Chunk

A chunk is a 384-block tall 16×16 segment of a world. Chunks are the method used by the world generator to divide maps into manageable pieces.

Generation
Chunks are 16 blocks wide, 16 blocks long, 384 blocks high, and 98,304 blocks total. They extend from the bottom of the world, Y=-64, all the way up to the build limit of Y=320. Chunks generate around players when they first enter the world. As they wander around the world, new chunks generate as needed.

Chunks generate with the help of the map seed, which means that the chunks are always the same if you would use the same seed again, as long as the map generator and version number remain the same.

Spawn chunks, a 19x19 set of chunks around the world spawn, are always loaded, so you can use that to your advantage when making automatic farms.

Chunk loading
Since Minecraft worlds are 30 million blocks in each cardinal direction and contain an extreme number of chunks, the game loads only certain chunks in order to make the game playable.

Unloaded chunks are unprocessed by the game and do not process any of the game aspects.

Events that can happen when a chunk is loaded are not the same as events that happen only around the player. For example, normal mob spawning and random tick (determines crop growth, etc.) can happen only near a player, and does not happen in loaded chunks far away from any players.

Tickets
Loading starts when a chunk receives ticket. All loaded chunks originate from the ticket.

Each load ticket has three properties: Level, Ticket type and (optionally) Time to Live.

Level and load type
Levels are numbers that determine what load type the chunk is.

Load levels range from 22 to 44 in regular gameplay, while only 22 to 33 are relevant. Load levels less than 22 (including negative levels) are valid but possible only with a modded game. Load levels above 44 are not possible in vanilla. However, it could be possible by modifying the in-game code.

For a given chunk, only its lowest level matters. (Lower levels = higher load type.)

There are four chunk load types; each load type has different properties. This excludes unloaded chunks.

Level propagation
Load levels "propagate" or flow from source chunk with a ticket to neighboring chunks, but each time it increases its Level by 1 until the maximum of 44.

The chunks that get load level from level expansion activate the assigned load type.

Ticket types
There are different ticket types, which come from various sources and activate chunks with different load types.

This ticket is caused by the player and is assigned level 31. A player ticket is given to each chunk within an area surrounding the player as defined by the formulas described below and propagates as described in the table above.
 * Player ticket

In singleplayer, the chunk loading distance is one less than the render distance, as configured in Options. In 1.18+, the lesser of simulation distance or render distance is used for chunk loading. It follows the formula $$(2r - 3)^2 = \mathsf{AoC}$$ where r is the selected render or simulation distance, and $$\mathsf{AoC}$$ is the area of chunks that are assigned an original player ticket (level 31 - entity ticking). The minimum AoC is 9, which means that there is no difference between having the render distance set to 2 or 3.

For example, in a single-player game with a simulation distance of 5 chunks, an area of 7×7 chunks centered around the player has a load type of entity ticking (level 31). The strip of chunks at the outer edge of a 9×9 perimeter surrounding the player have ticking (level 32), and the next enclosing chunks (11×11 perimeter) are border chunks (level 33).

In multiplayer, the chunk loading distance is equal to the simulation-distance, as configured in server.properties. It follows the formula $$(2s + 1)^2 = \mathsf{AoC}$$ where s is the configured simulation distance and $$\mathsf{AoC}$$ is the area of chunks that are assigned an original player ticket (level 31 &minus; entity ticking).

For example, in a multiplayer server with a configured simulation distance of 5 chunks, an area of 11×11 chunks centered around the player has a load type of entity ticking (level 31), while the immediately enclosing chunks. The strip of chunks at the outer edge of a 13×13 perimeter surrounding the player have ticking (level 32), and the next enclosing chunks (15×15 perimeter) are border chunks (level 33).

A ticket can also be created by using the command. It has a level of 31, so its propagation can be seen in the table above.
 * Forced ticket

Chunks remain force-loaded even after closing and opening the game.

Ticket created by the world spawn for the chunk it is located at ("spawn chunks"). Its position can be changed by a command. It has a level of 22, the lowest in the game. They are active whenever the player is in the Overworld and are otherwise permanently active.
 * Start ticket

Ticket created when an entity is teleported through a nether portal, given to the chunk at the other side of the portal. It has a level of 30, so it is stronger than the player/force loaded ticket.
 *  Portal ticket

It expires after 300 game ticks (equivalent to 15 seconds). Because they are created each time an entity passes through the portal, it is possible to create a "chunk loader". Perpetually keeping chunks loaded without the player being near, which can be used for various in-game mechanics such as farms, but can create lag.

