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

9.8 Unacceptability of applying mathematical operations to unit names 
 
Because it could possibly lead to confusion, mathematical operations are not applied to unit names 
but only to unit symbols. (See also Secs. 6.1.7 and 9.5.) 
Example:
joule per kilogram or J/kg or J·kg
−1
but not: 
joule/kilogram or
joule
·kilogram
−1
 
10 More on Printing and Using Symbols and Numbers in Scientific and Technical 
Documents
4 
By following the guidance given in this chapter, NIST authors can prepare manuscripts that are consistent 
with accepted typesetting practice. 
4
This chapter is adapted in part from Refs. [4: ISO 31-0], and [4: ISO 31-11].

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Guide for the Use of the International System of Units (SI) 
 
33 
10.1 Kinds of symbols 
 
Letter symbols are of three principal kinds: (a) symbols for quantities, (b) symbols for units, and (c) 
symbols for descriptive terms. Quantity symbols, which are always printed in italic (that is, sloping) type, 
are, with few exceptions, single letters of the Latin or Greek alphabets that may have subscripts or 
superscripts or other identifying signs. Symbols for units, in particular those for acceptable units, have been 
discussed in detail in earlier portions of this Guide. Symbols for descriptive terms include the symbols for 
the chemical elements, certain mathematical symbols, and modifying superscripts and subscripts on 
quantity symbols. 
 
10.1.1 Standardized quantity symbols 
 
The use of words, acronyms, or other ad hoc groups of letters as quantity symbols should be avoided 
by NIST authors. For example, use the quantity symbol Z
m
for mechanical impedance, not MI. In fact, there 
are nationally and internationally accepted symbols for literally hundreds of quantities used in the physical 
sciences and technology. Many of these are given in Refs. [4] and [5], and it is likely that symbols for the 
quantities used in most NIST publications can be found in these international standards or can readily be 
adapted from the symbols and principles given in these standards. Because of their international 
acceptance, NIST authors are urged to use the symbols of Refs. [4] and [5] to the fullest extent possible.
5
 
Examples: Ω (solid angle)
Z
m
(mechanical impedance) 
 
 
L
P
 (level of a power quantity)
Δ
r
(relative mass excess) 
 
 
p (pressure)
σ
tot
(total cross section) 
 
 
ĸ
T
 (isothermal compressibility)
Eu (Euler number) 
 
 
E (electric field strength)
T
N
(Néel temperature)