Chunk format

Chunks store the terrain and entities within a 16x16x128 area. They also store precomputed lighting, heightmap data for Minecraft's performance and other meta information.

Chunks were first introduced in Minecraft Infdev. Before the addition of the region file format in Beta 1.3, chunks were stored as individual chunk files. The file names contained the chunk's position encoded in Base36 and the extension ".dat".

The Data, SkyLight, and BlockLight are arrays of 4-bit values. The low bits of the first byte of one of these corresponds to the first block in the Blocks array.

As of 1.2.0 chunk sizes changed to 16x16x256, for higher building.

NBT Structure

 * TAG_Compound("Level"): Chunk data.
 * TAG_Byte_Array("Blocks"): 32768 bytes of block IDs defining the terrain. 8 bits per block. See Block Format below for byte ordering.
 * TAG_Byte_Array("Data"): 16384 bytes of block data additionally defining parts of the terrain. 4 bits per block.
 * TAG_Byte_Array("SkyLight"): 16384 bytes recording the amount of sun or moonlight hitting each block. 4 bits per block. Makes day/night transitions smoother compared to recomputing per level change.
 * TAG_Byte_Array("BlockLight"): 16384 bytes recording the amount of block-emitted light in each block. 4 bits per block. Makes load times faster compared to recomputing at load time.
 * TAG_Int_Array("HeightMap"): 1024 bytes(256 TAG_Int) of heightmap data. 16 x 16. Each byte records the lowest level in each column where the light from the sky is at full strength. Speeds computing of the SkyLight. Note: This array's indexes are ordered Z,X whereas the other array indexes are ordered X,Z,Y.
 * TAG_List("Entities"): Each TAG_Compound in this list defines an entity in the chunk. See Entity Format below.
 * TAG_List("TileEntities"): Each TAG_Compound in this list defines a tile entity in the chunk. See Tile Entity Format below.
 * TAG_List("TileTicks"): Each TAG_Compound in this list is an "active" block in this chunk waiting to be updated. These are used to save the state of redstone machines, falling sand or water, and other activity. See Tile Tick Format below.
 * TAG_Long("LastUpdate"): Tick when the chunk was last saved.
 * TAG_Int("xPos"): X position of the chunk. Should match the file name.
 * TAG_Int("zPos"): Z position of the chunk. Should match the file name.
 * TAG_Byte("TerrainPopulated"): 1 or not present (true/false) indicate whether the terrain in this chunk was populated with special things. (Ores, special blocks, trees, dungeons, flowers, waterfalls, etc.) If set to zero then minecraft will regenerate the chunk.

Block Format
Blocks are laid out in sets of vertical columns, with the rows going east-west through chunk, and columns going north-south. Blocks in each chunk are accessed via the following method:

unsigned char BlockID = Blocks[ y + z * ChunkSizeY(=256) + x * ChunkSizeY(=256) * ChunkSizeZ(=16) ];

The coordinate system is as follows:
 * X increases East, decreases West
 * Y increases upwards, decreases downwards
 * Z increases South, decreases North

The Data, BlockLight, and SkyLight arrays have four bits (a nibble) for each byte of the Blocks array. Note that these arrays use a kind of 'little endian' nibble order: e.g. the least significant four bits of the first byte of the Data, BlockLight, or SkyLight arrays correspond to the first byte of the Blocks array, and the most significant four bits correspond to the second byte, etc.

Tile Tick Format
(added in version RC2)


 * TAG_Compound: Tile Tick data
 * TAG_Int("i"): Block ID
 * TAG_Int("t"): Number of ticks until processing. May be negative when processing is overdue.
 * TAG_Int("x"): Position
 * TAG_Int("y"): Position
 * TAG_Int("z"): Position

Entity Format
Every entity is an unnamed TAG_Compound contained in the Entities list of a chunk file. The sole exception is the Player entity, stored in level.dat.

All entities share this base:


 * TAG_Compound: Entity data
 * TAG_String("id"): Entity ID
 * TAG_List("Pos"): 3 TAG_Doubles describing the current X,Y,Z position of the entity.
 * TAG_List("Motion"): 3 TAG_Doubles describing the current dX,dY,dZ velocity of the entity. (Note: Falling into the Void appears to set this to ridiculously high speeds. 0,0,0 is no motion.)
 * TAG_List("Rotation"): Two TAG_Floats representing rotation in degrees.
 * TAG_Float[0]: The entity's rotation clockwise around the Y axis (called yaw). Due west is 0. Can have large values because it accumulates all of the entity's lateral rotation throughout the game.
 * TAG_Float[1]: The entity's declination from the horizon (called pitch). Horizontal is 0. Positive values look downward. Does not exceed positive or negative 90 degrees.
 * TAG_Float("FallDistance"): Distance the entity has fallen. Larger values cause more damage when the entity lands.
 * TAG_Short("Fire"): Number of ticks until the fire is put out. Negative values reflect how long the entity can stand in fire before burning.
 * TAG_Short("Air"): How much air the entity has, in ticks. Fills to a maximum of 200 in air, giving 10 seconds submerged before the entity starts to drown, and a total of up to 20 seconds before the entity dies. Decreases while underwater. If 0 while underwater, the entity loses 1 health per second.
 * TAG_Byte("OnGround"): 1 if the entity is touching the ground.

