Units of Measure

APS Style follows CBE, 5th ed. and recommends the use of SI units (internationally accepted standard) in all publications. All units internationally recognized by CGMP, CBN, ICBN, and DBN and listed in CBE, 5th ed., p. 244-255, do not require definition. The copy editor is expected to correct inappropriate usage by the author and to include an explanatory query, if necessary.

Abbreviated vs. Spelled-Out

Units of measure are always abbreviated when used after a numeral. Do not add "s" to pluralize an abbreviated unit of measure.

Unit of measure should be spelled out when it follows a spelled-out number at the beginning of a sentence. Add "s" if plural. (Do not pluralize any form of the unit M [molar]. Molar describes a concentration, not a discrete unit.)

Twenty milliliters per second of the agent were administered...
Five micromolar CNQX gave an 80% reduction...

The agent (20 ml/s) was administered...
CNQX (5 μM) gave an 80% reduction...

If a shill is used, spell out units and change the shill to "per." Superscripts should be spelled out, e.g., cm² = "centimeters squared".

Spell out and pluralize, if needed, a unit of measure when a word comes between the number and the unit: 1 additional hour, 10 consecutive days.

As a broad generalization, actin research over the last 5 years has focused on detailed biochemistry, whereas...

Always spell out "gauge" (e.g., " a 20-gauge needle"). Authors frequently say "a 20-g needle"; in such cases, change "g" to "gauge" if you are sure of the meaning, and query the author.

Always spell out "liter" when it is used without other units, to avoid confusion with the numeral "1".

4-liter solution

not

4-l solution

however

4 mol/l solution
infusion of 3 g·l–1·h–1

Do not spell out a unit in parentheses; e.g., "(in mM)". In figure legends and in tables, do not spell out units even when they are not accompanying numbers.

The abbreviations for base pairs (bp) and kilobase (kb; referring to base pairs in sequences) do not need to be defined in APS journals, although they are often defined by authors at first use. Make sure that the k- prefix (kilo-) is lowercase.

The unit prefix " micro-" sometimes appears as " mc-" (e.g., mcg or mcm) and should be changed to " μ-" (e.g., μg or μm). If you must spell out " μm", use "micrometer", not "micron". Be alert for authors using a lowercase "u", which may also stand for "units" (U). In such cases, change to the correct unit and query the author for confirmation.

Do not repeat units of measure in a series of 2 or more items, in a range of values, with a ± symbol, with the combinations "from...to" and "between...and", with versus (vs.), or with a multiplication (×) symbol. However, when other words intervene, units can be repeated if necessary for clarity.

1, 2, and 3%
50-100 mg/kg
3 ± 1%
from 50 to 100 kDa
between 2.1 and 10.1 mg
10 vs. 100 mM
2 × 2°

Five protein species that range from 36 kDa (unglycosylated) to 46 kDa (fully glycosylated)...

Shills

A shill (or slash, also called slant line, solidus, virgule) is used with two units of measure, usually representing a rate (5 m/s) or a concentration (10 mmol/l). If a double unit is expressed with a center dot and superscripted "–1" (e.g., mg·kg^–1), change to a shill (i.e., mg/kg). This is particularly common in the "Methods" section of an article, where units are often expressed as rates or concentrations.

The shill is also used to express volume per volume and weight per volume as vol/vol and wt/vol, respectively. The numerical ratio that precedes vol/vol uses a colon.

(1:1:0.01, chloroform-methanol-HCl, vol/vol).

A shill should not be repeated with three or more units of measure. The expression 2 ml/kg/min is mathematically ambiguous. Instead, use 2 ml·kg^–1·min^–1. (See CBE, 5th ed.). However, do not change when the third element is not a unit of measure (e.g., "5 × 10⁵ cells/ml per dish").

For all other units of measure, follow manuscript for use of shill or center dots and superscript numbers.

Other Unit Symbols

Use the symbol "%" for percent: e.g., 2%, 10%, vol%, wt%, %body wt, etc. Note that the percent symbol is always bumped to the preceding or following term or value.

kDa is used as a unit of molecular mass. M_r (molecular weight) is an abbreviation (not a unit of measure) that does not have to be expanded (see Molecular Weight and Mass).

A mole (mol) is an SI unit of the amount of a substance; m- (milli-) is frequently used as a prefix with mol. Molar (M) is a unit of concentration; prefixes commonly used with M are m-, n-, and μ-. In SI units, a concentration would be expressed as mol/l. The copy editor must never change M to mol (with or without prefixes).

For components of solutions, if the author uses "(in mM/l)", he or she may be confusing the SI and non-SI units. Change to "(in mM)", or alternatively "(in mmol/l)", and query. This also applies to prefixes n- and μ- with M.

Do not use a single or double prime for seconds and minutes or inches and feet, repectively; e.g., change "20"´ video monitor" to "20-in. video monitor". Do not query.

Resistance units are frequently given incorrectly by authors and require special attention. Resistance is pressure divided by flow. Pressure can be expressed in mmHg, Torr, or cmH₂O. Flow is usually expressed in ml/min or other expressions of volume per unit of time. Hence, vascular resistance can be expressed as mmHg/(ml/min), which is equivalent to and should be correctly presented as mmHg·ml^–1·min. Note that only one occurrence of "^–1" is correct, unlike in typical combinations of three or more units. Other possible (and correct) unit combinations for resistance include:

Torr·ml^–¹·min·kg
cmH₂O·l^–¹·s
mmHg·ml^–¹·min·g (or kg; used for RVR, renal vascular resistance)

Conductance is flow divided by pressure (i.e., the inverse of resistance). It is typically expressed as ml·min^–¹·mmHg^–¹. If an author uses the abbreviation "mho", change to "S" (siemens).