Since the early 1990's most of consumer hard drive capacities are grouped in certain size classes measured in gigabytes. The exact capacity of a given drive is usually a number above or below the actual size.

Although most manufacturers of hard disk drives and flash-memory disk devices define 1 gigabyte as 1000000000bytes, the computer operating systems used by most users usually calculate size in gibibytes by dividing the total capacity in bytes by 1073741824, but report the result with the symbol GB. This practice can be a cause of confusion, as a hard disk with a manufacturer-rated capacity of 400 gigabytes may be reported by the operating system as only 372 GB.

This means that a perceived 300 GB  hard disk is actually only as 279 GB. If you are buying a PC with an operating system such as Windows 7 installed then the available capacity will be even less because of the space required by the operating system.