1457664 is the capacity of a DOS-formatted 3.5" IBM PC floppy disk in bytes. In the most common configuration, they have 512 bytes per sector, 18 sectors per track, 80 tracks per side, and two sides. This is 512*18*80*2=1474560. So where is the missing 16896 bytes? The answer is simple: A formatted floppy disk have exactly one boot sector and a 32-sector cluster which holds the FAT table and the root directory entry. That is exactly 512*33 = 16896 bytes.