Gigabyte

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

Because of a traditional inconsistency, "gigabytes" are often intended to mean gibibytes in common speech. This usage is not recommended as it creates confusion (see below).

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"^[1], and "One gigabyte, or GB, equals one billion bytes when referring to hard drive capacity"^[2], and similar quotes are found on the websites of other storage manufacturers.
• 1,073,741,824 bytes, equal to 1024³, or 2³⁰ bytes. This is the definition used for computer memory sizes, and most often used in computer engineering, computer science, and most aspects of computer operating systems. The IEC recommends that this unit should instead be called a gibibyte (abbreviated GiB), as it conflicts with SI units used for bus speeds and the like. HP states that Microsoft normally adheres to this definition ^[3]

Gigabytes vs gigabits

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

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 140×10⁹ or 140 billion bytes after formatting. Generally, operating systems calculate disk and file sizes using binary numbers, but present them using decimal SI symbols (e.g. GB, MB, kB), so this 140 GB drive would be reported as "130.38 GB". The correct presentation would be either "140 GB" or "130.38 GiB". The result is that there is a significant discrepancy between what the consumer 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[4] 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), GameCube (1.5 GB), Playstation 2, Xbox, Xbox 360, and Wii (8.5 GB), and Playstation 3 (25 GB)/(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
• Orders of magnitude (data)

Notes

1. ^ [1]
2. ^ [2]
3. ^ [3]

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