance to drop The Virus. If it does, you can take it and have two Viruses.
Similarly, just seeing set drops does not decrement them from other pools. For example, if you fight Lust in the Mini-Boss Room of Basement 1, it drops The Virus and the item remains uncollected, it will remain in the Treasure Room item pool and it can still be found in a Treasure Room later on in the run. Touching the drop will, as expected, remove it from all pools.

The set drops are as follows:

<3 - Drops from Gluttony
Anarchist Cookbook - Drops from Pride
Blood Bag - Drops from Blood Donation Machines
Bob's Curse - Drops from Super Sloth
Bob's Rotten Head - Drops from Sloth
Bucket of Lard - Drops from Super Gluttony
Crystal Ball - Drops from Fortune Telling Machines
Dead Cat - Drops from Super Pride
A Dollar - Drops from Slot Machines
Forever Alone - Drops from Super Envy
IV Bag - Drops from Blood Donation Machines
Mr. Boom - Drops from Wrath
Mr. Mega - Drops from Super Wrath
A Quarter - Drops from Super Greed
Shoop Da Whoop! - Drops from Envy
Skatole - Drops from the Shell Game
Steam Sale - Drops from Greed
The Virus - Drops from Lust
Yum Heart - Drops from Super Lust

Note that Blood Donation Machines will always spawn a Blood Bag instead of an IV Bag if you have already touched an IV Bag during the run.

Pseudo-Set Drops

Destroying a pot, mushroom, or skull has a 1% chance to spawn a specific collectible; see below.
Like set drops, these items are hard coded and exist outside of the item pools.
However, unlike set drops, these collectibles will not spawn if they have already been fully decremented out of their respective pools.
- For example, if a player sees A Quarter in the Basement 1 Treasure Room and does not touch it, then they will not be able to get A Quarter from an urn for the remainder of the run.
Furthermore, if a psuedo-set drop is successfully spawned from a urn/mushroom/skull, it will be also be decremented from the respective pool.
- For example, if a player gets A Quarter from destroying an urn, then they will not be able to get A Quarter from another urn for the remainder of the run.

The pseudo-set drops as as follows:

Blood Clot - 0.5% chance from destroying a polyp
Dry Baby - 0.5% chance from destroying a skull
Ghost Baby - 0.5% chance from destroying a skull
Magic Mushroom - 0.5% chance from destroying a mushroom
Mini Mush - 0.5% chance from destroying a mushroom
Placenta - 0.5% chance from destroying a polyp
A Quarter - 1% chance from destroying a urn

Notes

Item pools can be exhausted by having the game create every item in that specific pool. Items do not need to be picked up for this to happen, the game simply has to generate them (rerolling the item with The D6 for example).
- When an item pool is exhausted, the game will generate items from the Treasure Room pool instead.
- If the Treasure Room pool is exhausted, the only item that can be generated is Breakfast.

The Binding of Isaac: Rebirth
Achievements	Attributes	Bosses	Cards and Runes	Challenges	Chapters
Characters	Co-op	Items	Item pools	Monsters	Objects
Pickups	Pills	Rooms	Seeds	Transformations	Trinkets