Unit Prefix - Binary Prefixes

Binary Prefixes

A binary prefix indicates multiplication by a power of two. The binary multiple 210 (1024) is close to 1000. This has allowed the definition of a series of binary prefixes of comparable size to the metric kilo-, mega-, giga- et seq. Prefixes based on this series are commonly used in the information technology field prepended to the units of digital information, the bit and the byte.

Units of information are outside of the International System of Units and so the International Bureau of Weights and Measures makes no comment as to the prefixes appended to them. Computer professionals have historically used the same spelling, pronunciation and symbols for the binary series in the description of computer memory (with the exception the symbol for the binary kilo- is often capitalised). For example, in citations of main memory or RAM capacity, kilobyte, megabyte and gigabyte customarily mean 1024 (210), 1048576 (220) and 1073741824 (230) bytes respectively.

In the specifications of hard disk drive capacities and network transmission bit rates, on the other hand, decimal prefixes, consistent with the metric system, are used. For example, a 500-gigabyte hard drive holds 500000000000 bytes, and a 100-megabit-per-second Ethernet connexion transfers data at 100000000 bits per second. The amiguity has led to some confusion and even of lawsuits from purchasers who were expecting 220 or 230 and considered themselves shortchanged by the seller. (see Orin Safier v. Western Digital Corporation and Cho v. Seagate Technology (US) Holdings, Inc.). To protect themselves, some sellers write out the full term as "1000000".

With the aim of avoiding ambiguity the International Electrotechnical Commission (IEC) adopted new binary prefixes in 1998 (IEC 80000-13:2008 formerly subclauses 3.8 and 3.9 of IEC 60027-2:2005) Each binary prefix is formed from the first syllable of the decimal prefix with the similar value, and the syllable "bi". The symbols are the decimal symbol, always capitalised, followed by the letter "i". According to these standards, kilo-, mega-, giga- et seq. would only be used in the decimal sense, even when referring to data storage capacities: kilobyte and megabyte would denote one thousand and one million bytes respectively (consistent with the metric system), while new terms such as kibibyte, mebibyte and gibibyte, abbreviated KiB, MiB and GiB, would denote 210, 220 and 230 bytes respectively. However, as of 2012 adoption of the new terms has been slow and usage has been limited.