Ticket created at the start of the battle with the ender dragon and is given to the 0,0 chunk (the one that contains the exit portal). It has a level of 24.
 *  Dragon ticket

It expires after no more players participate in the battle, or the dragon dies.

Ticket created when entity is teleported either by going through the end portal or using or  commands. The command has a level of 32, whereas, for  and the end portal, it has a level of 33.
 * Post-teleport ticket

It expires after 5 game ticks.

Ticket created when an arbitrary piece of game code calls. If the game code says the chunk should be loaded, it places this ticket on that chunk (after possibly creating it).
 * Temporary ticket

The load level depends on what kind of chunk the game attempts to load (a fully world-generated chunk or not). It is always level 33 (border) or greater. In many cases, it involves world generation in some way.

For example, when a generic mob wandering AI is executing, it asks if a certain block has a solid top surface. As part of that check, that chunk has an "unknown" ticket with level 33 placed on it. Similarly, commands such as, ,  ,  and  generate a temporary ticket when reading data from or writing it to a previously loaded chunk; if run every tick, they can prevent the chunk from unloading until the world is reloaded.

It expires after 1 game tick.


 * Light ticket

Unknown, possibly world-generation related.

Limitations

 * Idle timeout


 * Each dimension has its own idle timeout. If there are players or forceloaded chunks in the dimension, the timeout is disabled. When entity leaves or enters this dimension, the timeout is reset.
 * If the timeout reaches 300 game ticks, the dimension stops processing certain actions. These include entity and block entity ticking, ender dragon fight, and global entities (lightning).


 * Some game aspects don't always get processed in loading chunks because there may be other conditions for their progress, which includes the following:
 * Chunk tick
 * Only chunks with centers within 128 blocks of a player are ticked on every game tick. See also Tick.
 * Entities
 * Hostile mobs instantly despawn if they spawn more than 128 blocks from any player.
 * This includes zombified piglins at Nether portals and witches in witch huts.
 * Passive mobs do not spawn naturally outside a 240 block X 240 block area around a player.
 * The passive mob spawn cap is limited by the number of friendly mobs loaded into memory, which means that any passive mobs present in the spawn chunks count toward the mob cap and usually prevents friendly mobs from naturally spawning anywhere else in the world. The only exception is when passive mobs spawn as part of a newly generated chunk.
 * See also Spawn.

Exceptions
Events in a chunk may affect blocks in outside chunks. If the outside chunk is inactive, the effects are suspended in most cases. Specifically,


 * Block changing on the edge of a ticking chunk can spread updates to blocks outside the ticking chunk and make them respond appropriately. The update may be propagated block by block until outside the border chunks, at which time it creates a temporary ticket to continue propagation.
 * Block in a chunk with a 33 level can request a scheduled tick, but it does not get processed until the chunk gets a load level of 32 or below.
 * Flowing water or lava can spread to the first adjacent block outside a ticking chunk, but the flow becomes suspended there until the border chunk has a greater load level.
 * Fire can spread to the first adjacent flammable block outside the ticking chunk. Like water and lava, it becomes suspended there. It cannot spread further until the outside chunk gets a level of 33 or below.
 * Grass and mycelium can spread to the first adjacent block outsider an entity ticking chunk.
 * Pumpkin and melon stem growing on the edge of an entity ticking chunk can place their fruits on an adjacent block outside an entity ticking chunk.
 * An entity (mob, minecart, arrow, etc.) that attempts to move into a ticking chunk from an entity ticking chunk becomes suspended as soon as it leaves the entity ticking chunk. When the ticking chunk gets a greater level, the entity resumes moving.
 * Exploding TNT in entity ticking chunks can damage or destroy blocks in a non-entity-ticking chunk.

Bedrock Edition chunk loading
All game aspects are active in loading chunks, including chunks within a player's simulation distance and chunks loaded by Commands/tickingarea. Unloaded chunks are unprocessed by the game and do not process any of the game aspects.

Type

 * Player
 * A round-like shape.
 * Chunks within the simulation distance of a player are loading.


 * Command
 * Create with the command.

Limit

 * Entities
 * Mob spawning is evaluated for every chunk within a 6 chunk cylindrical radius of the player that is loaded.

