# units(7) — Linux manual page

```units(7)            Miscellaneous Information Manual            units(7)
```

## NAME         top

```       units - decimal and binary prefixes
```

## DESCRIPTION         top

```   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
q        quecto   10^-30 = 0.000000000000000000000000000001
r        ronto    10^-27 = 0.000000000000000000000000001
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
R        ronna    10^27  = 1000000000000000000000000000
Q        quetta   10^30  = 1000000000000000000000000000000

The symbol for micro is the Greek letter mu, often written u in
an ASCII context where this Greek letter is not available.

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
Zi       zebi   2^70 = 1180591620717411303424
Yi       yobi   2^80 = 1208925819614629174706176

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.
```

```       The International System of Units
⟨https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf⟩.
```

## COLOPHON         top

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This page was obtained from the tarball man-pages-6.9.1.tar.gz
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Linux man-pages 6.9.1          2024-05-02                       units(7)
```