Large quantities of
bytes or
bits 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 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.