The
International System
of Units (SI) specifies a set of unit prefixes, known as
SI prefixes, also known as
metric
prefixes. An SI
prefix
is a name that precedes a basic unit of measure to indicate a
decimal multiple or
fraction of the unit. Each prefix has
a unique symbol that is prepended to the unit symbol. The
SI prefixes are standardized by the
International
Bureau of Weights and Measures in resolution dating from 1960
to 1991.
SI prefixes are used to reduce the number of zeros shown in
numerical quantities before or after the decimal point. For
example, an electrical current of , or onebillionth (
short scale) of an ampere, is written by using
the SIprefix
nano as
1 nanoampere or 1 nA.
List of SI prefixes
The International System of Units specifies twenty SI
prefixes:
Examples
General use of prefix names and symbols
Twenty SI prefixes are available to combine with units of measure.
For example, the prefix
kilo denotes a multiple of one
thousand, so 1
kilometre equals 1000
metres, 1
kilogram
equals 1000
grams, 1 kilowatt equals 1000
watts, and so on. Each SI prefix name has an
associated symbol which can be used in combination with the symbols
for units of measure. Thus, the "kilo" symbol, k, can be used to
produce km, kg, and kW, (kilometre, kilogram, and kilowatt). SI
prefixes are internationally recognized and also exist outside the
SI (many of them long predate SI, going back to the original
introduction of the metric system); prefixes may also be used in
combination with nonSI units; for example: milligauss (mG),
kilofoot (kft) and microinch (µin).
Prefixes may not be used in combination. This even applies for
mass, for which the
SI
base unit (which is the kilogram,
not the gram)
already contains a prefix. So milligram (mg) is used instead of
microkilogram (µkg), for example.
Prefixes corresponding to an
exponent that
is divisible by three are often recommended. Hence "100 m"
rather than "1 hm" (hectometre) or "10 dam" (decametres).
The "nonthree" prefixes (hecto, deca, deci, and centi) are
however more commonly used for everyday purposes than in
science.
SI prefixes with symbols for time and angles
Official
policies about the use of these prefixes vary slightly between the
Bureau International des Poids et Mesures (BIPM) and the American National
Institute of Standards and Technology (NIST); and some of the
policies of both bodies are at variance with everyday
practice. For instance, the NIST advises that "…to avoid
confusion, prefix symbols (and prefixes) are not used with the
timerelated unit symbols (names) min (minute), h (hour), d (day);
nor with the anglerelated symbols (names) ° (degree),
′ (minute), and
″ (second)." The
BIPM’s position on the use of SI prefixes with units of time larger
than the second is the same as that of the NIST but their position
with regard to angles differs: they state "However astronomers use
milliarcsecond, which they denote mas, and microarcsecond, µas,
which they use as units for measuring very small angles."
SI prefixes for temperature in °C
Official policy also varies from common practice for the degree
Celsius (°C). NIST
states "Prefix symbols may be used with the unit
symbol °C and prefixes may be used with the unit name 'degree
Celsius'. For example, 12 m°C (12 millidegrees Celsius) is
acceptable." However the use of prefixed forms of "°C" (such as
"µ°C") has not been adopted in science and engineering; prefixed
forms of the
kelvin (which are precisely
equivalent) are usually used instead.
Exponentiation of symbols
When units occur in
exponentiation,
for example, in square and cubic forms, any size prefix is
considered part of the unit, and thus included in the
exponentiation.
Pronunciation
There are two accepted pronunciations for the prefix
giga: and . According to the American writer
Kevin Self, in the 1920s a German committee
member of the
International
Electrotechnical Commission proposed giga as a prefix for
10
^{9}, drawing on a verse by the humorous poet Christian
Morgenstern that appeared in the third (1908) edition of
Galgenlieder (Gallows Songs). This suggests a hard German
g was originally intended as the pronunciation. Self was
unable to ascertain at what point the (soft
g)
pronunciation became accepted, but as of 1995 current practice had
returned to (hard
g).
