# units(7) — Linux manual page

```UNITS(7)                Linux Programmer's 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
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

⟨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.
```

## COLOPHON         top

```       This page is part of release 5.13 of the Linux man-pages project.
A description of the project, information about reporting bugs,