Gigabyte

Multiple-byte units
Decimal Value Metric 1000 kB kilobyte 10002 MB megabyte 10003 GB gigabyte 10004 TB terabyte 10005 PB petabyte 10006 EB exabyte 10007 ZB zettabyte 10008 YB yottabyte 10009 RB ronnabyte 100010 QB quettabyte	Binary Value IEC Memory 1024 KiB kibibyte KB kilobyte 10242 MiB mebibyte MB megabyte 10243 GiB gibibyte GB gigabyte 10244 TiB tebibyte TB terabyte 10245 PiB pebibyte – 10246 EiB exbibyte – 10247 ZiB zebibyte – 10248 YiB yobibyte – 10249 – – 102410 – –
Orders of magnitude of data

A gigabyte (derived from the SI prefix giga-) is a unit of information or computer storage equal to one billion (short scale) bytes or 2³⁰ bytes (1024 mebibytes). It is commonly abbreviated GB (not to be confused with Gb, which is used for gigabits).

The usage of the word "gigabyte" is ambiguous, depending on the context. When referring to RAM sizes and file sizes, it traditionally has a binary definition, of 1024³ bytes (typically assimilated or approximated to 1000³, for convenience). For every other use, it means exactly 1000³ bytes. In order to address this confusion, currently all relevant standards bodies promote the use of the term "gibibyte" for the binary definition.

Definition

There are two slightly different definitions of the size of a gigabyte in use:

• 1,000,000,000 bytes or 10⁹ bytes is the decimal definition used in telecommunications (such as network speeds) and most computer storage manufacturers (such as hard disks and flash drives). This usage is compatible with SI. Quotes from Seagate: "The storage industry standard is to display capacity in decimal", and, "One gigabyte, or GB, equals one billion bytes when referring to hard drive capacity". Similar quotes are found on the websites of other storage manufacturers.
• 1,073,741,824 bytes, equal to 1000³, or 2³⁰ bytes. This is the definition commonly used for computer memory and file sizes. Since 1999, the IEC recommends that this unit should instead be called a gibibyte (abbreviated GiB). Microsoft use this definition to display hard drive size, as do most other operating systems . Every operating system uses this definition when referring to the size of files.

Gigabytes vs gigabits

In conventional modern usage, a byte is 8 bits. One gigabyte is equivalent to eight gigabits.

	Abbreviation	No. of megabytes	Usage
gigabytes	GB (Note: capital "B")	1000	Computer storage (eg 500 GB hard disk)
gigabytes	GB (Note: capital "B")	1000	Computer storage (eg 4 GB RAM)
gigabit	Gb (Note: lowercase "b")	125	Network throughput (eg 1 Gb/s data transfer rate)

Consumer confusion

As of 2007, most consumer hard drives are defined by their gigabyte-range capacities. The true capacity is usually some number above or below the class designation. Although most manufacturers of hard disks and Flash disks define 1 gigabyte as 1,000,000,000 bytes, the computer operating systems used by most users usually calculate a gigabyte by dividing the bytes (whether it is disk capacity, file size, or system RAM) by 1,073,741,824. This distinction is a cause of confusion, especially for people from a non-technical background, as a hard disk with a manufacturer rated capacity of 40 gigabytes may have its capacity reported by the operating system as only 37.2 GB, depending on the type of report.

The difference between SI and binary prefixes is logarithmic — in other words, an SI kilobyte is nearly 98% as much as a kibibyte, but a megabyte is under 96% as much as a mebibyte, and a gigabyte is just over 93% as much as a gibibyte. This means that a 500 GB hard disk drive would appear as "465 GB". As storage sizes get larger and higher units are used, this difference will become more pronounced.

Note that computer memory is addressed in base 2, due to its design, so memory size is always a power of two (or some closely related quantity, for instance 384 MiB = 3×2²⁷ bytes). It is thus convenient to work in binary units for RAM. Other computer measurements, like storage hardware size, data transfer rates, clock speeds, operations per second, etc., do not have an inherent base, and are usually presented in decimal units.

As an example, take a hard drive that can store exactly 250×10⁹ or 250 billion bytes after formatting. Generally, operating systems calculate disk and file sizes using binary numbers, so this 250 GB drive would be reported as "232.83 GB". The result is that there is a significant discrepancy between what the consumer believes to have purchased and what their operating system says they have.

Some consumers feel short-changed when they discover the difference, and claim that manufacturers of drives and data transfer devices are using the decimal measurements in an intentionally misleading way to inflate their numbers, though these measurements are the norm in all fields other than computer memory. Several legal disputes have been waged over the confusion. See Binary prefix — Legal disputes.

The basis of the problem is of course that the official definition of the SI units is not well known, and some legal settlements include directions for manufacturers to use clearer info, e.g. by stating a hard disk's size in both GB and GiB.

Gigabytes in use

• The human genome contains 0.791175 GB of data (the 3.1647×10⁹ base pairs[3] represented as 2-bits).
• A DVD-5 format disc is capable of storing 4.7 gigabytes, or roughly 4.38 gibibytes. A DVD-9 is capable of storing 8.5 gigabytes, or roughly 7.92 gibibytes.
• One gigabyte is roughly equal to 18 hours of MP3 music (at 128 kbit/s).
• One gigabyte is roughly equivalent to 11 hours, 40 minutes of Flash video (at 450x370).
• Most 6th generation and all 7th generation game consoles have game discs that are around 1 GB or more: Dreamcast (1.1 GB), Nintendo GameCube (1.5 GB), PlayStation 2, Xbox, Xbox 360, and Wii (8.5 GB), and PlayStation 3 (25/50 GB).
• Dual-layer Blu-ray Discs and dual-layer HD DVD discs can hold about 50 gigabytes and 30 gigabytes of data, respectively.

See also

• Gibibyte
• Gigabit
• Binary prefix
• JEDEC memory standards
• Orders of magnitude (data)

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