When an SI prefix is affixed to a root word, the prefix carries the
stress, while the root drops
its stress but retains a full vowel in the syllable that is
stressed when the root word stands alone. For example,
gigabyte is , with stress on the first
syllable. However, words in common use outside the scientific
community may follow idiosyncratic stress rules. For example,
kilometre is commonly pronounced , with
reduced vowels on both syllables of
metre.
Other and obsolete prefixes
Obsolete prefixes such as
myrio and
myria, denoting a factor of 10,000, were
dropped before SI was adopted in 1960, probably because they did
not fit this pattern. A oneletter symbol was not available, as
M,
m, and
µ were already used, so the
twoletter symbols
mo and
ma were, albeit rarely,
used instead.
Double prefixes such as those formerly used in
micromicrofarads (picofarads),
hectokilometres (100 kilometres), and
millimicrons or
micromillimetres (both
nanometres) were also dropped with the introduction of the
SI.
Though in principle valid, many combinations of prefixes with
quantities are rarely used (in many cases because the quantity they
represent is larger or smaller than encountered in practice). In
most contexts only a few, i.e. the most common, standard
combination are established:
 Mass: hectogram, gram, milligram, microgram, and smaller are
common. However, megagram or larger are rarely used; tonnes (and
kilotonnes etc) or scientific
notation are used instead. Megagram is occasionally used to
disambiguate the (metric) tonne from the various (nonmetric)
tons.
 Volume in litres: litre, decilitre, centilitre, millilitre,
microlitre, and smaller are common. Larger volumes are sometimes
denoted in hectolitres; otherwise in cubic metres or cubic
kilometres. In Australia, large quantities of water are measured in
kilolitres, megalitres and gigalitres.
 Length: kilometre, metre, decimetre, centimetre, millimetre,
and smaller are common. The micrometre is often referred to by the
nonSI term micron. In some
fields such as chemistry, the ångström (equal to 0.1 nm) competes
with the nanometre. The femtometre, used
mainly in particle physics, is usually called a fermi. For large scales, megametre, gigametre,
and larger are rarely used. Often used are astronomical units, light years, and parsecs;
the astronomical unit is mentioned in the SI standards as an
accepted nonSI unit.
 Time: second, millisecond, microsecond, and shorter are common.
The kilosecond and megasecond also have some use, though for these
and longer times one usually uses either scientific notation or
minutes, hours, and so on.
^{†} the United Kingdom, Ireland, Australia and New Zealand
previously used the
long scale
number name conventions, but have now at least partly switched to
the
short scale usage. In
particular, above a million and below a millionth, the
same name has different values in the two naming systems,
so
billion and
trillion (for example) have
unfortunately become potentially ambiguous terms internationally.
Using the SI prefixes can circumvent this problem.
NonSI units
 Prefixes go back to the introduction of the metric system in the 1790s, long before the SI
was introduced in 1960. The prefixes (including those introduced
after the introduction of SI) are used with any metric units, SI or
not (e.g. millidynes).
 SI prefixes rarely appear coupled with imperial units or English units except in some specialised cases
(e.g. microinches, kilofeet, kilopound or 'kip').
 They are also used with other specialized units used in
particular fields (e.g. megaelectronvolts,
gigaparsecs).
 They are also occasionally used with currency units (e.g.,
gigadollar), mainly by people who are familiar with the prefixes
from scientific usage.
Similar prefixes as abbreviations
The symbol
K is often used informally to mean a multiple
of thousand in many contexts. For example, one may talk of a
40K salary (40 000), or call the
Year 2000 problem as
Y2K problem.
In these cases an uppercase K is often used, although the uppercase
K is the official symbol of the kelvin.
In other financial and business contexts, the letter
M is
often used to denote multiplication by 1000, in recognition of the
Latin term
mille, also used in
roman numerals. In these situations
one
million is often written as
1 MM. Similar usage of
M occurs in the term CPM (
Cost
per mille) used in advertising.
