AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Expressing Multiplication and Division
Mathematical Composition

The product of 2 or more terms, including units of measure, is conventionally indicated by a raised multiplication dot (⋅) (eg, 7 kg ⋅ m²) or by 2 or more characters closed up (eg, y = mx  +  b). However, in scientific notation the times sign (×) is used (eg, 3 × 10⁻¹⁰ cm) (see 17.4.4, Units of Measure, Use of Numerals With Units, Multiplication of Numbers). An asterisk should not be used to represent multiplication despite its use in this role in computer programs. However, there may be occasions on which the asterisk may be used to provide the reader with the exact equation used in the analysis (eg, regression models).

A forward slash, a horizontal line, a negative exponent, or the word per may be used to express rates, which are generally obtained by dividing one unit by another. For example, velocity (meters per second) may be expressed as follows:

Complex rates involve division of a rate by another unit. Complex rates that are used frequently are conventionally indicated by 2 forward slashes in the same expression.

The dose was 25 mg/kg/d.

Plasma renin activity was 1.3 ng/mL/h.

Acceleration at the surface of the earth is 9.8 m/s/s (or 9.8 m/s²).

Negative exponents may also be used to express such a rate when appropriate: 2 mL ⋅ kg⁻¹ ⋅ min⁻¹ (see 17.2.2, Products and Quotients of Unit Symbols). Common sense and clarity should guide this decision.