Decimal 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.
Prefix Name Value
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
µ 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. See
also
⟨http://physics.nist.gov/cuu/Units/prefixes.html⟩
Binary prefixes
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".
Prefix Name Value
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
See also
⟨http://physics.nist.gov/cuu/Units/binary.html⟩
Discussion
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=1000 k, G=1000 M.
The situation was messy: in the 14k4 modems, k=1000; in the
1.44 MB diskettes, M=1024000; and so on. In 1998 the IEC
approved the standard that defines the binary prefixes given
above, enabling people to be precise and unambiguous.
Thus, today, MB = 1000000 B and MiB = 1048576 B.
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.