Units used in computing and telecommunications
The International System of Units does not define symbols for the
storage size units
bit and
byte and this has allowed ambiguities to emerge in
combination with SI prefixes. The bit is often given the symbol
bit or
b, while byte is usually written as
byte,
B, and occasionally as
b. Thus,
kb/s sometimes means
kilobits per second, but may
sometimes refer to
kilobytes per second.
The National Institute of Standards and Technology in the United
States has suggested the use of
bit for bits and
B for bytes.
In nonstandard use,
K is often used as a symbol prefix to
the units
bit and
byte to
designate the
binary prefix kibi = 2
^{10} = 1024.
Binary prefixes
The prefixes
kilo,
mega,
giga and
greater are often used in combination with the storage size units
bit and byte.
Because 2
^{10} = 1024 which is close to the value (1000) of
the prefix
kilo, a 1024byte amount of
computer memory is often referred to as a
kilobyte even though this does not conform to the strict
definition of
kilo. Likewise 2
^{20} is 1 048 576
which is close to 1 000 000 and this has led to 1 048 576 bytes
often being called a
megabyte. This has led to some
confusion because
megabyte is also used to refer to 1 000
000 bytes, e.g., in descriptions of
hard
disk drive capacities and network transmission
bit rates.
To eliminate this ambiguity the
International
Electrotechnical Commission (IEC) adopted new
binary prefixes in 1998 (
IEC 8000013:2008 formerly subclauses 3.8 and
3.9 of
IEC 600272:2005). Each binary
prefix is formed from the first syllable of the decimal prefix with
the similar value, and the syllable 'bi' (pronounced 'bee'). Its
symbol is the decimal symbol, always capitalized, followed by the
letter 'i'.
According to this standard one
kilobyte (1
kB) is 1000 bytes, whereas one
kibibyte (1
KiB) is 1024 bytes. Likewise
mebi (Mi;
2
^{20}),
gibi (Gi; 2
^{30}),
tebi (Ti; 2
^{40}),
pebi (Pi; 2
^{50}),
exbi
(Ei; 2
^{60}),
zebi (Zi; 2
^{70})
and
yobi (Yi; 2
^{80}).
The use of these new binary prefixes is increasing but is largely
limited to technical literature and new computer software.
Proposed changes
There are proposals for further harmonisation of the
capitalisation. Therefore the symbols for kilo, hecto, and deca
would be changed from ‘k’ to ‘K’, from ‘h’ to ‘H’, and from ‘da’ to
‘D’. Likewise some lobby for the removal of prefixes that do not
fit the 10
^{±3n} scheme, namely hecto, deca, deci,
and centi. The
CGPM has postponed its decision
on both matters for now.
An unsolved (and maybe unsolvable) issue is the application of
prefixes to units with exponents other than ±1. The prefix is
always applied before the exponent. This eventually led to the
introduction of special units for area and volume without exponents
in the original
metric system:
 1 are (a) = 100 m^{2} (10 m × 10 m =
1 dam × 1 dam = 1 dam^{2})
 ⇒ 1 ca = 1 m^{2} (1 m × 1 m)
 ⇒ 1 ha = 10 000 m^{2} (100 m × 100 m = 1 hm × 1 hm = 1
hm^{2})
 1 stere (st) = 1 m^{3}
 1 litre (l or L) = 1 dm^{3} = 1
mst = 0.001 m^{3}
Of these the litre and the hectare are the most common. Litre
designations are sometimes used to differentiate a volume of liquid
(as opposed to a gas, or solid which are usually designated as
cubic volumes). Hectares are widely used as a metric alternative to
the acre (approximately 2.5 acres to the hectare).
See also
References
 four resolutions
 http://physics.nist.gov/Pubs/SP811/sec06.html

http://www.bipm.fr/en/si/si_brochure/chapter3/prefixes.html
 Ambler Thompson, Barry N. Taylor. (2008). Guide for the Use of the International System of Units
(SI). Gaithersburg, MD: National
Institute of Standards and Technology p. 74. This source
recommends B as a symbol for byte, but is silent concerning
bits.
External links