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The gigabyte is a multiple of the unit byte for digital information. The prefix giga means 109 in the International System of Units (SI), therefore one gigabyte is 1000000000bytes. The unit symbol for the gigabyte is GB.

This definition is used in all contexts of science, engineering, business, and many areas of computing, including hard drive, solid state drive, and tape capacities, as well as data transmission speeds. However, the term is also used in some fields of computer science and information technology to denote 1073741824 (10243 or 230) bytes, particularly for sizes of RAM. The use of gigabyte is thus ambiguous. For semiconductor RAM, the gigabyte denotes 1073741824bytes. For hard drive capacities as described and marketed by the drive manufacturers, the gigabyte denotes 1000000000bytes, but when a 500-GB drive's capacity is displayed by, for example, Microsoft Windows, it is reported as 465 GB, where GB then means 1073741824bytes.

To address this ambiguity, the binary prefixes are standardized in the International System of Quantities, each binary prefix denoting an integer power of 1024. With these prefixes, a memory module that is labeled as having the size 1GB is designated as 1GiB (one gibibyte).




This 2.5" hard drive can hold 500 GB of data.

The term gigabyte is commonly used to mean either 10003 bytes or 10243 bytes. The latter binary usage originated as compromise technical jargon for byte multiples that needed to be expressed in a power of 2, but lacked a convenient name. As 1024 (210) is approximately 1000 (103), roughly corresponding to SI multiples, it was used for binary multiples as well.

In 1998 the International Electrotechnical Commission (IEC) published standards for binary prefixes, requiring that the gigabyte strictly denote 10003 bytes and gibibyte denote 10243 bytes. By the end of 2007, the IEC Standard had been adopted by the IEEE, EU, and NIST, and in 2009 it was incorporated in the International System of Quantities. Nevertheless, the term gigabyte continues to be widely used with the following two different meanings:

Decimal definition

  • 1 GB = 1000000000 bytes (= 10003 B = 109 B)

Based on powers of 10, this definition uses the prefix as defined in the International System of Units (SI). This is the recommended definition by the International Electrotechnical Commission (IEC). This definition is used in networking contexts and most storage media, particularly hard drives, flash-based storage, and DVDs, and is also consistent with the other uses of the SI prefix in computing, such as CPU clock speeds or measures of performance. The Mac OS X file manager from version 10.6 and higher is a notable example of this usage in software. Since Snow Leopard, file sizes are reported in decimal units.

Binary definition

  • 1 GiB = 1073741824 bytes (= 10243 B = 230 B).

The binary definition uses powers of the base 2, as is the architectural principle of binary computers. This usage is widely promulgated by some operating systems, such as Microsoft Windows in reference to computer memory (e.g., RAM). This definition is synonymous with the unambiguous unit gibibyte.

Consumer confusion

Since the early 2000s, disk drive manufacturers based most consumer hard drive capacities in certain size classes expressed in decimal gigabytes. The exact capacity of a given drive model is close to the class designation. Most manufacturers of hard disk drives and flash-memory disk devices define one gigabyte as 1000000000bytes, which is displayed on the packaging. Some operating systems now express hard drive capacity or file size using decimal multipliers (integer powers of 1000), while others, such as Microsoft Windows, still report size in gigabytes by dividing the total capacity in bytes by 1073741824 (230 bytes= 1 gibibyte), while still reporting the result with the symbol GB. This discrepancy causes confusion, as a disk with an advertised capacity of, for example, 400 GB (meaning 400000000000bytes) might be reported by the operating system as 372 GB, meaning 372 GiB. Other software, like Mac OS X 10.6 and some components of the Linux kernel use decimal prefixes.

The JEDEC memory standards use IEEE 100 nomenclature which quote the gigabyte as 1073741824bytes (230 bytes).

The difference between units based on decimal and binary prefixes increases as a semi-logarithmic (linear-log) function—for example, the decimal kilobyte value is nearly 98% of the kibibyte, a megabyte is under 96% of a mebibyte, and a gigabyte is just over 93% of a gibibyte value. This means that a 300 GB (279 GiB) hard disk might be indicated variously as 300 GB, 279 GB or 279 GiB, depending on the operating system. As storage sizes increase and larger units are used, these differences become even more pronounced. Some legal challenges have been waged over this confusion such as a lawsuit against drive manufacturer Western Digital. Western Digital settled the challenge and added explicit disclaimers to products that the usable capacity may differ from the advertised capacity. Seagate was sued on similar grounds and also settled.

Because of its physical design, the capacity of modern computer random access memory devices, such as DIMM modules, is always a multiple of a power of 1024. It is thus convenient to use prefixes denoting powers of 1024, known as binary prefixes, in describing them. For example, a memory capacity of 1073741824bytes is conveniently expressed as 1 GiB rather than as 1.074 GB. The former specification is, however, often quoted as 1 GB when applied to random access memory.

Software allocates memory in varying degrees of granularity as needed to fulfill data structure requirements and binary multiples are usually not required. Other computer capacities and rates, like storage hardware size, data transfer rates, clock speeds, operations per second, etc., do not depend on an inherent base, and are usually presented in decimal units. For example, the manufacturer of a "300 GB" hard drive is claiming a capacity of 300000000000bytes, not 300x10243 (which would be 322122547200) bytes.

Examples of gigabyte-sized storage

  • One hour of SDTV video at 2.2 Mbit/s is approximately 1 GB.
  • Seven minutes of HDTV video at 19.39 Mbit/s is approximately 1 GB.
  • 114 minutes of uncompressed CD-quality audio at 1.4 Mbit/s is approximately 1 GB.
  • A single layer DVD+R disc can hold about 4.7 GB.
  • A dual-layered DVD+R disc can hold about 8.5 GB.
  • A single layer Blu-ray disc can hold about 25 GB.
  • A dual-layered Blu-ray disc can hold about 50 GB.

Source: Wikipedia