The basic unit of hard drive capacity is the byte. Hard drive capacities currently range from billions of bytes (gigabytes) up to approximately a trillion bytes (a terabyte).

Microsoft Windows reports the capacity of a computer's hard drive on its Disk Properties page. In the sample page at right, Windows shows the capacity of the built-in hard drive as 100,027,596,800 bytes, and also as 93.1 GB.

The apparent difference between these two capacity values for this hard drive reflects two different concepts of the size of a gigabyte (GB), when referring to hard drives.

To specify the capacity of a hard drive in gigabytes, hard drive manufacturers divide the total number of bytes by one variety of gigabyte -- 1,000,000,000 (1 billion) bytes:

        100,027,596,800 bytes / 1,000,000,000 = 100 GB

The product literature of computer manufacturers, including Toshiba, generally follows hard drive manufacturers' product specifications. Toshiba describes this particular hard drive, therefore, as having a capacity of 100 GB.

Microsoft Windows, however, indicates the capacity of a hard drive in gigabytes by dividing the total number of bytes on the drive by a different gigabyte value -- 1,073,741,824 bytes:

        100,027,596,800 bytes / 1,073,741,824 = 93.1 GB
          
This gigabyte value is rooted in the binary organization of computer memory (RAM) which increases in capacity by powers of two. Computer memory has always been measured in kilobytes (1024 bytes), megabytes (1024 x 1024 bytes), and gigabytes (1024 x 1024 x 1024 bytes = 1,073,741,824 bytes).

Windows applies this larger memory-based gigabyte value not only to the computer's memory, but also to its hard drive.

Windows divides the total number of bytes on the hard drive by this larger gigabyte (1,073,741,824 bytes), resulting in the smaller hard disk capacity value reported by the Windows Disk Properties page (93.1 GB) compared to the manufacturer's specification for its capacity (100 GB) -- in this particular example.

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The hard drives in some Toshiba computers have a special partition that is invisible to Microsoft Windows. The capacity values displayed on the Disk Properties page do not include the size of this partition. This partition is used for special Toshiba applications, including the Express Media Player and the Hard Drive Recovery program.

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