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, if there are 200 items in a given item pool, and each item has a weight of 1, that means there is a 1 in 200 (0.5%) chance to get any particular item from that pool.
- However, if an item has a weight of 0.5, that means that item is 50% as likely to spawn compared to an item that has a weight of 1. In a pool of 200 items that has only 2 items with a weight of 0.5, that would result in the total weight being 199. ((1 * 198) + (2 * 0.5) = 199) This means items with a weight of 1 have a 1 in 199 (0.5%) chance to spawn, while an item with a weight of 0.5 has a 1 in 398 (0.25%) chance to spawn.
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.
Item weights were adjusted more extensively in Repentance.
- Treasure Room:
  - Astral Projection: 0.2
  - The Body: 0.2
  - Death's Touch: 0.2
  - Epic Fetus: 0.1
  - Esau Jr.: 0.5
  - Eternal D6: 0.5
  - Gello: 0.5
  - The Halo: 0.5
  - Holy Light: 0.2
  - Holy Mantle: 0.2
  - Magic Mushroom: 0.5
  - Mega Mush: 0.1
  - The Mind: 0.2
  - Mom's Knife: 0.2
  - Plum Flute: 0.5
  - Pyro: 0.1
  - The Relic: 0.2
  - The Soul: 0.2
  - Skeleton Key: 0.1
  - Spindown Dice: 0.1
- Shop:
  - Blank Card: 0.5
  - Card Reading: 0.5
  - Chaos: 0.5
  - Crooked Penny: 0.5
  - Keeper's Box: 0.5
  - Magic Skin: 0.1
  - Mama Mega!: 0.5
  - Plan C: 0.2
  - Sack Head: 0.5
- Library:
  - Book of Virtues: 0.2
  - Lemegeton: 0.2
- Curse Room:
  - False PHD: 0.5
  - Heartbreak: 0.5
  - Plan C: 0.2
  - Vanishing Twin: 0.5
- Boss Room:
  - PJs: 0.5
  - Super Bandage: 0.5
- Secret Room:
  - Sacred Orb: 0.5
  - Vanishing Twin: 0.5
- Devil Room:
  - Abyss: 0.5
  - Berserk!: 0.5
  - Dark Arts: 0.5
  - Forget Me Now: 0.2
  - Heartbreak: 0.5
  - Lemegeton: 0.5
  - Mega Blast: 0.2
  - Plan C: 0.2
  - Void: 0.5
- Angel Room:
  - Book of Virtues: 0.5
  - Delirious: 0.4
  - Sacred Orb: 0.5
  - Void: 0.5
- Golden Chest:
  - Fate: 0.5
  - Lead Pencil: 0.1
  - Orphan Socks: 0.1
  - PJs: 0.5
  - Schoolbag: 0.5
- Red Chest:
  - Blood Puppy: 0.5
  - False PHD: 0.2
  - Plan C: 0.1
  - Red Key: 0.1
- Devil Beggar:
  - Abaddon: 0.2
  - A Pound of Flesh: 0.5
  - Forget Me Now: 0.5
  - Lord of the Pit: 0.2
  - Plan C: 0.2
  - Spirit of the Night: 0.2
- Bomb Bum:
  - Brimstone Bombs: 0.2
  - Pyro: 0.2
- Rotten Beggar:
  - Dead Onion: 0.5
  - Rotten Tomato: 0.5
- Battery Bum:
  - 120 Volt: 0.1
  - Jacob's Ladder: 0.1
- Old Chest:
  - Dad's Ring: 0.2
  - Mom's Knife: 0.2
- Mom's Chest:
  - Mom's Knife: 0.1
  - Red Key: 0.5
- Greed Mode Treasure Room:
  - Anima Sola: 0.1
  - The Body: 0.2
  - Death's Touch: 0.2
  - Epic Fetus: 0.1
  - Esau Jr.: 0.5
  - Eternal D6: 0.5
  - Gello: 0.5
  - The Halo: 0.5
  - Holy Light: 0.2
  - Holy Mantle: 0.2
  - Keeper's Box: 0.2
  - Magic Mushroom: 0.5
  - Mega Mush: 0.5
  - The Mind: 0.2
  - Missing No.: 0.1
  - Mom's Knife: 0.2
  - Plum Flute: 0.5
  - Pyro: 0.1
  - The Relic: 0.2
  - Skeleton Key: 0.1
  - The Soul: 0.2
  - Spindown Dice: 0.1
- Greed Mode Shop:
  - Anima Sola: 0.1
  - Blank Card: 0.5
  - Chaos: 0.5
  - D infinity: 0.1
  - Echo Chamber: 0.1
  - Esau Jr.: 0.1
  - Flip: 0.1
  - Glitched Crown: 0.1
  - Inner Child: 0.1
  - Isaac's Tomb: 0.1
  - Mama Mega!: 0.5
  - Plan C: 0.2
  - R Key: 0.1
  - Sacred Orb: 0.1
  - Spindown Dice: 0.1
  - Spirit Shackles: 0.1
  - Strawman: 0.1
  - TMTRAINER: 0.1
- Greed Mode Curse Room:
  - False PHD: 0.5
  - Heartbreak: 0.5
  - Plan C: 0.2
- Greed Mode Boss Room:
  - Magic Mushroom: 0.2
  - PJs: 0.5
  - Super Bandage: 0.5
- Greed Mode Secret Room:
  - Sacred Orb: 0.5
- Greed Mode Devil Room:
  - Abyss: 0.5
  - Berserk!: 0.5
  - Dark Arts: 0.5
  - Heartbreak: 0.5
  - Lemegeton: 0.5
  - Mega Blast: 0.2
  - Technology: 0.2
- Greed Mode Angel Room:
  - Book of Virtues: 0.5
  - Sacred Orb: 0.5

