A gigabyte (derived from the SI prefix giga- + byte) is a unit of information or computer storage meaning either exactly 1 billion (short scale) bytes (1000³, or 10⁹) or approximately 1.07 billion bytes (1024³, or 2³⁰).

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. When referring to disk storage capacities and the transmission of data over telecommunication lines, 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).

Definition

• 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. Quote from Seagate: "For drive storage capacity, 1 gigabyte = 1,000,000,000 bytes (or one billion bytes).", 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 commonly used for computer memory and file sizes. Microsoft uses this definition to display hard drive sizes, as do most other operating systems and programs. Since 1999, the IEC recommends that this unit should instead be called a gibibyte (abbreviated GiB).

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.

Gigabytes vs. gigabits

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

In computer networking the conventional SI units are followed. Manufacturers of networking equipment always use 1000-bit kilobits as their basic unit of measurement.

Consumer confusion

Since 2001, 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, as a hard disk with a manufacturer rated capacity of 400 gigabytes may have its capacity reported by the operating system as only 372 GB, depending on the type of report.