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 1991.
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 3,000 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" (in practice, "non-three" prefixes generally appear in civil, not scientific, uses).
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; the two-letter symbols mo and ma 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:
† 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.
There are two accepted pronunciations for the prefix giga-: [ˈgɪgə] and [ˈdʒɪgə]. According to the American writer Self, in the 1920s a German committee member of the International Electrotechnical Commission proposed giga- as a prefix for 109, 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 /dʒ/ (soft g) pronunciation became accepted, but as of 1995 current practice had returned to /g/ (hard g).
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 /ˈɡɪɡəbaɪt/. 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, kilometre (or in the USA, kilometer) may also be pronounced /kɨˈlɒmɨtɚ/.
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 210 = 1024, and 103 = 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 computer memory, as follows: kilo (k) : ≈ kibi = Ki = 210 = 10241 = 1 024; mega (M) : ≈ mebi = Mi = 220 = 10242 = 1 048 576; giga (G) : ≈ gibi = Gi = 230 = 10243 = 1 073 741 824; tera (T) : ≈ tebi = Ti = 240 = 10244 = 1 099 511 627 776 (= 1 024 Gi); peta (P) : ≈ pebi = Pi = 250 = 10245 = 1 125 899 906 842 624 (= 1 048 576 Gi); exa (E) : ≈ exbi = Ei = 260 = 10246 = 1 152 921 504 606 846 976 (= 1 073 741 824 Gi).
These prefixes, however, usually retain their powers-of-1000 meanings when used to describe either disk storage or rates of data transmission (bit rates): 10 Mbit/s Ethernet runs at 10,000,000 bit/s, not 10,485,760 bit/s. This creates confusion when the two usages in memory or filesystems and in transmission or disk storage are colliding, notably for monitoring file transmission rates.
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. The baud symbol is normally "Bd" but it is often confused with bits per second and abbreviated with just its initial, in either letter case, even though it does not directly measure the effective binary data rate but the way this data is encoded and modulated over a transmission link.
It is recommended by several standards bodies to not abbreviate bit and to use B for byte, in order to keep the units very distinct, as in kbit or MiB. French-speakers often use "o" for "octet", today a near synonym for the byte.
Consequently, the International Electrotechnical Commission (IEC) adopted new binary prefixes in 1998 (IEC 80000-13:2008 formerly subclauses 3.8 and 3.9 of IEC 60027-2:2005), formed from the first syllable of the decimal prefix plus 'bi' (pronounced 'bee'). The symbol is the decimal symbol (converted to capital for the first prefix) plus 'i'.
The practical use of these binary prefixes is growing slowly and is largely limited to expert literature. Their use in marketing literature is rare.
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:
Of these the litre and the hectare are the most ubiquitous in common use: 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).