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.
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2 IEC standard prefixes 3 External links |
| 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.
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.
IEC standard prefixes
| 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-.
See also: