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Dictionary of Units / Measurements

# Prefixes of SI

The S I allows the sizes of units to be made bigger or smaller by the use of appropriate prefixes. For example, the electrical unit of a watt is not a big unit even in terms of ordinary household use, so it is generally used in terms of 1000 watts at a time. The prefix for 1000 is kilo so we use kilowatts [kW] as our unit of measurement. For makers of electricity, or bigger users such as industry, it is common to use megawatts [MW] or even gigawatts [GW]. The full range of prefixes with their [symbols or abbreviations] and their multiplying factors which are also given in other forms is
 yotta [Y] 1 000 000 000 000 000 000 000 000     = 10^24 zetta [Z] 1 000 000 000 000 000 000 000         = 10^21 exa   [E] 1 000 000 000 000 000 000             = 10^18 peta  [P] 1 000 000 000 000 000                 = 10^15 tera  [T] 1 000 000 000 000                     = 10^12 giga  [G] 1 000 000 000                  (a thousand millions = a billion) mega  [M] 1 000 000                      (a million) kilo  [k] 1 000                          (a thousand) hecto [h] 100                              (a hundred) deca  [da]10                               (ten)           1 deci  [d] 0.1                              (a tenth) centi [c] 0.01                             (a hundredth) milli [m] 0.001                          (a thousandth) micro [µ] 0.000 001                      (a millionth) nano  [n] 0.000 000 001                  (a thousand millionth) pico  [p] 0.000 000 000 001                   = 10^-12 femto [f] 0.000 000 000 000 001                      = 10^-15 atto  [a] 0.000 000 000 000 000 001           = 10^-18 zepto [z] 0.000 000 000 000 000 000 001              = 10^-21 yocto [y] 0.000 000 000 000 000 000 000 001   = 10^-24

[µ] the symbol used for micro is the Greek letter known as 'mu'
Nearly all of the S I prefixes are multiples (kilo to yotta) or sub-multiples (milli to yocto) of 1000.

However, these are inconvenient for many purposes and so hecto, deca, deci, and centi are also used.

deca also appears as deka [da] or [dk] in the USA and Continental Europe. So much for standards!