Minecraft wiki entry for Region_file_format, associated text: Location information for a chunk consists of four bytes split into two fields: the first three bytes are a (big-endian) offset in 4KiB sectors from the start of the file, and a remaining byte that gives the length of the chunk (also in 4KiB sectors, rounded up). Chunks are always less than 1MiB in size. If a chunk isn't present in the region file (e.g. because it hasn't been generated or migrated yet), both fields are zero.