The true capacity of hard disks are usually some number above or
below the class designation. Although most manufacturers of hard
disks and Flash disks define 1 gigabyte as 1,000,000,000 bytes, the
computer operating systems used by most users usually calculate a
gigabyte by dividing the bytes (whether it is disk capacity, file
size, or system RAM) by 1,073,741,824. This distinction is a cause
of confusion, as a hard disk with a manufacturer rated capacity of
400 gigabytes may have its capacity reported by the operating
system as only 372 GB, depending on the type of report.
The difference between units based on SI and binary prefixes
increases exponentially — in other words, an SI kilobyte is nearly
98% as much as a kibibyte, but a megabyte is under 96% as much as a
mebibyte, and a gigabyte is just over 93% as much as a
gibibyte. This means that a 500 GB hard disk drive
would appear as "465 GB". As storage sizes get larger and
higher units are used, this difference will become more
pronounced.