AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020
Expressing Unit Names and Symbols
Units of Measure
The SI includes conventions for expressing unit names and abbreviations (often referred to as symbols) and for displaying them in text.
17.2.1 Capitalization.
The SI unit names are written lowercase (eg, kilogram) when spelled out, except for Celsius (as in “degrees Celsius”), which is capitalized. Abbreviations or symbols for SI units also are written lowercase, with the following exceptions:
■Abbreviations derived from a proper name should be capitalized (eg, N for newton, K for kelvin, A for ampere), although nonabbreviated SI unit names derived from a proper name are not capitalized (eg, newtons, amperes).
■An uppercase letter L is used as the abbreviation for liter to avoid confusion with the lowercase letter l and the number 1.
■Certain SI prefixes are capitalized to distinguish them from similar lowercase abbreviations:
✵M denotes the prefix mega (10^{6}), whereas m denotes the prefix milli (10^{−3})
✵P denotes the prefix peta (10^{15}), whereas p denotes the prefix pico (10^{−12})
17.2.2 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 20.5, 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.
Avoid: |
newton · m or N · meter |
Better: |
newton meter is expressed as newton meter or N m or N · m |
When numerals are used to denote a quantity of measurement, it is preferable to use the abbreviated form of the SI unit.
Avoid: |
50 newton meter |
Better: |
50 N · m |
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.
Avoid: |
μg per L |
Better: |
μg/L or μg L^{−1} or μg · L^{−1} |
When the unit names are spelled out in a quotient or in text, the word per should be used.
Avoid: |
The power output was measured in joules/second [or J/s]. |
Better: |
The power output was measured in joules per second. |
Expressions with 2 or more units of measure may require use of the forward slash, dot products, negative exponents, or parentheses (see 20.5, Expressing Multiplication and Division).
mL · kg^{−1} · min^{−1} or mL/kg/min
m^{2} · kg · s^{−2} · A^{−2} or (m^{2} · kg)/(s^{2} · A^{2})