Exception
Events in a ticking area may affect blocks in outside chunks. If the outside chunk is inactive, the effects are suspended in most cases. Specifically,


 * Block changing on the edge of a ticking area can spread updates to blocks outside the ticking area and respond appropriately.
 * Flowing water or lava can spread to the first adjacent block in an outside chunk, but the flow becomes suspended there until the outside chunk becomes active.
 * Fire can spread to the first adjacent flammable block outside the ticking area. Like water and lava, it becomes suspended there; although visible, its animation does not run, and it cannot spread further until the outside chunk becomes active.
 * Grass and mycelium can spread to the first adjacent block in an outside chunk, but the affected block does not actually change its appearance until its chunk becomes active; it then changes instantly. Grass and mycelium cannot spread beyond the first such block, nor from such a block into the ticking area until the outside chunk becomes active.
 * Pumpkin and melon stem growing on the edge of a ticking area can place their fruits on an adjacent block in an outside chunk.
 * An entity (mob, minecart, arrow, etc.) that attempts to move into an outside chunk becomes suspended as soon as it leaves the ticking area. It remains visible but motionless. When the outside block becomes active, the entity resumes moving.
 * Exploding TNT can damage or destroy blocks in an inactive chunk, and unlike other events, its effects are not limited to adjacent blocks. However, secondary effects in the outside chunk are suspended until the chunk becomes active. For instance, if an explosion destroys a block that supported sand or gravel, the sand or gravel does not fall immediately. The same thing happens with items that were attached to destroyed blocks, such as item frames and Redstone torches; they do not drop until the chunk is activated.
 * Primed TNT launched into an inactive chunk is suspended in mid-air within the first outside block it enters. It disappears until the outer chunk becomes active, at which time it resumes its flight and countdown.

Finding chunk edges


X and Z coordinates that are divisible by 16 represent the boundaries between chunks. EG: (96, -32) is a corner where four chunks meet. One of those chunks is between X coordinates 80 to 96 and Z coordinates -48 to -32. Another one is between X coordinates 96 to 112 and Z coordinates -32 to -16, and so on. When either X or Z crosses a multiple of 16, the player is moving across chunks.

Essentially, the player is in the top-left corner (north-western) of a chunk when both X and Z coordinates are divisible by 16.

Additionally, the player can know the chunk they are on by this formula: The X of a chunk is floor(X coordinate / 16) The Z of a chunk is floor(Z coordinate / 16) Where floor is the largest previous integer. E.g. Floor( 27.9561 ) is 27 In other words, if X was 27, Z was −15 the chunk is chunk (Floor(27/16), Floor(−15/16)), meaning that the player is on chunk (1, −1). Also, the coordinates of a block within a chunk can be found by taking the coordinate mod 16.

$$, the key can be used to display chunk boundaries. Alternately, pressing the "F3" button opens the Debug screen that shows the player's X, Y, and Z coordinates, in addition to the "chunk" variable. These coordinates change as the player moves around. The player can know the chunk they are in by the variable "chunk".

$$, when toggling fancy graphics, the world renders again, loading only the chunk the player is in for a split second, briefly showing the chunk boundaries. When the player changes the render distance rapidly, chunk barriers appear as a blue line. Also, if in mid-air and bridging with full blocks, the next block placed fades into view when a chunk border is intersected, showing the chunk border. This is sometimes unreliable, and it happens only on chunk borders. This does not happen underground or when the block placed is close to more than one block.

Slime chunks
Slime chunks are specific chunks capable of spawning slimes below Y=40 regardless of Light. These chunks are otherwise same as all other chunks.

Java Edition
Slime chunks generate throughout the world (except in mushroom islands). 1 chunk in 10 is a slime chunk. These slime chunks are determined pseudo-randomly by combining their chunk coordinates with the seed of the world: Random rnd = new Random(   seed +    (int) (xPosition * xPosition * 0x4c1906) +    (int) (xPosition * 0x5ac0db) +     (int) (zPosition * zPosition) * 0x4307a7L +    (int) (zPosition * 0x5f24f) ^ 0x3ad8025f ); return rnd.nextInt(10) == 0; That is, using the chunk coordinates to help generate a seed, a random number between 0 and 9 inclusive is generated, and if that number is 0, the chunk can spawn slimes. To convert world coordinates to chunk coordinates, divide by 16 and round down. Note that are 32-bit integers ( int s).

Bedrock Edition
The slime chunk algorithm $$ is different from $$. The algorithm doesn't depend on the world seed, thus the chunks that slimes can naturally spawn in are the same for every world.

Trivia

 * If a player stands in a chunk that has not generated yet, the world immediately becomes invisible until they are in a valid chunk. This does not happen if the Y coordinates are beyond the chunk boundaries.
 * $$, if one of the sixteen 16×16×16 sections of a chunk doesn't have any blocks in it, placing a block there shows the same animation as a chunk loading. This bug makes the block that the player placed to turn to the same color of the sky in the direction the player is facing for about 1 second and starts fading away into the normal block texture. If the time is sunrise or sunset and the player places a block there, it actually changes colors constantly for 1 second if the player keeps bobbing their head up and down. The block also makes blocks behind it appear invisible.