Gigabyte

The gigabyte 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 1 000  000  000  bytes. The unit symbol for the gigabyte is GB or Gbyte, but not Gb (lower case b) which is typically used for the gigabit.

Historically, the term has also been used in some fields of computer science and information technology to denote the gibibyte, or (10243 or 230) bytes. 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 of usage. When referring to disk storage capacities it usually means 10003 bytes. 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.

Consumer confusion
Since the early 2000s most of 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 1 000  000  000  bytes, software like Microsoft Windows reports size in gigabytes by dividing the total capacity in bytes by 1  073  741  824, while still reporting the result with the symbol "GB". This practice is a cause of confusion, as a hard disk with a manufacturer-rated capacity of 400 gigabytes might be reported by the operating system as only "372 GB", for instance. Other software, like Mac OS X 10.6 and some components of the Linux kernel measure using the decimal units.

In the Content Delivery Network industry, where billing is often done by the Gigabyte, there can be a difference up to 7% between vendors based on the definition of the size. Some companies including 3Crowd and BitGravity, have referenced agreed upon definitions for 1024 Megabytes multiplied by 1000 as a Barretbyte GbB, referencing BitGravity co-founder Barrett Lyon for billing purposes.

The JEDEC memory standards uses the IEEE 100 nomenclatures which defines a gigabyte as 1 073  741  824  bytes (or 230 bytes).

The difference between units based on SI and binary prefixes increases as a semi-logarithmic (linear-log) function—for example, the SI 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 is indicated only as 279 GB. As storage sizes increase and larger units are used, this difference becomes 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 is addressed in multiples 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 using DIMM memory boards. Software application, however, allocate memory, usually virtual 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 4.7 GB.
 * A dual-layered Blu-ray disc can hold 50 GB.
 * A Universal Media Disc can hold 0.9 GB of data. (1.8 GB on dual-layered discs.)