An 'SI prefix' (also known as a 'metric prefix') is a name or associated symbol that precedes a unit of measure (or its symbol) to form a
decimal multiple or submultiple. The abbreviation ''SI'' is from the French language name ''Système International d’Unités'' (also known as ''
International System of Units). '' SI
prefixes are used to reduce the quantity of zeroes in numerical equivalencies. For example, one-billionth of an ampere (a small electrical current) can be written as ''0.000 000 001 ampere.'' In symbol form, this is written as ''0.000 000 001 A.'' Using an SI prefix, these are equivalent to ''1 nanoampere'' or ''1 nA.'' The SI prefixes are governed by the
Bureau International des Poids et Mesures (BIPM, also known as the
International Bureau of Weights and Measures) and are the product of
four resolutions dating from 1960 to 1992.
List of SI prefixes
The twenty SI prefixes are shown in the chart below.
Usage
General use of prefix names and symbols
Twenty SI prefixes are available to combine with units of measure. For example, the prefix name ''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 may also be used in combination with non-SI units; for example: milligauss (mG), kilofoot (kft) and microinch (µin).
Prohibition of multiple prefixes
The kilogram is the only SI base unit that has an SI prefix as part of its unit name and symbol. Because multiple prefixes may not be used (such as microkilogram or µkg), the prefixes are used with the unit ''gram'' and its symbol ''g'' (e.g. milligram or mg).
SI prefixes with symbols for time and angles
Officially endorsed policies of the BIPM and the
National Institute of Standards and Technology (NIST) vary slightly with respect to the use of the SI prefixes—both between their respective agencies and from real–world practice. For instance, the NIST
advises that “…to avoid confusion, prefix symbols (and prefixes) are not used with the time-related unit symbols (names) min (minute), h (hour), d (day); nor with the angle-related 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 with °C
A similar difference between officially endorsed policy and actual practice exists with regard to the symbol for degree Celsius (°C). The 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.” Notwithstanding this official endorsement, the practice of using prefixed forms of “°C” (such as “µ°C”) has not been well-adopted in science and engineering; prefixed forms of the
kelvin are usually used instead.
Details
Examples:
★ 5 cm = 5 × 10
−2 m = 5 × 0.01 m = 0.05 m
★ 3 MW = 3 × 10
6 W = 3 × 1,000,000 W = 3,000,000 W
The prefix always takes precedence over any exponentiation; thus "km²" means ''square kilometre'' and not ''kilo–square metre''. For example, 3 km² is equal to 3,000,000 m² and ''not'' to 3000 m² (''nor'' to 9,000,000 m²). Thus the SI prefixes provide steps of a factor one million instead of one thousand in the case of an exponent 2, of a thousand million in the case of an exponent 3, etc. As a result large numbers may be needed, even if the prefixes are fully used, or intermediate units (like the
litre) are introduced.
Prefixes where the
exponent is divisible by three are often recommended. Hence "100 m" rather than "1 hm".
The
obsolete prefixes such as ''
myrio-'' and ''
myria-'' were dropped before SI was adopted in 1960, probably because they did not fit this pattern, no one-letter symbol was available (M, m, and µ already being used, while two-letter mo and ma symbols were used instead) and were rarely used anyway.
The prefix ''kilo'' derives from the
Greek word ''χίλια'' (''khilia'' or ''chilia'') = thousand.
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 legal, many combinations of prefixes with quantities are rarely used. 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 or
scientific notation are used instead. Megagram is sometimes used to disambiguate the (metric) tonne from the various (non-metric) 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 and megalitres.
★ Length: kilometre, metre, decimetre, centimetre, millimetre, and smaller are common. The micrometre is often referred to by the non-SI term ''
micron''. In some fields such as
chemistry, the
angstrom (equal to 0.1 nm) competes with the nanometre. The
femtometre, used mainly in particle physics, is usually called a
fermi. At 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 non-SI 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.
Pronunciation
There are two accepted pronunciations for the prefix ''giga-'': and . According to the American writer 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 suggest 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''.
[1]
[1]
When any SI prefix is affixed to a root word, the prefix carries the primary
stress, and the root word carries a secondary stress on the same syllable that is stressed when the root word stands alone. For example, the pronunciation and stress of ''
gigabyte'' is . However, when a word with an SI prefix is also commonly used outside the scientific community, it may adopt other pronunciations that do not follow this rule. For example, ''kilometer'' may also be pronounced .
Use outside SI
The symbol "K" is often used informally to mean a multiple of (a) thousand, so one may talk of "a 40K salary" (40 000), or the
Y2K problem. In these cases an uppercase K is often used, although using an uppercase K is never correct when writing under the rules of the SI. Also, it is often used as a prefix to designate the
binary prefix kilo = 2
10 = 1024, although this is now non-standard.
Non-SI 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 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.
