Minecraft wiki entry for History, associated text: The format of each chunk in the chunk index is only 4 bytes. Ex: 15 51 01 00 . The first byte describes how many 4kb sectors make up the chunk's data. In practice, this is always 21 (0x15). The next three bytes is a little-endian number describing how many 4kb sectors from the start of the file exist before the chunk data begins. If a chunk is not present, all 4 bytes will be zero 00 00 00 00.