Quick Description:
You get a drive, "formatted capacity" as it is called by marketing people will always be less. 80gb becomes 74. 300gb becomes 280. and so on.
Long Description: (from Wikipedia)
Hard drive manufacturers typically specify drive capacity using 'SI prefixes', that is, the SI definition of the prefixes "giga" and "mega." This is largely for historical reasons, since disk drive storage capacities exceeded millions of bytes[1] long before there were standard 'binary prefixes' (even before there were the SI prefixes, 1960). The IEC only standardized 'binary prefixes' in 1999. As it turned out, many practitioners early on in the computer and semiconductor industries adopted the term kilobyte to describe 210 (1024) bytes because 1024 is "close enough" to the metric prefix kilo, which is defined as 103 or 1000. Sometimes this non-SI conforming usage include a qualifier such as '"1 kB = 1,024 Bytes"' but this qualifier was frequently omitted, particularly in marketing literature. This trend became habit and continued to be applied to the prefixes "mega," "giga," "tera," and even "peta."
Operating systems and their utilities, particularly visual operating systems such as Microsoft Windows, frequently report capacity using binary prefixes which results in a discrepancy between the drive manufacturer's stated capacity and the system's reported capacity. Obviously the difference becomes much more noticeable in reported capacities in the multiple gigabyte range, and users will often notice that the volume capacity reported by their OS is significantly less than that advertised by the hard drive manufacturer. For example, Microsoft's Windows 2000 reports drive capacity both in decimal to 12 or more significant digits and with binary prefixes to 3 significant digits. Thus a disk drive specified by a drive manufacturer as a '30 GB' drive has its capacity reported by Windows 2000 both as '30,065,098,568 bytes' and '28.0 GB'. The drive manufacturer has used the SI definition of "giga," 109 and can be considered as an approximation of a gibibyte. Since utilities provided by the operating system probably define a gigabyte as 230, or 1073741824, bytes, the reported capacity of the drive will be closer to 28.0 GB. For this very reason, many utilities that report capacity have begun to use the aforementioned IEC standard binary prefixes (e.g. KiB, MiB, GiB) since their definitions are unambiguous.
Many people mistakenly attribute the discrepancy in reported and specified capacities to reserved space used for file system and partition accounting information. However, for large (several GiB) filesystems, this data rarely occupies more than a few MiB, and therefore cannot possibly account for the apparent "loss" of tens of GBs.
The capacity of a hard drive can be calculated by multiplying the number of cylinders by the number of heads by the number of sectors by 512 bytes/sector.
