The capacity of an HDD can be calculated by multiplying the number of cylinders by the number of heads by the number of sectors by the number of bytes/sector (most commonly 512). On ATA drives bigger than 8 gigabytes, the values are set to 16383 cylinder, 16 heads, 63 sectors for compatibility with older operating systems. It should be noted that the values for cylinder, head & sector reported by a modern drive are not the actual physical parameters since, amongst other things, with zone bit recording the number of sectors varies by zone.
Hard disk drive manufacturers specify disk capacity using the SI prefixes mega, giga, and tera and their abbreviations M, G and T, respectively. Byte is typically abbreviated B.
Operating systems frequently report capacity using the same abbreviations but in reference to binary-based units. For instance, the prefix mega in the context of data storage can mean 220 (1,048,576), which is approximately equal to the actual value of the SI prefix mega, 106 (1,000,000). Similar usage has been applied to prefixes of greater magnitude. This results in a discrepancy between the disk manufacturer's stated capacity and the apparent capacity of the drive when examined from the operating system. The difference becomes much more noticeable in the multi-gigabyte range. For example, Microsoft Windows reports disk capacity both in decimal-based units to 12 or more significant digits and with binary-based units to 3 significant digits. Thus a disk specified by a disk manufacturer as a 30 GB disk might have its capacity reported by Windows 2000 both as "30,065,098,568 bytes" and "28.0 GB" The disk manufacturer used the SI definition of "giga", 109 to arrive at 30 GB; however, because the utilities provided by Windows define a gigabyte as 1,073,741,824 bytes (230 bytes, often referred to as a gibibyte, or GiB), the operating system reports capacity of the disk drive as 28.0 GB.