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The SI prefixes are defined as follows:

Factor Symbol Prefix   Prefix Symbol Factor
10-1 d deci   deca da 10
10-2 c centi   hecto h 102
10-3 m milli   kilo k 103
10-6 μ micro   mega M 106
10-9 n nano   giga G 109
10-12 p pico   tera T 1012
10-15 f femto   peta P 1015
10-18 a atto   exa E 1018
10-21 z zepto   zetta Z 1021
10-24 y yocto   yotta Y 1024

The above SI prefixes can be applied to all SI units, except those associated with computer binary data. For example:

    Old Definitions:
  • 1 kb = (210) bits = 1,024 bits
  • 1 MB = (210)2 Bytes = 1,048,576 Bytes

To make the SI prefixes universal and to avoid confusion in the future, International Electrotechnical Commission (IEC), the leading international organization for worldwide standardization in electrotechnology, approved in December 1998 standard names and symbols of prefixes for binary multiples to be used in the fields of data processing and transmission. The prefixes are:

Name Prefix Symbol Factor
kilobinary kibi Ki 210
megabinary mebi Mi (210)2
gigabinary gibi Gi (210)3
terabinary tebi Ti (210)4
petabinary pebi Pi (210)5
exabinary exbi Ei (210)6

Based on the new definitions, the SI prefixes, k, M, and G, are back to their original meaning of 1,000, 1,000,000, and 1,000,000,000, respectively. The commonly known binary prefixes are written as Ki, Mi, and Gi and pronounced as kibi, mebi, and gibi, respectively.

    New Definitions: (See the unit conversion calculator for more binary data units.)
  • 1 kilo-bit = 1 kb = 103 bits = 1,000 bits
  • 1 kilobinarybit = 1 kibibit = 1 Kib = (210) bits = 1,024 bits
  • 1 megabyte = 1 MB = 106 Bytes = 1,000,000 Bytes
  • 1 megabinarybyte = 1 mebibyte = 1 MiB = (210)2 Bytes = 1,048,576 Bytes

The IEEE Standards Board decided that IEEE standards will be using the conventional, internationally adopted, definitions of the base-ten SI prefixes, except that the base-two definition may be used if such usage is explicitly pointed out on a case-by-case basis.  


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