The usage of the word "gigabyte" is ambiguous: the value depends on the context. When referring to RAM sizes and file sizes, it traditionally has a binary definition, of 10243 bytes. For general definitional purposes, it means exactly 10003 bytes, since the prefix "giga-" refers to 1000 "megas". In order to address this confusion, currently the International Electrotechnical Commission (IEC) promotes the use of the term "gibibyte" for the binary definition. This position is endorsed by other standards organizations including the Institute of Electrical and Electronics Engineers (IEEE), the International Committee for Weights and Measures (CPIM) and the National Institute of Standards and Technology (NIST).
It is commonly abbreviated GB or Gbyte (not to be confused with Gb, which is used for a gigabit).
The IEC's recommendations are frequently ignored amongst computer professionals, and "gigabyte" is used colloquially to mean 10243 bytes. The standard body JEDEC Solid State Technology Association acknowledges the conflict by noting that in light of the existence of the IEC recommendations this usage is deprecated but, in recognition of the widespread colloquial usage, it continues to recognize the definition of 10243 bytes solely in the context of semiconductor storage capacities.
In computer networking the conventional SI units are followed. Manufacturers of networking equipment always use 1000-bit kilobits as their basic unit of measurement.
|Abbreviation||No. of bytes||Usage|
|gigabytes||GB (Note: uppercase "B")||10003||Computer storage (e.g., 500 GB hard disk)|
|gigabytes||GB (Note: uppercase "B")||10243||Computer memory (e.g., 4 GB RAM)|
|gibibytes||GiB (Note: uppercase "B")||10243||Computer storage (e.g., 34 GiB file)|
|gigabit||Gbit or Gb||125*10002||Network throughput (eg 1 Gbit/s data transfer rate)|
The difference between units based on SI and binary prefixes increases exponentially — 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 at the hardware level (for example, in using DIMM memory units). RAM memory size as seen by application software has no consistent bias towards power of two units, as the operating system will allocate memory in other granularities. 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.
An example, take a hard drive that can store exactly 250 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 they 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. Several legal disputes have been waged over the confusion.
To further complicate matters, flash memory chips are organized in multiples of 2, but retail flash memory products have available capacities specified by multiples of 10. Removable flash storage products contain file systems that make the devices behave like hard disks instead of RAM, yet it is called 'memory'. In operating systems like Windows Vista, flash memory can indeed be treated like RAM.
The basis of the problem is that the official definition of the SI units is not well known, and some legal settlements include directions for manufacturers to use clearer information, e.g. by stating a hard disk's size in both GB and GiB. However, JEDEC memory standards still use the IEEE 100 nomenclatures.