Products and Quotients of Unit Symbols
18.2.2 Products and Quotients of Unit Symbols
The product of 2 or more SI units should be indicated by a space between them or by a raised multiplication dot. The multiplication dot must be positioned properly to distinguish it from a decimal point, which is set on the baseline. (See 21.6, Mathematical Composition, Expressing Multiplication and Division.) When the unit of measure is the product of 2 or more units, either abbreviations (symbols) or nonabbreviated units should be used. Abbreviated and nonabbreviated forms should not be combined in products.
Preferred: |
newton meter is expressed as newton meter or N m or N · m |
Avoid: |
newton · m or N · meter |
When numerals are used to denote a quantity of measurement, it is preferable to use the abbreviated form of the SI unit.
Preferred: |
50 N · m |
Avoid: |
50 newton meter |
The quotient of SI unit symbols may be expressed by the forward slash or virgule (/) or by the use of negative exponents. If the derived unit is formed by 2 abbreviated units of measure (eg, μg/L), the quotient also may be expressed by means of the forward slash or negative exponents.
Preferred: |
μg/L or μg L−1 or μg · L−1 |
Avoid: |
μg per L |
When the unit names are spelled out in a quotient or in text, the word per should be used.
Preferred: |
The power output was measured in joules per second. |
Avoid: |
The power output was measured in joules/second [or J/s]. |
Expressions with 2 or more units of measure may require use of the forward slash, dot products, negative exponents, or parentheses. (See 21.6, Mathematical Composition, Expressing Multiplication and Division.)
mL · kg−1 · min−1 or mL/kg/min
m2 · kg · s−2 · A−2 or (m2 · kg)/(s2 · A2)