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man:units

UNITS(7) Linux Programmer's Manual 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
``` 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 tak-
ing 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`
``` 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 con-
form.  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 4.16 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/.```

Linux 2017-09-15 UNITS(7)