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Binary prefix

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Large quantities of bytes, bits, or bits per second (bit/s) are often given using prefixes derived from the SI prefixes, so the prefixes K, M, and G are used for kilobyte, megabyte, gigabyte. For example, a 50 byte text string, 100 KB (kilobytes) files, 128 MB (megabytes) of RAM, or 30 GB (gigabytes) of disk storage.

As popularly used, these prefixes often indicate multiples that are similar, but not equal to, factors denoted by their SI counterparts. Specifically, popular usage in computing often denotes whole powers of two, while SI prefixes are powers of ten. The exact numbers are listed below:

Prefixes in colloquial computing usage
NameAbbrFactorSI size
kiloK210 = 1024103 = 1000
megaM220 = 1 048 576106 = 1 000 000
gigaG230 = 1 073 741 824109 = 1 000 000 000
teraT240 = 1 099 511 627 7761012 = 1 000 000 000 000
petaP250 = 1 125 899 906 842 6241015 = 1 000 000 000 000 000
exaE260 = 1 152 921 504 606 846 9761018 = 1 000 000 000 000 000 000
zettaZ270 = 1 180 591 620 717 411 303 4241021 = 1 000 000 000 000 000 000 000
yottaY280 = 1 208 925 819 614 629 174 706 1761024 = 1 000 000 000 000 000 000 000 000

These are identical to SI prefixes, except for "K", which is lowercase in SI.

It is widely regarded as confusing that common usage of kilobyte means 1024 bytes, while the "correct" value is 1000 bytes. Hard disk manufacturers are the only group in computing that habitually uses the lower SI factors, so what is advertised as a 30 GB hard disk will actually only hold about 28 × 230 bytes. Telecommunications also uses the SI factors, so a 1 Mbit/s connection transfers 106 bits per second. Floppy disk manufacturers are even more confusing. The prefix "M" means (1000 × 1000) in SI, and (1024 × 1024) in standard computing. However, the standard "1.44 MB" floppy holds (1.44 × 1000 × 1024) bytes.

In the days of 32K RAM computers, this confusion was not serious, since the different between 210 and 103 was approximately 2%. However, as computing equipment have become increasingly capable, these differences have become an increasingly large error when expressed as percentages.

In 1999, the International Electrotechnical Commission (IEC) published Amendment 2 to "IEC 60027-2: Letter symbols to be used in electrical technology - Part 2: Telecommunications and electronics". This standard, which had been approved in 1998, introduced the prefixes kibi-, mebi-, gibi-, tebi-, pebi-, exbi-, to be used in specifying binary multiples of a quantity. The names come from shortened versions of the original SI prefixes and bi which is short for binary. It also clarifies that, from the point of view of the IEC, the SI prefixes only have their base-10 meaning and never have a base-2 meaning.

New IEC Standard Prefixes
NameAbbrFactor
kibiKi210 = 1024
mebiMi220 = 1 048 576
gibiGi230 = 1 073 741 824
tebiTi240 = 1 099 511 627 776
pebiPi250 = 1 125 899 906 842 624
exbiEi260 = 1 152 921 504 606 846 976

As of 2002 this naming convention has not yet gained widespread use. The IEC did not give names for the prefixes beyond exa-, but if they had given them names, they would probably be zebi- and yobi-.

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