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

Guide for the Use of the International System of Units (SI) 
 
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6
Rules and Style Conventions for Printing and Using Units 
 
6.1 Rules and style conventions for unit symbols 
 
The following eight sections give rules and style conventions related to the symbols for units. 
6.1.1 Typeface 
 
Unit symbols are printed in roman (upright) type regardless of the type used in the surrounding text. 
(See also Sec. 10.2 and Secs. 10.2.1 to 10.2.4.) 
6.1.2 Capitalization 
 
Unit symbols are printed in lower-case letters except that: 
(a) the symbol or the first letter of the symbol is an upper-case letter when the name of the unit is derived 
from the name of a person; and 
(b) the recommended symbol for the liter in the United States is L. (See Table 6, footnote (b).) 
Examples: m (meter) s (second)
V (volt) 
Pa (pascal) lm (lumen)
Wb (weber) 
 
6.1.3 Plurals 
 
Unit symbols are unaltered in the plural. 
Example:
l = 75 cm
but not:
l = 75 cms 
 
Note: l is the quantity symbol for length. (The rules and style conventions for expressing the values of 
quantities are discussed in detail in Chapter 7.) 
6.1.4 Punctuation 
Unit symbols are not followed by a period unless at the end of a sentence. 
Example: 
“Its length is 75 cm.” or “It is 75 cm long.” but not: “It is 75 cm. long.” 
6.1.5 Unit symbols obtained by multiplication 
Symbols for units formed from other units by multiplication are indicated by means of either a half-
high (that is, centered) dot or a space. However, this Guide, as does Ref. [6], prefers the half-high dot 
because it is less likely to lead to confusion.
Example: N 
· m or N m 
 
Notes: 
1. A half-high dot or space is usually imperative. For example, m · s
−1
is the symbol for the meter per 
second while ms−1 is the symbol for the reciprocal millisecond (10
3
s
−1
— see Sec. 6.2.3). 
2. Reference [4: ISO 31-0] suggests that if a space is used to indicate units formed by multiplication, 
the space may be omitted if it does not cause confusion. This possibility is reflected in the 
common practice of using the symbol kWh rather than kW · h or kW h for the kilowatt hour. 
Nevertheless, this Guide takes the position that a half-high dot or a space should always be used to 

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