# Man Pages

units(7) - phpMan units(7) - phpMan

Command: man perldoc info search(apropos)

```UNITS(7)                   Linux Programmer's Manual                  UNITS(7)

NAME
units, kilo, kibi, mega, mebi, giga, gibi - decimal and binary prefixes

DESCRIPTION
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
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.  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 num-
bers.  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 unambigu-
ous.

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.

COLOPHON
This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux                             2001-12-22                          UNITS(7)
```