SkeletonFace.png Mobs
Known Mob Entity ids: Blaze, CaveSpider, Chicken, Cow, Creeper, EnderDragon, Enderman, Ghast, Giant, LavaSlime, Mob, Monster, MushroomCow, Ozelot, Pig, Sheep, Silverfish, Skeleton, Slime, SnowMan, Spider, Squid, Villager, Wolf, Zombie, and PigZombie

Additional fields for mobs:


 * TAG_Short("AttackTime"): Number of ticks the entity's "invincibility shield" is lasting after the entity was last struck.
 * TAG_Short("DeathTime"): Number of ticks the entity has been dead for. Controls death animations.
 * TAG_Short("Health"): Amount of health the entity has. Players and enemies normally have up to 20 health. Livestock has up to 10 health.
 * TAG_Short("HurtTime"): how long the mob turns red for after being hit

Pig has one additional field:


 * TAG_Byte("Saddle"): 1 or 0 (true/false) - true if there is a saddle on the pig.

Sheep has two additional fields:


 * TAG_Byte("Sheared"): 1 or 0 (true/false) - true if the sheep has been shorn.
 * TAG_Byte("Color"): 0 to 15 - see wool data values for a mapping to colors.

Creeper has one additional/optional field:


 * TAG_Byte("powered"): 1 or 0 (true/false) - true if the creeper was struck by lightning (doubles the explosive power). This tag is only written if value is 1 (true).

Slime has one additional field, and it can't be used as the Entity ID of a Monster Spawner:


 * TAG_Int("Size"): The size of the slime

Wolf has three additional fields:


 * TAG_String("Owner"): Name of the player that owns this wolf. Empty string if no owner.
 * TAG_Byte("Sitting"): 1 or 0 (true/false) - true if the wolf is sitting.
 * TAG_Byte("Angry"): 1 or 0 (true/false) - true if the wolf is angry.

PigZombie has one additional field:


 * TAG_Short("Anger"): Anger level. Determines the agressivity of the creature towards players.

Enderman has two additional fields:


 * TAG_Short("carried"): Id of the block carried by the Enderman.
 * TAG_Short("carriedData"): Additional data about the block carried by the Enderman.

Arrow.png Projectiles
Known Projectile Entity ids: Arrow, Snowball, Egg, Fireball, SmallFireball, ThrownEnderpearl

Additional fields for Arrow, Snowball, Egg, Fireball (from Ghasts), SmallFireball (from Blazes), ThrownEnderpearl, and fire charge:


 * TAG_Short("xTile"): X coordinate of the item's position in the chunk.
 * TAG_Short("yTile"): Y coordinate of the item's position in the chunk.
 * TAG_Short("zTile"): Z coordinate of the item's position in the chunk.
 * TAG_Byte("inTile"): For arrows being stuck into blocks.
 * TAG_Byte("shake"): The "shake" when arrows hit a block.
 * TAG_Byte("inGround"): 1 or 0 (true/false) - Unknown

Stone.png Items
Known Item Entity ids: Item, Painting, XPOrb

Additional fields for Item:


 * TAG_Short("Health"): Starts at 5, and currently only decreases as the item takes fire damage. When health reaches 0, the item is destroyed.
 * TAG_Short("Age"): The amount of time an item has been "untouched" on the ground. After 6000 'ticks' (5 minutes ) the item is destroyed.
 * TAG_Compound("Item"): Item data
 * TAG_Short("id"): Item or Block ID.
 * TAG_Short("Damage"): The amount of wear each item has suffered. 0 means undamaged. When the Damage exceeds the item's durability, it breaks and disappears. Only tools and armor accumulate damage normally.
 * TAG_Byte("Count"): Number of items stacked in this inventory slot. Any item can be stacked, including tools, armor, and vehicles. Range is 1-255. Values above 127 are not displayed in-game.
 * TAG_Compound("tag")
 * TAG_List("ench"): Contains enchantments for this item
 * Tag_Compound: Make one of these for each enchantment
 * TAG_Short("id"): Id of the Enchantment
 * TAG_Short("lvl"): Level of the Enchantment

Additional fields for Painting:


 * TAG_Byte("Dir"): Direction the painting faces: 0 is east, 1 is north, 2 is west, and 3 is south.
 * TAG_String("Motive"): The name of this Painting's art.
 * TAG_Int("TileX"): X coordinate of the block the painting is hanging on.
 * TAG_Int("TileY"): Y coordinate of the block the painting is hanging on.
 * TAG_Int("TileZ"): Z coordinate of the block the painting is hanging on.

