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Binary prefixes are often used to quantify large amounts of bytes, bits, or bits per second (bit/s), and are derived, but slightly different, from the SI prefixes such as kilo-, mega-, giga- and so on.

We can abbreviate these to get the prefixes K, M, and G for kilobyte, megabyte, gigabyte. Mbit/s, Kbit/s etc. are often used to abbreviate "Megabits per second", "Kilobits per second", etc. However, terms such as "three megabytes" are often abbreviated inaccurately to "3M", and then the prefix in fact appears to become a suffix, but in fact still prefixes the quantity (three megabytes). For example: a 50 byte text string, a 100 KB (kilobyte) file, 128 MB (megabytes) of RAM, or 30 GB (gigabytes) of disk storage.

## Conventional usage

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:

 Name Abbr Factor SI size kilo K 210 = 1024 103 = 1000 mega M 220 = 1 048 576 106 = 1 000 000 giga G 230 = 1 073 741 824 109 = 1 000 000 000 tera T 240 = 1 099 511 627 776 1012 = 1 000 000 000 000 peta P 250 = 1 125 899 906 842 624 1015 = 1 000 000 000 000 000 exa E 260 = 1 152 921 504 606 846 976 1018 = 1 000 000 000 000 000 000 zetta Z 270 = 1 180 591 620 717 411 303 424 1021 = 1 000 000 000 000 000 000 000 yotta Y 280 = 1 208 925 819 614 629 174 706 176 1024 = 1 000 000 000 000 000 000 000 000

These are identical to SI prefixes, except for "K", which corresponds to "k" in SI.

It is widely regarded as confusing that the 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 difference between 210 and 103 was approximately 2%. However, as computing equipment has grown in memory capacity, these differences have led to increasingly large errors when expressed as percentages.

## IEC standard prefixes

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.

 Name Abbr Factor kibi Ki 210 = 1024 mebi Mi 220 = 1 048 576 gibi Gi 230 = 1 073 741 824 tebi Ti 240 = 1 099 511 627 776 pebi Pi 250 = 1 125 899 906 842 624 exbi Ei 260 = 1 152 921 504 606 846 976

As of 2003 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- as well as nobi- and dogbi-.