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.
If a pedestal cycles between multiple items, such as from Glitched Crown, each item option will count as being seen.

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

All other item pools not listed above are fully decremented on sight.

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 spawn another Notched Axe in the run. (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.
- All items in the shop pool are fully decremented on sight. Thus, if you use a Restock Machine to reroll a Treasure Map, it will never appear again for the rest of the run.
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 from the Seven Deadly Sins 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.
Set drops from most Machines can be obtained multiple times.
- With the exception of IV Bag, Redemption, and Skatole.
  - If an IV Bag has already been touched ( or seen), then Blood Donation Machines will only spawn Blood Bags after.
Set drops from bosses will always replace the Boss Room Item if the set drop wasn't already obtained or depleted from pools, with some exceptions.
- Famine, Pestilence, War, and Death will always drop Cube of Meat or Ball of Bandages, even if either was already obtained.
  - Respectively, if A Pony or White Pony have been touched, then Headless Horseman and Conquest will drop a Boss Room Item instead.
  - Respectively, if A Pony or White Pony have been touched, then Headless Horseman and Conquest will always drop either Cube of Meat or Ball of Bandages.
- Krampus will always spawn either Head of Krampus or A Lump of Coal, even if either was already obtained.
- Krampus will only spawn Head of Krampus once, then will only spawn A Lump of Coal after, even if it was already obtained.
- Mom will always drop The Polaroid and The Negative if they are unlocked, even if either was already obtained.
- Bumbino will drop his items in addition to the boss room item, and will always drop The Poop once all his drops are obtained.
- Hornfel will drop his items in addition to the boss room item, and will only drop 3 bombs once all his drops are seen.
Sacrificial Altar's drops are deceptively not from the Devil Room Pool, and are considered set drops.
- Once all of its drops are seen, it will always drop Pentagram.
All other set drops not mentioned above can only spawn if the item hasn't already been obtained or depleted from pools.

