Gigabyte is an SI-multiple of the unit byte for digital information storage. Since the giga prefix means 10⁹, gigabyte means (1000³, 10⁹). However, this term is also often used meaning (1024³, 2³⁰).

Originally the binary use of SI prefixes (kilo, mega, giga, etc.) was confined to contexts where the quantities were inherently determined in powers of two by the computer word or address size, like memory or disk sizes, so that confusion was unlikely. Later, disk design was not constrained by address word sizes or other physical details and disk blocks were numbered consecutively in decimal numbers (logical block addressing), creating the opportunity for confusion when size was still reported with binary interpretation of the prefix.

Today the usage of the word "gigabyte" is ambiguous: the value depends on the context. When referring to RAM sizes it traditionally has a binary interpretation of 1024³ bytes. Some operating systems list file sizes in SI units, but using the binary interpretation. Today, when referring to disk storage capacities it usually means 1000³ bytes. This also applies to data transmission volumes over telecommunication lines, as the telecommunications industry has always used the SI prefixes with their standards-based meaning.

In order to address this confusion, the International Electrotechnical Commission (IEC) has been promoting 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 (CIPM) and the National Institute of Standards and Technology (NIST) but has seen limited acceptance.

Gigabyte is commonly abbreviated GB or Gbyte (not to be confused with Gb, which is used for a gigabit) while Gibibyte is abbreviated GiB.

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 the early 2000s most 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 , the computer operating systems used by most users usually calculate size in gigabytes by dividing the total capacity in bytes (whether it is disk capacity, file size, or system RAM) by . This distinction can be a cause of confusion, as a hard disk with a manufacturer-rated capacity of 400 gigabytes may be reported by the operating system as only 372 GB large, depending on the type of report. The JEDEC memory standards uses the IEEE 100 nomenclatures which defines a gigabyte as .