We calculate 80 GB is equal to 80000 MB but when a operating system scan this disc space it calculates 1 GB = 1024 MB so 80 GB H.D.D. Shows only 76 GB .
Usable space of an 80Gb hard disk a almost 74 to 76 Gb. The reason is partition table. When you partitioned your disk the information about the partion and file system is kept in a partition table. That table require some space to which you are asking.