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

6.2 Rules and style conventions for SI prefixes 
 
The following eight sections give rules and style conventions related to the SI prefixes. 
6.2.1 Typeface and spacing 
 
Prefix names and symbols are printed in roman (upright) type regardless of the type used in the 
surrounding text, and are attached to unit symbols without a space between the prefix name or symbol and 
the unit name or symbol. This last rule also applies to prefixes attached to unit names.
Examples: mL (milliliter) 
pm (picometer)
GΩ (gigaohm)
THz 
(terahertz) 
 
 
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Guide for the Use of the International System of Units (SI) 
 
14 
6.2.2 Capitalization 
 
The prefix symbols Y (yotta), Z (zetta), E (exa), P (peta), T (tera), G (giga), and M (mega) are printed in 
upper-case letters while all other prefix symbols are printed in lower-case letters (see Table 5). Prefix 
names are normally printed in lowercase letters. 
6.2.3 Inseparability of prefix and unit 
 
The grouping formed by a prefix symbol attached to a unit symbol constitutes a new inseparable 
symbol (forming a multiple or submultiple of the unit concerned) which can be raised to a positive or 
negative power and which can be combined with other unit symbols to form compound unit symbols. 
Examples: 2.3 
cm
3
= 2.3 (cm)
3
= 2.3 (10
−2
m)
3
= 2.3 × 10
−6
m
3
1 
cm
−1
= 1 (cm)
−1
= 1 (10
−2
m)
−1
= 10
2 
m
−1
5000 
µs
−1
= 5000 (µs)
−1
= 5000 (10
−6
s)
−1
= 5000 × 10
6
s
−1
= 5 × 10
9
s
−1
1 V/cm = (1 V)/(10
−2 
m) = 10
2
V/m 
Prefix names are also inseparable from the unit names to which they are attached. Thus, for example, 
millimeter, micropascal, and meganewton are single words.