A gigabyte (symbol GB) is a unit of measurement in computers of one thousand million bytes (the same as one billion bytes in the short scale usage). However, because computers work on the binary system, rather than a gigabyte being 10³ megabytes (1000 MBs), the term gigabyte actually means 2¹⁰ megabytes (1024 MBs).

Because of differences in definition between the standard SI implementation of prefixes and the computer implementations, the exact number in common practice could be either one of the following:

• 1,000,000,000 bytes or 10⁹ bytes is the definition used by telecommunications engineers and some storage manufacturers. This is consistent with the SI prefix "giga-".
• 1,073,741,824 bytes, equal to 1024³, or 2³⁰. This is the definition most often used in computer engineering, computer science, computer programming, and almost all computer operating systems. It has been suggested that this measure can be abbreviated as GiB (gibibyte) to avoid ambiguity, as defined in IEC 60027-2 but is not commonly done as such.

See integral data type.

Likewise, a terabyte is either equal to 1024 gigabytes or to 1000 gigabytes depending on the usages.

As a result of this confusion, the unadorned term gigabyte is useful only where just one digit of precision is required. In technical specifications, the first usage is typically expanded to remove the ambiguity ("GB is one billion bytes"). The only exception is RAM, where sizes are always given in the power-of-two units natural to this domain.

Thus, to convert metric gigabytes into binary gigabytes (for example a 100GB drive contains 93GiB when installed), follow this formula:

where y is size of drive in metric gigabytes

megabyte << gigabyte << terabyte

Gigabytes in use

As of 2005, most consumer hard drives are measured in gigabyte-range capacities. Per-gigabyte costs are 0.50-0.80 USD.

In speech, gigabyte is often abbreviated to gig, as in "This is a ten-gig hard drive". The initial G in giga- is usually pronounced hard as in geek, not soft as in giant, however both pronounciations are equally valid.

A gigabit, which should not be confused with gigabyte, is 1/8th of a gigabyte and is mainly used to describe bandwidth, e.g. 2 gigabit/s is the speed of current Fibre Channel interfaces.

Unicode has a symbol for Gigabyte: (㎇).

Distinction between 1000 and 1024 megabytes

Main article: Binary prefix

To clarify the distinction between decimal and binary prefixes, the International Electrotechnical Commission (IEC), a standards body, in 1997 proposed short unions of the International System of Units (SI) prefixes with the word "binary". Thus meaning (2) would be called a gibibyte (GiB). This naming convention has not yet been widely accepted.

Origin of prefix

The prefix "giga" comes from the Greek word γίγας (gigas) meaning "giant", and was chosen because 10⁹ can be described as a "gigantic" number.

See also

• 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