The set drops are as follows:

From the Seven Deadly Sins:

<3 - Drops from Gluttony
Anarchist Cookbook - Drops from Pride
Bob's Curse - Drops from Super Sloth
Bob's Rotten Head - Drops from Sloth
Bucket of Lard - Drops from Super Gluttony
Dead Cat - Drops from Super Pride
Eternal D6 - Drops from Ultra Pride when Florian is killed last
Forever alone - Drops from Super Envy
Mr. Boom - Drops from Wrath
Mr. Mega - Drops from Super Wrath
A Quarter - Drops from Super Greed
Shoop da Whoop! - Drops from Envy
Steam Sale - Drops from Greed
The Virus - Drops from Lust
Yum Heart - Drops from Super Lust

From Bosses:

Ball of Bandages - Drops from Famine, Pestilence, War, and Death
Cube of Meat - Drops from Famine, Pestilence, War, and Death
Head of Krampus - Drops from Krampus
Key Piece 1 - Drops from Uriel and Gabriel
Key Piece 2 - Drops from Gabriel and Uriel
Little C.H.A.D. - Drops from C.H.A.D.
Little Gish - Drops from Gish
Little Steven - Drops from Steven
A Lump of Coal - Drops from Krampus
The Negative - Drops from Mom
Plum Flute - Drops from Baby Plum after not damaging her for 30 seconds
The Polaroid - Drops from Mom
A Pony - Drops from Headless Horseman
Steven - Drops from Steven
White Pony - Drops from Conquest

From Machines:

Blood Bag - Drops from Blood Donation Machines
Crystal Ball - Drops from Fortune Telling Machines
A Dollar - Drops from Slot Machines
IV Bag - Drops from Blood Donation Machines
Mom's Underwear - Drops from Mom's Dressing Table
Mom's Heels - Drops from Mom's Dressing Table
Mom's Lipstick - Drops from Mom's Dressing Table
Mom's Bra - Drops from Mom's Dressing Table
Mom's Pad - Drops from Mom's Dressing Table
Mom's Eyeshadow - Drops from Mom's Dressing Table
Mom's Contacts - Drops from Mom's Dressing Table
Mom's Wig - Drops from Mom's Dressing Table
Mom's Perfume - Drops from Mom's Dressing Table
Mom's Pearls - Drops from Mom's Dressing Table
Redemption - Drops from Confessionals after taking a Devil Deal
Skatole - Drops from the Shell Game

From Rocks:

Blood Clot - Drops from destroying polyps
Dry Baby - Drops from destroying skulls
Ghost Baby - Drops from destroying skulls
Leech - Drops from destroying buckets in Downpour
Magic Mushroom - Drops from destroying mushrooms
Mini Mush - Drops from destroying mushrooms
Placenta - Drops from destroying polyps
The Poop - Drops from destroying buckets in Dross
A Quarter - Drops from destroying urns
Rotten Baby - Drops from destroying polyps in the Corpse
Rotten Meat - Drops from destroying polyps in the Corpse
The Small Rock - Drops from destroying tinted rocks and super special rocks
Wavy Cap - Drops from destroying mushrooms in a Secret Room

From Shopkeepers:

From Items and Trinkets:

The Bible - Drops from Pandora's Box on Womb II
Red Key - Drops from Pandora's Box on Home
A Lump of Coal - Drops from Mystery Gift
Lunch - Drops from Bag Lunch
The Poop - Drops from Mystery Gift

From Rooms:

Broken Shovel - Found by bombing the first starting room after defeating the first boss in under 1 minute
Broken Shovel - Found by defeating Boss Rush while holding Broken Shovel
Dad's Note - Found in Mausoleum and Gehenna II's Boss Room after entering The Strange Door
Flush! - Found in a rare room in the Mines
Inner Child - Found in the hidden closet in Home
Knife Piece 1 - Found in the Mirrored World's Treasure Room
Knife Piece 2 - Found at the end of the Mine Cart Room area
The Poop - Found in a rare room in Downpour and Dross
Redemption - Drops from Sacrifice Rooms after taking a Devil Deal
We Need To Go Deeper! - Found in Grave Rooms

From Bumbino after he collects 12 cents:

Chance from Hornfel after defeating his 2nd phase before he escapes:

From sacrificing familiars with Sacrificial Altar

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 from it is Breakfast.
Even though you technically cannot see items during Curse of the Blind, their weights are still being reduced upon encountering them.
- The same applies to blind items in alternative chapters.

The Binding of Isaac: Rebirth