Gigabyte is an SI-multiple of the unit byte for digital information storage. Since the giga prefix means 109, gigabyte means 1000000000bytes (10003, 109). However, this term is also often used meaning 1073741824bytes (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.

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