Definitions of the SI units: The proposed binary prefixes

Joseph S. Barrera III (
Wed, 17 Mar 1999 09:13:28 -0800

Proposed prefixes for binary multiples

From: Bruce Barrow, "A Lesson in Megabytes," IEEE Standards Bearer, Janua=
1997, page 5, with minor editorial changes.

Institute of Electrical and Electronics Engineers (IEEE) Standards
Coordinating Committee 14 (SCC 14), Quantities, Units and Letter Symbols,
has begun to work with the IEEE Computer Society, the International
Organization for Standardization (ISO), and the International
Electrotechnical Commission (IEC) to find acceptable names for prefixes t=
are related to powers of two. A proposal being circulated internationally=
the IEC would introduce the new prefixes kibi, mebi, gibi and tebi derive=
as short unions of the SI prefixes with the word "binary." The proposed n=
prefix symbols are Ki, Mi, Gi, and Ti. Thus we would have a gibibyte of 2=
bytes and a gigabyte of 10^9 bytes, and the 90 mm (3 1/2 inch) diskette
would be formatted for 1440 KiB.
Current proposed prefixes for binary multiples for use in information
technology are listed in the following table. These are only proposed
prefixes; final recommendations will not be made for a few years. (It is
suggested that in English, the first syllable should be pronounced in the
same way as the first syllable in the corresponding SI prefix, and that t=
second syllable should be pronounced as "bee.")

Motivation for the proposed prefixes for binary multiples

Once upon a time, computer professionals noticed that 2^10 was very nearl=
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.
Then data storage for gigabytes, and even terabytes, became practical, an=
the storage devices were not constructed on binary trees, which meant tha=
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 u=
megabyte to mean 2^20 =3D 1 048 576 bytes, but the manufacturers of compu=
storage devices usually use the term to mean 1 000 000 bytes. Some design=
of local area networks have used megabit per second to mean 1 048 576 bit=
but all telecommunications engineers use it to mean 10^6 bit/s. And if tw=
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.=
MB" diskette. The confusion is real, as is the potential for incompatibil=
in standards and in implemented systems.
Faced with this reality, the IEEE Standards Board decided that IEEE
standards will use the conventional, internationally adopted, definitions=
the metric prefixes. Mega will mean 1 000 000, except that the base-two
definition may be used during an interim period if such usage is explicit=
pointed out on a case-by-case basis.

Return to SI prefixes
"A Lesson in Megabytes" reprinted from IEEE Standards Bearer Copyright =A9
1997 by the Institute of Electrical and Electronics Engineers, Inc. The I=
disclaims any responsibility or liability resulting from the placement an=
use in the described manner. Information is reprinted with the permission=
the IEEE.