Minecraft wiki entry for History, associated text: As an example, if a chunk in the chunk index at 1,0 (X,Z) reads 15 51 01 00 we can locate the chunk data within the file. First we get the decimal representation of the last three bytes which are in little-endian 51 01 00 equals 337 in decimal. Next we calculate the offset by multiplying our result by the size of a sector 337 * 4,096. Last we get the decimal representation of the first byte which is always 21 (0x15) and multiply it by the size of a sector 21 * 4,096. As a result, we know this chunk is located at the decimal offset of 1,380,352 and is 86,016 bytes in size.