Guide for the Use of the International System of Units (SI)

6.2.4 Unacceptability of compound prefixes 
 
Compound prefix names or symbols, that is, prefix names or symbols formed by the juxtaposition of 
two or more prefix names or symbols, are not permitted. 
Example: nm 
(nanometer) but not: mµm (millimicrometer) 
6.2.5 Use of multiple prefixes 
 
In a derived unit formed by division, the use of a prefix symbol (or a prefix name) in both the 
numerator and the denominator can cause confusion. Thus, for example, 10 kV/mm is acceptable, but 
10 MV/m is often considered preferable because it contains only one prefix symbol and it is in the 
numerator. 
In a derived unit formed by multiplication, the use of more than one prefix symbol (or more than one 
prefix name) can also cause confusion. Thus, for example, 10 MV 
· ms is acceptable, but 10 kV · s is often 
considered preferable. 
Note: Such considerations usually do not apply if the derived unit involves the kilogram. For example, 
0.13 mmol/g is not considered preferable to 0.13 mol/kg. 
6.2.6 Unacceptability of stand-alone prefixes 
 
Prefix symbols cannot stand alone and thus cannot be attached to the number 1, the symbol for the 
unit one. In a similar vein, prefix names cannot be attached to the name of the unit one, that is, to the word 
“one.” (See Sec. 7.10 for a discussion of the unit one.) 
Example: 
the number density of Pb atoms is 5 
×10
6
/m
3
but not: the number density of Pb atoms 
is 5 M/m
3
6.2.7 Prefixes and the kilogram 
 
For historical reasons, the name “kilogram” for the SI base unit of mass contains the name “kilo,” 
the SI prefix for 10
3
. Thus, because compound prefixes are unacceptable (see Sec. 6.2.4), symbols for 
decimal multiples and submultiples of the unit of mass are formed by attaching SI prefix symbols to g, the 

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