Capacity Measurements

In December 1998, the International Electrotechnical Commission (IEC)the leading international organization for worldwide standardization in electrotechnologyapproved as an IEC International Standard names and symbols for prefixes for binary multiples for use in the fields of data processing and data transmission. Prior to this, a lot of confusion had existed as to whether a megabyte stood for 1 million bytes (10⁶) or 1,048,576 bytes (2²⁰). Even so, these new prefixes have yet to be widely adopted and confusion still reigns. The industry-standard abbreviations for the units used to measure the capacity of magnetic (and other) drives are shown in Table 8.4.

Table 8.4. Standard Abbreviations and Meanings

Abbreviation	Description	Power	Value
K	Kilo	103	1,000
Ki	Kibi	210	1,024
M	Mega	106	1,000,000
Mi	Mebi	220	1,048,576
G	Giga	109	1,000,000,000
Gi	Gibi	230	1,073,741,824
T	Tera	1012	1,000,000,000,000
Ti	Tebi	240	1,099,511,627,776
P	Peta	1015	1,000,000,000,000,000
Pi	Pebi	250	1,125,899,906,842,624

According to this prefix standard, 1 mebibyte (1 MiB = 2²⁰ B = 1,048,576 B) and 1 megabyte (1MB = 10⁶ B = 1,000,000 B) are not equal. Because these prefixes are not in widespread use (and they might never be), M in most cases can indicate both decimal millions of bytes and binary megabytes. Similarly, G is often used to refer to decimal billions of bytes and binary gigabytes. In general, memory values are expressed by using the binary values, although disk capacities can go either way. This often leads to confusion in reporting disk capacities because many manufacturers tend to use whichever value makes their products look better. For example, drive capacities are often rated in decimal billions (G - Giga), whereas most BIOS chips and operating system utilities, such as the Windows FDISK, rate the same drive in binary gigabytes (Gi - Gibi). Note also that when bits and bytes are used as part of some other measurement, the difference between bits and bytes is often distinguished by the use of a lower- or uppercase B. For example, megabits are typically abbreviated with a lowercase b, resulting in the abbreviation Mbps for megabits per second, whereas MBps indicates megabytes per second.