As popularly used, these prefixes indicate multiplies that are similar, but not equal to, factors denoted by their SI counterparts. Specifically, popular usage in computing denotes whole powers of two, while SI prefixes are powers of ten. The exact numbers are listed below:
name abbr factor in colloquial computing usage SI size kilo K 2^10 == 1,024 10^3 == 1,000 mega M 2^20 == 1,048,576 10^6 == 1,000,000 giga G 2^30 == 1,073,741,824 10^9 == 1,000,000,000 tera T 2^40 == 1,099,511,627,776 10^12 == 1,000,000,000,000 peta P 2^50 == 1,125,899,906,842,624 10^15 == 1,000,000,000,000,000 exa E 2^60 == 1,152,921,504,606,846,976 10^18 == 1,000,000,000,000,000,000
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 harddisk will actually only hold about 28 * 2^30 bytes. Telecommunications also uses the SI factors, so a 1 Mbps connection transfers 10^6 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 "1.44MB" floppy holds (1.44 * 1000 * 1024) bytes.
In 1999 the IEC? published Amendment 2 to "IEC 60027-2: Letter symbols to be used in electrical technology - Part 2: Telecommunications and electronics". This standard introduced the prefixes kibi?, mebi?, gibi?, tebi?, pebi?, exbi?, to be used in specifying binary multiples of a quantity.
name abbr factor kibi Ki 2^10 == 1024 mebi Mi 2^20 == 1048576 gibi Gi 2^30 == 1073741824 tebi Ti 2^40 == 1099511627776 pebi Pi 2^50 == 1125899906842624 exbi Ei 2^60 == 1152921504606846976
As of 2001 these terms have not yet gained widespread use.
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