Unit multiples of the byte are defined in a metric system based on the
powers of 10, following the
International System of Units (SI), which defines, for example, the prefix
kilo as 1000 (103), as well as a binary system based on
powers of two. Historically, the binary system used the identical prefixes of the metric system, but quantified differently. The nomenclature of the latter system has led to confusion. Systems based on powers of 10 use standard
SI prefixes (
kilo,
mega,
giga, ...) and their corresponding symbols (k, M, G, ...). The modern binary system uses prefixes
kibi,
mebi,
gibi, etc., and their corresponding symbols (Ki, Mi, Gi, ...). Historical usage for the binary system still uses the prefixes K, M, and G. While the difference between the decimal and binary interpretations is relatively small for the kilobyte (about 2% smaller than the kibibyte), the systems deviate increasingly as units grow larger (the relative deviation grows by 2.4% for each three orders of magnitude). For example, a power-of-10-based terabyte is about 9% smaller than power-of-2-based tebibyte.
Units based on powers of 10 (SI Prefixes) Definition of prefixes using powers of 10—in which 1
kilobyte (symbol kB) is defined to equal 1,000 bytes—is recommended by the International Electrotechnical Commission (IEC). The IEC standard defines eight such multiples, up to 1 yottabyte (YB), equal to 10008 bytes. The additional prefixes
ronna- for 10009 and
quetta- for 100010 were adopted by the
International Bureau of Weights and Measures (BIPM) in 2022. This definition is most commonly used for
data-rate units in
computer networks, internal bus, hard drive and flash media transfer speeds,
ISP contracts, and for the capacities of most
storage media, particularly
hard drives and
SSDs,
flash-based storage,
CDs,
DVDs and
Blu-rays.
Operating systems that use this definition include
Android (and derivatives),
macOS,
iOS, and
Debian,
ChromeOS,
HarmonyOS and the
Gnome desktop environment. It is also consistent with the other uses of the
SI prefixes in computing, such as
CPU clock speeds or
measures of performance. The IBM System 360 and the related disk and tape systems set the byte at 8 bits and documented capacities in decimal units. The early 8-, 5.25- and 3.5-inch floppies gave capacities in multiples of 1024, using "KB" rather than the more accurate "KiB". The later, larger, 8-, 5.25- and 3.5-inch floppies gave capacities in a hybrid notation, i.e., multiples of 1024,000, using and Early 5.25-inch disks used decimal even though they used 128-byte and 256-byte sectors. Hard disks used mostly 256-byte and then 512-byte before 4096-byte blocks became standard.
Units based on powers of 2 (IEC Prefixes) A system of units based on
powers of 2 in which 1 kibibyte (KiB) is equal to 1,024 (i.e., 210) bytes was created by the IEC to solve the confusion over incorrect usage of SI prefixes. It is defined by international standard IEC 80000-13, which is supported by national and international standards bodies (
BIPM,
IEC,
NIST). The IEC standard defines ten such multiples, up to 1 quebibyte (QiB), equal to 102410 bytes. These unit symbols are rarely used in practice. Notable exceptions are
KDE and applications based on the
Qt toolkit but the KDE settings app allows switching to SI. A historic convention of nomenclature for the same units, in which 1
kilobyte (KB) is equal to 1,024 bytes, 1
megabyte (MB) is equal to 10242 bytes and 1
gigabyte (GB) is equal to 10243 bytes is mentioned by a 1990s
JEDEC standard which is used for
RAM. Only the first three multiples (up to GB) are mentioned by the JEDEC standard, which makes no mention of TB and larger. These units can only be recognized through comparison using different software. Sometimes the capitalization of the K in KB can be an indicator. While confusing and incorrect, this convention is used by the
Microsoft Windows operating system and
random-access memory capacity, such as main memory and
CPU cache size, and in marketing and billing by some telecommunication companies, such as
Vodafone,
AT&T,
Orange and
Telstra. For
storage capacity, the historic convention was used by macOS and iOS through Mac OS X 10.5 Leopard and iOS 10, after which they switched to units based on powers of 10. and was advertised as "110 Kbyte", using the 1000 convention. Likewise, the 8-inch
DEC RX01 floppy (1975) held bytes formatted, and was advertised as "256k". Some devices were advertised using a
mixture of the two definitions: most notably, floppy disks advertised as "1.44 MB" have an actual capacity of , the equivalent of 1.47 MB or 1.41 MiB. In 1995, the
International Union of Pure and Applied Chemistry's (IUPAC) Interdivisional Committee on Nomenclature and Symbols attempted to resolve this ambiguity by proposing a set of
binary prefixes for the powers of 1024, including kibi (kilobinary), mebi (megabinary), and gibi (gigabinary). In December 1998, the
IEC addressed such multiple usages and definitions by adopting the IUPAC's proposed prefixes (kibi, mebi, gibi, etc.) to unambiguously denote powers of 1024. Thus one kibibyte (1 KiB) is 10241 bytes = 1024 bytes, one mebibyte (1 MiB) is 10242 bytes = bytes, and so on. In 1999,
Donald Knuth suggested calling the kibibyte a "large kilobyte" (
KKB).
Modern standard definitions The IEC adopted the IUPAC proposal and published the standard in January 1999. The IEC prefixes are part of the
International System of Quantities. The IEC further specified that the kilobyte should only be used to refer to bytes.
Lawsuits over definition Lawsuits arising from alleged consumer confusion over the binary and decimal definitions of multiples of the byte have generally ended in favor of the manufacturers, with courts holding that the legal definition of gigabyte or GB is 1 GB = (109) bytes (the decimal definition), rather than the binary definition (230, i.e., ). Specifically, the United States District Court for the Northern District of California held that "the U.S. Congress has deemed the decimal definition of gigabyte to be the 'preferred' one for the purposes of 'U.S. trade and commerce' [...] The California Legislature has likewise adopted the decimal system for all 'transactions in this state. Earlier lawsuits had ended in settlement with no court ruling on the question, such as a lawsuit against drive manufacturer
Western Digital. Western Digital settled the challenge and added explicit disclaimers to products that the usable capacity may differ from the advertised capacity.
Practical examples == Common uses ==