Additional fields for XPOrb:


 * TAG_Short("Health"): Starts at 5, and currently only decreases as the item takes fire damage. When health reaches 0, the item is destroyed.
 * TAG_Short("Age"): The amount of time an orb has been "untouched" on the ground. After a certain amount of ticks, the orb is destroyed.
 * TAG_Short("Value"): The amount of experience the orb gives when picked up.

Boat.png Vehicles
Known Vehicle Entity ids: Minecart, Boat

Additional fields for Minecart:


 * TAG_Int("Type"): The type of the cart: 0 - empty, 1 - with a chest, 2 - with a furnace.

For a minecart with a furnace there are three additional fields:


 * TAG_Double("PushX"): Unknown
 * TAG_Double("PushZ"): Unknown
 * TAG_Short("Fuel"): Unknown, but probably number of ticks until fuel runs out.[ assumption ]

For a minecart with a chest there is one additional field:


 * TAG_List("Items"): List of items in the slots, same as for a chest.

Dynamic Tiles
Known Dynamic Tile Entity ids: PrimedTnt, FallingSand

Additional fields for PrimedTnt


 * TAG_Byte("Fuse"): Ticks until explosion.

Additional fields for FallingSand:


 * TAG_Byte("Tile"): Block ID.

Tile Entity Format
Known TileEntity ids: Furnace, Sign, MobSpawner, Chest, Music, Trap, RecordPlayer, Piston, Cauldron, EnchantTable, and Airportal

All tile entities share this base:
 * TAG_Compound: Tile entity data
 * TAG_String("id"): Tile entity ID
 * TAG_Int("x"):
 * TAG_Int("y"):
 * TAG_Int("z"): Coordinates of the TileEntity.

A tile entity has additional fields depending on its id:

Furnace.png Furnace
Note that the total burning time for the currently consumed fuel isn't stored, so if you save a world with a half-flame in the furnace, it will be restored into a full flame (but it will be consumed twice as fast).
 * TAG_Short("BurnTime"): Fuel time = Number of ticks left for the current fuel. Decremented each game tick.
 * TAG_Short("CookTime"): Cooking time = Number of ticks that the item has been smelted. When it reaches 200, the item gets smelted and another burn operation starts. Incremented each game tick if the furnace is burning.
 * TAG_List("Items"): List of items in the furnace slots. Each item is a TAG_Compound identical to the ones in the Inventory list. The first entry corresponds to the burning item, the second one is for the fuel and the last one for the result. The list may be empty.

Sign.png Sign

 * TAG_String("Text1"): First row of text
 * TAG_String("Text2"): Second row of text
 * TAG_String("Text3"): Third row of text
 * TAG_String("Text4"): Fourth row of text

Monster Spawner.png MobSpawner

 * TAG_String("EntityId"): The id of the mob.
 * TAG_Short("Delay"): Ticks until next spawn.

Chest.png Chest

 * TAG_List("Items"): List of items in the chest. Each item is a TAG_Compound identical to the ones in the Inventory list. Chest slots are numbered 0-26 with 0 in the top left corner.

Note Block.png Music

 * TAG_Byte("note"): Pitch (number of right-clicks)

Dispenser.png Trap

 * TAG_List("Items"): List of items in the dispenser, numbered 0-8.

Jukebox.png RecordPlayer

 * TAG_Int("Record"): Record currently playing. 0 is no record. Max value currently used is 2.

Piston.png Piston
This tile entity is that of Block 36, not any part of the piston itself.


 * TAG_Int("blockId"): ID of the block being moved
 * TAG_Int("blockData"): Additional data about the block being moved
 * TAG_Int("facing"): Direction in which the block is being pushed
 * TAG_Float("progress"): How far the block as been pushed
 * TAG_Byte("extending"): Boolean value indicating if the piston is extending (1) or contracting (0).

Brewing Stand.png Cauldron
Note that despite being called "Cauldron", this Tile Entity only appears on Brewing Stands as of 1.9pre3.


 * TAG_List("Items"): List of items in the brewing stand. Each item is a TAG_Compound identical to the ones in the Inventory list. The slots are numbered from 0-3.
 * TAG_Int("BrewTime"): Time brewing in ticks.

Enchantment Table.png EnchantTable
The enchantment table has no extra tags besides the base ones. It is used for the rotation and opening/closing of the floating book.

End Portal.png Airportal
This is the tile entity associated with the enterable End Portal (not the End Portal Frame). It has no extra tags.