SI Prefixes


SI prefixes refer to a series of prefixes approved for use in the International System of Units (SI). In the following source, one can find the rules associated with the use of SI prefixes:


Le Systéme International d’Unités (SI), The International System of Units (SI), 8th Edition (Bur. Intl. Poids et Mesures, Sèvres, France, 2006)


The following text is from the above source and interested users are recommended to see this source to find more about SI prefixes and in general about SI:


Prefix symbols are printed in roman (upright) type, as are unit symbols, regardless of the type used in the surrounding text, and are attached to unit symbols without a space between the prefix symbol and the unit symbol. With the exception of da (deca), h (hecto), and k (kilo), all multiple prefix symbols are capital (upper case) letters, and all submultiple prefix symbols are lower case letters. All prefix names are printed in lower case letters, except at the beginning of a sentence.


The grouping formed by a prefix symbol attached to a unit symbol constitutes a new inseparable unit symbol (forming a multiple or submultiple of the unit concerned) that can be raised to a positive or negative power and that can be combined with other unit symbols to form compound unit symbols.




2.3 cm3 = 2.3 (cm)3 = 2.3 (10-2 m)3 = 2.3×10-6 m3

1 cm-1 = 1 (cm)-1 = 1 (10-2 m)-1 = 102 m-1 = 100 m-1

1 V/cm = (1 V)/(10-2 m) = 102 V/m = 100 V/m

5000 μs-1 = 5000 (μs)-1 = 5000 (10-6 s)-1 = 5×109 s-1


Similarly prefix names are also inseparable from the unit names to which they are attached. Thus, for example, millimetre, micropascal, and meganewton are single words.


Compound prefix symbols, that is, prefix symbols formed by the juxtaposition of two or more prefix symbols, are not permitted. This rule also applies to compound prefix names.


Prefix symbols can neither stand alone nor be attached to the number 1, the symbol for the unit one. Similarly, prefix names cannot be attached to the name of the unit one, that is, to the word “one”.


Prefix names and symbols are used with a number of non-SI units, but they are never used with the units of time: minute, min; hour, h; day, d. However astronomers use milliarcsecond, which they denote mas, and microarcsecond, μas, which they use as units for measuring very small angles.


Among the base units of the International System, the kilogram is the only one whose name and symbol, for historical reasons, include a prefix. Names and symbols for decimal multiples and submultiples of the unit of mass are formed by attaching prefix names to the unit name “gram”, and prefix symbols to the unit symbol “g”.




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