for binary multiples
1998 the International Electrotechnical Commission (IEC), the leading international
organization for worldwide standardization in electrotechnology, approved
as an IEC International Standard names and symbols for prefixes for binary
multiples for use in the fields of data processing and data transmission.
The prefixes are as follows:
for binary multiples
|Examples and comparisons with SI prefixes
|| 1 Kibit = 210 bit = 1024 bit
|| 1 kbit = 103 bit = 1000 bit
|| 1 MiB = 220 B = 1 048 576 B
|| 1 MB = 106 B = 1 000 000 B
|| 1 GiB = 230 B = 1 073 741 824
|| 1 GB = 109 B = 1 000 000 000 B
suggested that in English, the first syllable of the name of the binary-multiple
prefix should be pronounced in the same way as the first syllable of the
name of the corresponding SI prefix, and that the second syllable should
be pronounced as "bee."
important to recognize that the new prefixes for binary multiples are not
part of the International System of Units (SI), the modern metric system.
However, for ease of understanding and recall, they were derived from the
SI prefixes for positive powers of ten. As can be seen from the above table,
the name of each new prefix is derived from the name of the corresponding
SI prefix by retaining the first two letters of the name of the SI prefix
and adding the letters "bi," which recalls the word "binary." Similarly,
the symbol of each new prefix is derived from the symbol of the corresponding
SI prefix by adding the letter "i," which again recalls the word "binary."
(For consistency with the other prefixes for binary multiples, the symbol
Ki is used for 210 rather than ki.)
prefixes for binary multiples, which were developed by IEC Technical Committee
(TC) 25, Quantities and units, and their letter symbols, with the strong
support of the International Committee for Weights and Measures (CIPM)
and the Institute of Electrical and Electronics Engineers (IEEE), were first
adopted by the IEC as Amendment 2 to IEC International Standard
IEC 60027-2: Letter symbols to be used in electrical technology -
Part 2: Telecommunications and electronics. The full content of
Amendment 2, which has a publication date of 1999-01, is reflected
in the tables above and the suggestion regarding pronunciation. Subsequently
the contents of this Amendment were incorportated in the second edition of
IEC 60027-2, which has a publication date of 2000-11 (the first edition
was published in 1972). The complete citation for this revised standard is
IEC 60027-2, Second edition, 2000-11, Letter symbols to be used in
electrical technology - Part 2: Telecommunications and electronics.
upon a time, computer professionals noticed that 210 was very
nearly equal to 1000 and started using the SI prefix "kilo" to mean 1024.
That worked well enough for a decade or two because everybody who talked
kilobytes knew that the term implied 1024 bytes. But, almost overnight
a much more numerous "everybody" bought computers, and the trade computer
professionals needed to talk to physicists and engineers and even to ordinary
people, most of whom know that a kilometer is 1000 meters and a kilogram
is 1000 grams.
data storage for gigabytes, and even terabytes, became practical, and the
storage devices were not constructed on binary trees, which meant that,
for many practical purposes, binary arithmetic was less convenient than
decimal arithmetic. The result is that today "everybody" does not "know"
what a megabyte is. When discussing computer memory, most manufacturers
use megabyte to mean
220 = 1 048 576 bytes, but the
manufacturers of computer storage devices usually use the term to mean
1 000 000 bytes. Some designers of local area networks
have used megabit per second to mean 1 048 576 bit/s, but all
telecommunications engineers use it to mean 106 bit/s. And if two
definitions of the megabyte are not enough, a third megabyte of
1 024 000 bytes is the megabyte used to format the familiar
90 mm (3 1/2 inch), "1.44 MB" diskette. The confusion is
real, as is the potential for incompatibility in standards and in implemented
with this reality, the IEEE Standards Board decided that IEEE standards
will use the conventional, internationally adopted, definitions of the
SI prefixes. Mega will mean 1 000 000, except that the base-two
definition may be used (if such usage is explicitly pointed out on a case-by-case
basis) until such time that prefixes for binary multiples are adopted by
an appropriate standards body.
Return to SI prefixes
*Historical context adapted from: Bruce Barrow, "A Lesson in
Megabytes," IEEE Standards Bearer, January 1997, page 5. Portions
copyright © 1997 by the Institute of Electrical and Electronics Engineers,
Inc. The IEEE disclaims any responsibility or liability resulting from the
placement and use in the described manner. Information is reprinted with the
permission of the IEEE.