APS House Style :: Mathematics
If a unit of measure is not preceded by a number, write it out (unless it is in parentheses).
If the author uses "K" to stand for 1,000s, change it to the full number (e.g., change "4K" to "4,000"); this does not, however, apply to computer terms for file size (kilobits). Remember that kilo- as a prefix is always lowercase, e.g., kHz, kg, kDa, etc.
Both "1 of 10" and "1/10" are permitted to express a proportion; follow the author's text verbatim. However, the phrase "1 out of 10" should be changed to "1 of 10" ("out" is not needed). You may, as needed, change single-digit fractions to words in text, i.e., change "1/3" to "one-third," etc.
Units of measure are assumed to correspond with all values preceding the unit. As such, you should change, e.g., 10 Hz and 25 Hz to 10 and 25 Hz (at manuscript stage only). However, it would not be correct to alter sentences where different types of units are listed concurrently; for example, do not alter the sentence "…pH, arterial pressure, and venous pressure of 7.5, 80 mmHg, and 110 mmHg."
APS uses the International System of Units (SI units) for all measurements. In a nutshell, this means that APS follows a basic system of measurement unit rules. This system is as follows:
Unit abbreviations are all lowercase. Symbols are all written in lowercase except for symbols derived from the name of a person (e.g., a unit of pressure is named after B. Pascal, so its symbol is written as "Pa," whereas the unit itself is written as "pascal").
The unit of "liter" is written out completely if it is the only unit because its abbreviation (l) is too easily confused with the number 1 in print. Some authors capitalize the "L" for liter measurements to avoid this confusion; you will have to change this to the lowercase form or write out "liters."
Unit abbreviations do not indicate pluralization. Unit values are assumed to automatically reflect the correct form of pluralization. This means that the abbreviations should not include the "-s" suffix.
Unit abbreviations do not have punctuation to indicate that they are abbreviations. This is considered to be redundant. The only exception to this rule is the abbreviation "in." for "inches" to avoid confusion with "in" used as a preposition.
Values and units are separated by a space. A space should separate the numerical value and its units, e.g., "2.21 kg," "7.3 × 102 m2," and "54 M." The only exception is for degree symbol (°) to indicate temperature or angle.
Units are styled with shills and chem points. Units with ≤2 discrete measurements are bumped together with a shill ("/") to indicate division or a chem point ("•") to indicate multiplication; you should change "ml•kg«minus»1" to "ml/kg.". Units with ≥3 discrete measurement values are joined using a chem point and appropriate use of the «minus»1 exponent; for example, change "ml/kg/min" to "ml•kg«minus»1•min«minus»1."
APS does not use ' and " to indicate seconds and minutes of time, respectively. Instead, use the abbreviations "s" (for seconds) and "min" (for minutes).
Always change "sec" to "s" and "hr" to "h."
Always change "dynes" to "dyn," which is the correct form of the unit abbreviation.
Always change "cc" (for cubic centimeters) to "ml" (for milliliters).
Resistance units are frequently given incorrectly by authors and require special attention. Resistance is pressure divided by flow. Pressure can be expressed as mmHg, Torr, or cmH2O. Flow is usually expressed in ml/min. Therefore, resistance can be expressed as, e.g., mmHg/(ml/min), which is equivalent to (and correctly presented as) mmHg•ml«minus»1•min. Note that only one occurrence of "«minus»1" is correct, unlike in typical combinations of three or more units. For safety's sake, query the author to verify that any altered units are correct.
Another common error made by authors is to use the abbreviation "nM" (nanomolars) instead of "nm" (nanometers). The unit nanomolar denotes concentration; nanometers is distance. If you notice that excitation/emission light wavelengths or a radius/diameter is in nM, then change the unit to "nm" and query the author for verification.
See the Resources section for more information regarding commonly encountered units of measurement.
Values should be associated with units of measurements unless they are proportions/ratios. If you have any question as to whether a value needs a unit of measure or if the unit of measurement has been styled correctly, simply include a small author query for verification. In general, only change or add units if it is obviously scientifically necessary.