Computing
Main articles: Binary prefix
The prefixes k and greater are common in
computing, where they are applied to information and storage units like the
bit and the
byte. Since 2
10 = 1024, and 10
3 = 1000, this led to the SI prefix letters being used to denote "binary" powers. Although these are incorrect usages according to the SI standards it seems common to apply base 10 prefixes, when relating to computers, as follows:
; k:= 2
10 = 1 024
; M:= 2
20 = 1 048 576
; G:= 2
30 = 1 073 741 824
; T:= 2
40 = 1 099 511 627 776
; P:= 2
50 = 1 125 899 906 842 624
These prefixes, however, usually retain their powers-of-1000 meanings when used to describe rates of data transmission (
bit rates): 10
Mbit/s Ethernet runs at 10,000,000 bit/s, not 10,485,760 bit/s. The confusion is compounded by the fact that the units of information (the
bit and the
byte) are not part of SI, where the bit, byte,
octet,
baud or
symbol rate would rather be given in
hertz.
Although some use "bit" for the
bit and "b" for the
byte, "b" is often used for bit and "B" for byte instead. (In SI, B stands for the
bel, although its sub-unit, the decibel ("dB"), is almost universally used instead, preventing confusion between the symbols.) It is recommended by several standards bodies to use 'bit' and 'B' to keep the units very distinct, as in ''kbit'' or ''MiB''. French-speakers often use "o" for "octet", nowadays a synonym for byte, but this is unacceptable in SI because of the risk of confusion with the zero.
Consequently, the
International Electrotechnical Commission (IEC) adopted new
binary prefixes in 1998, formed from the first syllable of the decimal prefix plus 'bi' (pronounced 'bee'). The symbol is the decimal symbol plus 'i'. So now, one
kilobyte (1 kB) equals 1000 bytes, whereas one
kibibyte (1 KiB) equals 2
10 = 1024 bytes. Likewise
mebi (Mi; 2
20),
gibi (Gi; 2
30),
tebi (Ti; 2
40),
pebi (Pi; 2
50), and
exbi (Ei; 2
60). Although the IEC standard does not mention them, the sequence can be readily extended to
zebi (Zi; 2
70) and
yobi (Yi; 2
80). The practical use of these binary prefixes is growing only very slowly, is largely limited to expert literature, and they remain mostly ignored by marketing literature. It can be argued that the main purpose of the binary prefixes is to clarify that, according to national and international standards, the traditional SI prefixes ''always'' refer to powers of ten, even in the context of information technology.
Proposed changes
Extension
Continuing backwards in the alphabet, after ''zetta'' and ''yotta'', proposals for the next large number include ''xenta'' and ''xona'' (among others), the latter as an alteration of the
Latin-derived
numerical prefix , and the next ''small'' number would also start with an ‘x’.
One proposed extension is, after ''zetta'' and ''yotta'', ''xona'', ''weka'', ''vunda'', ''uda'', and ''treda''.
[3]
Preserving the rule on abbreviating the prefixes (a
Latin capital for the large number and a lower-case letter for the small number), even without consensus on the full name the following prefix symbols could be used without ambiguity: ‘X’, ‘W’, ‘V’, ‘x’, ‘w’, ‘v’. The logically next small prefix symbol, ‘u’, was formerly the accepted substitution for ‘µ’ (now withdrawn), the symbol for “micro”.
[4][5]
Another proposal for xenta/xona is ''novetta'', from the Italian . This does not have the convenience of backward alphabetic order. Also, the symbol 'n' is already in use for "nano".
Harmonization
There are also proposals for further harmonization of the capitalisation. Therefore the symbols for kilo, hecto, and deka 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
±3''n'' scheme, namely hecto, deka, 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 m² (10 m × 10 m = 1 dam × 1 dam = 1 dam²)
★
★ ⇒ 1 ca = 1 m² (1 m × 1 m)
★
★ ⇒ 1 ha = 10,000 m² (100 m × 100 m = 1 hm × 1 hm = 1 hm²)
★ 1
stere (st) = 1 m³
★ 1
litre (l or L) = 1 dm³ = 1 mst = 0.001 m³
Of these the litre and the hectare are the most ubiquitous in common use: Litre designations are sometimes used to differentiate a volume of fluid (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
★
Binary prefix
★
Engineering notation
★
Metric system
★
Non-SI unit prefix
★
Number names
★
Numbers in various languages (for comparison/etymology)
★
Orders of magnitude
★
Scientific notation
★
SI base unit
★
SI derived unit
References
1. Technically speaking, , Kevin, Self, Spectrum,
2. Technically speaking, , Kevin, Self, Spectrum,
3. Slide 2 on the presentation at http://bt.pa.msu.edu/TM/BocaRaton2006/talks/davis.pdf
4. International Standard ISO 2955: "Information processing - Representation of SI and other units in Systems with limited character sets (2nd ed.)
5. A search at http://www.iso.org/iso/en/CatalogueListPage.CatalogueList?scopelist=CATALOGUE for standard number 2955 shows this standard is withdrawn (accessed 2006-07-26).
External links
Standards organizations
★
The International Bureau of Weights and Measures (BIPM): SI prefixes
★
US NIST "Definitions of the SI units: The twenty SI prefixes"
★
US NIST "Definitions of the SI units: The binary prefixes"
★
International Standard ISO 2955: "Information processing - Representation of SI and other units in Systems with limited Character sets"
Other proposals
★
Proposal for an extension of the SI-prefix system to even larger and smaller units
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