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NIST Guide to the SI, Chapter 6: Rules and Style Conventions for Printing and Using Units

6.1 Rules and style  

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).)

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 halfhigh (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


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 [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 avoid possible confusion; 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.



ms fraction

, or m · s-1

However, to avoid ambiguity, the solidus must not be repeated on the same line unless parentheses are used.

Example: m/s2 or m · s-2but 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 Secs. 9.5 and 9.8.)

Example: C/kg, C · kg-1, or coulomb per kilogram but not:  coulomb/kg; coulomb per kg; C/kilogram; coulomb · kg-1; 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 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)

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 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

Prefix names 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 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 Images/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 Images/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 ×106/m3but 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 prefix names 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 ºC and prefix names may be used with the unit name "degree Celsius." For example, 12 m ºC (12 millidegrees Celsius) is acceptable. However, to avoid confusion, prefix symbols (and prefix names) 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 prefix names) may be used with the unit symbols (names) L (liter), t (metric ton), eV (electronvolt), u (unified atomic mass unit), Da (dalton) (see Tables 6 and 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).

Created January 28, 2016, Updated February 17, 2022