Binary prefix

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In computing, a binary prefix is a specifier or mnemonic that is prepended to the units of digital information, the bit and the byte, to indicate multiplication by a power of 2. In practice the powers used are multiples of 10, so the prefixes denote powers of 1024 = 210.

The computer industry uses terms such as "kilobyte," "megabyte," and "gigabyte," and corresponding abbreviations "KB", "MB", and "GB", in two different ways. In citations of main memory or RAM capacity, "gigabyte" customarily means 1073741824 bytes. This is a power of 2, specifically 230, so this usage is referred to as a "binary unit" or "binary prefix."

In other contexts, the industry traditionally uses "kilo", "mega", "giga", etc., in a manner consistent with their meaning in the International System of Units (SI): as powers of 1000. For example, a "500 gigabyte" hard drive holds 500000000000 bytes, and a "100 megabit per second" Ethernet connection runs at 100000000 bit/s.

Starting in about 1998, a number of standards and trade organizations approved standards and recommendations for a new set of binary prefixes, proposed earlier by the International Electrotechnical Commission (IEC), that would refer unambiguously to powers of 1024. According to these, the SI prefixes would only be used in the decimal sense, even when referring to data storage capacities: kilobyte and megabyte would denote one thousand bytes and one million bytes respectively (consistent with SI), while new terms such as kibibyte, mebibyte and gibibyte, abbreviated KiB, MiB, and GiB, would denote 1024 bytes, 1048576 bytes, and 1073741824 bytes respectively.[1]

In practice, the IEC binary prefixes have seen little use by the press or the US computing industry and marketplace. However, they are starting to appear in academic textbooks, in the EU computing industry and marketplace (as required by EU law since 2007),[2][3] certain US and International Government contexts (as required by contract or internal policy),[4] and for file sizes as measured by recent versions of the operating systems Linux and OS X but not Windows.[5]