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 Sec. 10.2.1 to
Sec. 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)
Pa (pascal) |
s (second)
lm (lumen) |
V (volt)
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. [8], 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 (103 s-1 - see
Sec. 6.2.3). |
| 2 |
Reference [6: 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 kWh
for the kilowatt hour. Nevertheless, this Guide takes the position that
a half-high dot or a space should always be used to avoid possible confusion;
and that for this same reason, only one of these two allowed forms should be
used in any given manuscript. |
6.1.6 Unit symbols obtained by division
Symbols for units formed from other units by division are indicated by means of
a solidus (oblique stroke, /), a horizontal line, or negative exponents.
Example: m/s, 
,
or m · s-1
However, to avoid ambiguity, the solidus must not be repeated on the same line
unless parentheses are used.
| Examples: |
m/s2 or |
m · s-2 |
but not: m/s/s |
| m · kg/(s3 · A) or |
m · kg · s-3 · A-1 |
but not: m · kg/s3/A |
Negative exponents should be used in complicated cases.
6.1.7 Unacceptability of unit symbols and unit names together
Unit symbols and unit names are not used together. (See also
Sec. 9.5 and
Sec. 9.8.)
| Example: |
C/kg,
C · kg-1, or
coulomb per kilogram |
but not: |
coulomb/kg;
coulomb per kg;
C/kilogram;
coulomb · kg-1; or
C per kg; coulomb/kilogram |
6.1.8 Unacceptability of abbreviations for units
Because acceptable units generally have internationally recognized symbols and
names, it is not permissible to use abbreviations for their unit symbols or
names, such as sec (for either s or second), sq. mm (for either mm2
or square millimeter), cc (for either cm3 or cubic centimeter),
mins (for either min or minutes), hrs (for either h or hours), lit (for either
L or liter), amps (for either A or amperes), AMU (for either u or unified
atomic mass unit), or mps (for either m/s or meter per second). Although the
values of quantities are normally expressed using symbols for numbers and
symbols for units (see Sec. 7.6), if for some
reason the name of a unit is more appropriate than the unit symbol (see
Sec. 7.6, note 3), the name of the unit
should be spelled out in full.
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 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 symbol and the unit symbol. This last rule also applies to
prefixes attached to unit names.
Examples: mL (milliliter) pm (picometer)
GΩ (gigaohm)
THz (terahertz)
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). Prefixes 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 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 |
| 5000 µs-1 = 5000 (µs)-1
= 5000 (10-6 s)-1
= 5000 × 106 s-1
= 5 × 109 s-1 |
| 1 V/cm = (1 V)/(10-2 m) = 102 V/m |
Prefixes are also inseparable from the unit names to which they are attached.
Thus, for example, millimeter, micropascal, and meganewton are single words.
6.2.4 Unacceptability of compound prefixes
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 prefixes.
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)
in both the numerator and the denominator may 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) may 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, prefixes 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 × 106/m3 |
but not : |
the number density of Pb atoms is 5 M/m3 |
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 103. 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 unit symbol for gram, and the names of such multiples and
submultiples are formed by attaching SI prefixes to the name "gram."
Example: 10-6 kg = 1 mg (1 milligram)
but not: 10-6 kg = 1 µkg (1 microkilogram)
6.2.8 Prefixes with the degree Celsius and units accepted for use with the
SI
Prefix symbols may be used with the unit symbol °C and prefixes may be used
with the unit name "degree Celsius." For example, 12 m°C
(12 millidegrees Celsius) is acceptable. However, to avoid confusion,
prefix symbols (and prefixes) are not used with the time-related unit symbols
(names) min (minute), h (hour), d (day); nor with the
angle-related symbols (names) ° (degree), ′ (minute),
and ″ (second) (see Table 6).
Prefix symbols (and prefixes) may be used with the unit symbols (names)
L (liter), t (metric ton), eV (electronvolt), and
u (unified atomic mass unit) (see Table 6
and Table 7). However, although submultiples
of the liter such as mL (milliliter) and dL (deciliter) are in common
use, multiples of the liter such as kL (kiloliter) and ML (megaliter)
are not. Similarly, although multiples of the metric ton such as
kt (kilometric ton) are commonly used, submultiples such as
mt (millimetric ton), which is equal to the kilogram (kg), are not.
Examples of the use of prefix symbols with eV and u are 80 MeV
(80 megaelectronvolts) and 15 nu (15 nanounified atomic mass
units).
