Chunk Format

Revision as of 10:10, 12 April 2016 by Fenhl (talk | contribs) (added missing info about a value of exactly 9, which should use the global palette based on previous versions of this article; formatting)
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Warning.png This information, particularly about the palette and when it can be omitted, is incorrect. The article will be corrected as this format is researched and reverse-engineered. Sorry for any inconvenience.

This article describes the Chunk Section format used in the Chunk Data packet (Play, 0x20, clientbound).


  • Chunk Section: a 16×16×16 area, sometimes also called chunk.
  • Chunk Column: 16 chunks aligned vertically (totalling 16×256×16).


A Chunk Section is defined in terms of other data types. A Chunk Section consists of the following fields:

Field Name Field Type Notes
Bits Per Block Unsigned Byte Determines how many bits are used to encode a block. Note that not all numbers are valid here. This also changes whether the palette is present.
Palette Length VarInt Length of the following array. May be 0.
Palette Array of VarInt Mapping of block state IDs in the global palette to indices of this array
Data Array Length VarInt Number of bytes in the following array
Data Array Long Array List of 4096 indices pointing to state IDs in the Palette
Block Light Byte Array Half byte per block
Sky Light Optional Byte Array Only if in the Overworld; half byte per block

Data Array, Block Light, and Sky Light are given for each block with increasing x coordinates, within rows of increasing z coordinates, within layers of increasing y coordinates.

There are several values that can be used for the bits per block value. In most cases, invalid values will be treated as if it is a different value when parsed by the Notchian client, meaning that chunk data will be parsed incorrectly if you use an invalid bits per block. Servers must make sure that the bits per block value is correct.

  • 4: Blocks are encoded as 4 bits. The palette array is used and sent.
  • 5 to 8: Blocks are encoded with the given number of bits. The palette array is used and sent.
  • 9 and above: Blocks are encoded by their whole ID in the global palette, with bits per block being set as the base 2 logarithm of the number of block states, rounded up. For the current vanilla release, this is 13 bits per block.

The global palette encodes a block as 13 bits. It uses the block ID for the first 9 bits, and the block damage value for the last 4 bits. For example, diorite (block ID 1 for minecraft:stone with damage 3) would be encoded as 000000001 0011. If a block is not found in the global palette (either due to not having a valid damage value or due to not being a valid ID), it will be treated as air.

In half-byte arrays, two values are packed into each byte. Even-indexed items are packed into the low bits, odd-indexed into the high bits.


The following implement the previous (before 1.9) format: