units(7) -- Linux man page
units, kilo, kibi, mega, mebi, giga, gibi - decimal and binary prefixes
The SI system of units uses prefixes that indicate powers of ten.
A kilometer is 1000 meter, and a megawatt is 1000000 watt.
Below the standard prefixes.
|y||yocto||10^-24 = 0.000000000000000000000001|
|z||zepto||10^-21 = 0.000000000000000000001|
|a||atto||10^-18 = 0.000000000000000001|
|f||femto||10^-15 = 0.000000000000001|
|p||pico||10^-12 = 0.000000000001|
|n||nano||10^-9 = 0.000000001|
|u||micro||10^-6 = 0.000001|
|m||milli||10^-3 = 0.001|
|c||centi||10^-2 = 0.01|
|d||deci||10^-1 = 0.1|
|da||deka||10^ 1 = 10|
|h||hecto||10^ 2 = 100|
|k||kilo||10^ 3 = 1000|
|M||mega||10^ 6 = 1000000|
|G||giga||10^ 9 = 1000000000|
|T||tera||10^12 = 1000000000000|
|P||peta||10^15 = 1000000000000000|
|E||exa||10^18 = 1000000000000000000|
|Z||zetta||10^21 = 1000000000000000000000|
|Y||yotta||10^24 = 1000000000000000000000000|
The symbol for micro is the Greek letter mu, often written u
in an ASCII context where this Greek letter is not available.
The binary prefixes resemble the decimal ones, but have an additional 'i'
(and "Ki" starts with a capital 'K'). The names are formed by taking the
first syllable of the names of the decimal prefix with roughly the same
size, followed by "bi" for "binary".
|Ki||kibi||2^10 = 1024|
|Mi||mebi||2^20 = 1048576|
|Gi||gibi||2^30 = 1073741824|
|Ti||tebi||2^40 = 1099511627776|
|Pi||pebi||2^50 = 1125899906842624|
|Ei||exbi||2^60 = 1152921504606846976|
Before these binary prefixes were introduced, it was fairly
common to use k=1000 and K=1024, just like b=bit, B=byte.
Unfortunately, the M is capital already, and cannot be
capitalized to indicate binary-ness.
At first that didn't matter too much, since memory modules
and disks came in sizes that were powers of two, so everyone
knew that in such contexts "kilobyte" and "megabyte" meant
1024 and 1048576 bytes, respectively. What originally was a
sloppy use of the prefixes "kilo" and "mega" started to become
regarded as the "real true meaning" when computers were involved.
But then disk technology changed, and disk sizes became arbitrary numbers.
After a period of uncertainty all disk manufacturers settled on the
standard, namely k=1000, M=1000k, G=1000M.
The situation was messy: in the 14k4 modems, k=1000; in the 1.44MB
diskettes, M=1024000; etc. In 1998 the IEC approved the standard
that defines the binary prefixes given above, enabling people
to be precise and unambiguous.
Thus, today, MB = 1000000B and MiB = 1048576B.
In the free software world programs are slowly
being changed to conform. When the Linux kernel boots and says
hda: 120064896 sectors (61473 MB) w/2048KiB Cache
the MB are megabytes and the KiB are kibibytes.