For example, the prefix "kilo" multiplies by one thousand, so a kilometre is 1,000 metres, and a kilowatt is 1,000 watts. The prefix "milli" subdivides by a thousand, so a millimetre is one thousandth of a metre (1,000 millimetres in a metre), and a millilitre is one thousandth of a litre. The way that the same prefixes can be applied to any SI unit is one of the strengths of SI, since it considerable simplifies the learning and use of it.
The most commonly used are:
The full table follows below.
(Sub)multiple  Prefix  Symbol  Name (Americas)  Name (European) 

10^{24}[?]  yotta  Y  Septillion  Quadrillion 
10^{21}[?]  zetta  Z  Sextillion  Thousand trillion (Trilliard) 
10^{18}[?]  exa  E  Quintillion  Trillion 
10^{15}[?]  peta  P  Quadrillion  Thousand billion (Billiard[?]) 
10^{12}[?]  tera  T  Trillion  Billion 
10^{9}[?]  giga  G  Billion  Thousand million (Milliard) 
10^{6}[?]  mega  M  Million  
10^{3}[?]  kilo  k  Thousand  
10^{2}[?]  hecto  h  Hundred  
10^{1}[?]  deca or deka  da  Ten  
10^{1}[?]  deci  d  Tenth  
10^{2}[?]  centi  c  Hundredth  
10^{3}[?]  milli  m  Thousandth  
10^{6}[?]  micro  μ  Millionth  
10^{9[?]}  nano  n  Billionth  Milliardth 
10^{12}[?]  pico  p  Trillionth  Billionth 
10^{15}[?]  femto  f  Quadrillionth  Billiardth 
10^{18}[?]  atto  a  Quintillionth  Trillionth 
10^{21}[?]  zepto  z  Sextillionth  Trilliardth 
10^{24}[?]  yocto  y  Septillionth  Quadrillionth 
Examples:
The prefix always takes precedence over any exponentiation; thus km^{2} means square kilometre and not kilo  square metre. For example, 3 km^{2} is equal to 3,000,000 m^{2} and not to 3,000 m^{2} (nor to 9,000,000 m^{2}).
Prefixes where the exponent is divisible by three are recommended. Hence '100 metres' rather than 'one hectometre'. Notable exceptions include centimetre, hectare (hectoare), centilitre, and 1 dm^{3} (equivalent to one litre).
The accepted pronunciation of the initial G of "giga" was once soft, /jī'gə/ (like "gigantic"), but now the hard pronunciation, /gig'ə/, is probably more common. [Is this true of Commonwealth countries?]
Note that the formal SI metric prefix for 1000 is lower case "k"; some use this strictly, reserving "K" for multiplication by 1024 (KB is thus "kilobytes").
Use outside SI The abbreviation "k" is often used to mean a multiple of a thousand, so one may talk of "a 40K salary" (40,000), or the Y2K problem.
However, these prefixes retain their powersof1000 meanings when used to describe rates of data communication: 10Mbps Ethernet runs at 10,000,000 bits per second, not 10,485,760 bits per second.
This inconsistency was not relevant when computers had little storage and communication links were relatively slow, but the increasing capacity of computing systems and speed of network links began making this inconsistency a more serious problem.
Thus, new binary prefixes have recently been adopted by the International Electrotechnical Commission, 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) is 1000 bytes, whereas one kibibyte (1 KiB) is 2^{10} = 1024 bytes. Likewise mebi (2^{20}), gibi (2^{30}), tebi (2^{40}), pebi (2^{50}), and exbi (2^{60}). For example, at 1 MB/s = 10^{6} bytes per second, it would take slightly longer than one second to transfer an object 1 MiB = 2^{20} bytes in size.
For more information on these poweroftwo prefixes, see Binary prefixes.
See also Orders of magnitude.
This article (or an earlier version of it) contains material from FOLDOC, used with permission.
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