Binary Prefix

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 1024.

The computer industry currently uses terms such as kilobyte, megabyte, and gigabyte, and corresponding symbols 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 1024 (specifically 10243), and 1024 is a power of 2 (specifically 210), therefore this usage is referred to as a binary prefix.

In most other contexts, the industry 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 transfers data at 100000000 bit/s. In contrast with "binary prefix", this usage is referred to as a "decimal prefix", as 1000 is a power of 10.

Using the same prefixes to mean two different things within the same industry has caused some confusion. Starting around 1998, a number of standards and trade organizations approved standards and recommendations for a new set of binary prefixes 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.