Byte/Prefixes

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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.