units(7)

NAME

```   units - 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
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

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

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=1000k, G=1000M.

The situation was messy: in the 14k4  modems,  k=1000;  in  the  1.44MB
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 = 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 4.09 of the Linux  man-pages  project.   A
description  of  the project, information about reporting bugs, and the
latest    version    of    this    page,    can     be     found     at
https://www.kernel.org/doc/man-pages/.
```

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