Gigabyte

This article is about the unit of measurement. For the computer hardware manufacturer, see Gigabyte Technology. For the cartoon character, see ReBoot. For the virus writer, see Gigabyte (virus writer).

Multiples of bytes
SI decimal prefixes		Binary usage	IEC binary prefixes
Name (Symbol)	Value		Name (Symbol)	Value
kilobyte (kB)	103	210	kibibyte (KiB)	210
megabyte (MB)	106	220	mebibyte (MiB)	220
gigabyte (GB)	109	230	gibibyte (GiB)	230
terabyte (TB)	1012	240	tebibyte (TiB)	240
petabyte (PB)	1015	250	pebibyte (PiB)	250
exabyte (EB)	1018	260	exbibyte (EiB)	260
zettabyte (ZB)	1021	270	zebibyte (ZiB)	270
yottabyte (YB)	1024	280	yobibyte (YiB)	280
See also: Multiples of bits · Orders of magnitude of data

The gigabyte (pron.: /ˈɡɪɡəbaɪt/ GIG-ə-byt or /ˈdʒɪɡəbaɪt/) is a multiple of the unit byte for digital information storage.

The prefix giga means 10⁹ in the International System of Units (SI), therefore 1 gigabyte is 1000000000bytes. The unit symbol for the gigabyte is GB.

Historically, the term has also been used in some fields of computer science and information technology to denote the gibibyte, or 1073741824 (1024³ or 2³⁰) bytes. For instance, the Institute of Electrical and Electronics Engineers (IEEE) defined the unit accordingly for the use in power switchgear. In 2000, however, IEEE adopted the International Electrotechnical Commission (IEC) recommendation, which uses the metric prefix interpretation.

Today the usage of the unit gigabyte continues to depend on the context. When referring to disk capacities it usually means 10⁹ bytes, often stated explicitly on the manufacturer's permanent sticker. This also applies to data transmission quantities over telecommunication circuits, as the telecommunications and computer networking industries have always used the SI prefixes with their standards-based meaning. When referring to RAM sizes it most often (see binary prefix adoption) has a binary interpretation of 1024³ bytes, i.e. as an alias for gibibyte. File systems and software often list file sizes or free space in some mixture of SI units and binary units; they sometimes use SI prefixes to refer to binary interpretation – that is using a label of gigabyte or GB for a number computed in terms of gibibytes (GiB), continuing the confusion.

In order to address this the International Electrotechnical Commission has been promoting the use of the term gibibyte for the binary definition. This position is endorsed by other standards organizations including the IEEE, the International Committee for Weights and Measures (CIPM) and the U.S. National Institute of Standards and Technology (NIST), but the binary prefixes have seen limited acceptance. The JEDEC industry consortium continues to recommend the IEEE 100 nomenclature of using the metric prefixes kilo, mega and giga in their binary interpretation for memory manufacturing designations.

Definition

The term "gigabyte" is commonly used to mean either 1000³ bytes or 1024³ bytes. This originated as compromise technical jargon for the byte multiples that needed to be expressed by the powers of 2 but lacked a convenient name. As 1024 (2¹⁰) approximates 1000 (10³), roughly corresponding SI multiples, it was used for binary multiples as well. In 1998 the International Electrotechnical Commission (IEC) proposed standards for binary prefixes and requiring the use of gigabyte to strictly denote 1000³ bytes and gibibyte to denote 1024³ bytes. By the end of 2007, the IEC Standard had been adopted by the IEEE, EU, and NIST. Nevertheless, the term kilobyte continues to be widely used with the following two different meanings:

Base 10 definition

• 1 GB = 1000000000 bytes (= 1000³ B = 10⁹ B) is the definition recommended by the International System of Units (SI) and the International Electrotechnical Commission (IEC). This definition is used in networking contexts and most storage media, particularly hard drives, Flash-based storage, and DVDs, and is also consistent with the other uses of the SI prefix in computing, such as CPU clock speeds or measures of performance. The Mac OS X file manager from version 10.6 and higher is a notable example of this usage in software. Since Snow Leopard, file sizes are reported in decimal units.

Base 2 definition

• 1 GB = 1073741824 bytes (= 1024³ B = 2³⁰ B) is the definition used by Microsoft Windows in reference to computer memory (e.g., RAM). This definition is synonymous with the unambiguous IEC standard name gibibyte.

Consumer confusion

Since the early 2000s most consumer hard drive capacities are grouped in certain size classes measured in gigabytes. The exact capacity of a given drive is usually some number above or below the class designation. Although most manufacturers of hard disk drives and flash-memory disk devices define 1 gigabyte as 1000000000bytes, software like Microsoft Windows reports size in gigabytes by dividing the total capacity in bytes by 1073741824 (2³⁰ = 1 gibibyte), while still reporting the result with the symbol "GB". This practice causes confusion, as a hard disk with an advertised capacity of, for example, "400 GB" (meaning 400000000000bytes) might be reported by the operating system as only "372 GB" (meaning 372 GiB). Other software, like Mac OS X 10.6 and some components of the Linux kernel measure using the decimal units. The JEDEC memory standards uses the IEEE 100 nomenclatures which defines a gigabyte as 1073741824bytes (or 2³⁰ bytes).

The difference between units based on decimal and binary prefixes increases as a semi-logarithmic (linear-log) function—for example, the decimal kilobyte value is nearly 98% of the kibibyte, a megabyte is under 96% of a mebibyte, and a gigabyte is just over 93% of a gibibyte value. This means that a 300 GB (279 GiB) hard disk might be indicated variously as 300 GB, 279 GB or 279 GiB, depending on the operating system. As storage sizes increase and larger units are used, these difference become even more pronounced. Some legal challenges have been waged over this confusion such as a suit against Western Digital. Western Digital settled the challenge and added explicit disclaimers to products that the usable capacity may differ from the advertised capacity.

Because of its physical design, computer memory capacity is a multiple of base 2, thus, memory size at the hardware level can always be factored by a power of two. It is thus convenient to use binary units for non-disk memory devices at the hardware level, for example, in boards using DIMM memory. That is, a memory capacity of 1073741824bytes, for example, is conveniently expressed as 1 GiB as opposed to 1.074 GB. Software applications, however, allocate memory in varying degrees of granularity as needed to fulfill data structure requirements and binary multiples are usually not required. Other computer measurements, like storage hardware size, data transfer rates, clock speeds, operations per second, etc., do not depend on an inherent base, and are usually presented in decimal units.

Examples of gigabyte-sized storage

• One hour of SDTV video at 2.2 Mbit/s is approximately 1 GB.
• Seven minutes of HDTV video at 19.39 Mbit/s is approximately 1 GB.
• 114 minutes of uncompressed CD-quality audio at 1.4 Mbit/s is approximately 1 GB.
• A DVD-R can hold about 4.7 GB.
• A dual-layered Blu-ray disc can hold about 50 GB.

See also

• Orders of magnitude (data)
• Binary prefix

External links

• http://physics.nist.gov/cuu/Units/binary.html
• http://www.iec.ch/zone/si/si_bytes.htm
• http://www.quinion.com/words/turnsofphrase/tp-kib1.htm
• http://www.nist.gov/public_affairs/techbeat/tb9903.htm