Here in the United States, we use US customary units of measurement, such as pounds, feet, and inches, for most of our daily activities. However, we are also accustomed to seeing—and sometimes using—the metric system adopted by most other countries. So, should we use metric or US customary units of measurement in our writing?
Before answering that question, let’s take a closer look at the names US customary units of measurement and the metric system.
The Names: The Metric System and US Customary Units of Measurement
The metric system is officially called the International System of Units, commonly abbreviated as SI units. However, unless your writing is highly technical or scientific, you can safely follow the lead of one of our primary scientific style guides, the Publication Manual of the American Psychological Association (APA style), which uses the name metric instead of SI.1
US customary units of measurement are based on the historical imperial units of measurement previously used in the United Kingdom.2 Although not identical, US customary units of measurement are sometimes called imperial units or English units. In addition, they are sometimes called US standard units or conventional units.
Three Guidelines for Using Metric and US Customary Units of Measurement
The three guidelines below explain when to use metric and when to use US customary units of measurement in writing.
Note that if you are following a specific style guide, that guide’s recommendations should always override the information presented here.
1. Use the metric system in technical, scientific, and medical writing.
Technical, scientific, and medical publications almost always use the metric system abbreviations rather than US customary units.
Part A is attached 5 cm behind Part B.
The patient lost 4 kg of body weight per month.
In fact, the following style guides require the metric system for most lengths and weights:
Publication Manual of the American Psychological Association (APA style)3 (See the Style Guide Alert below.)
Scientific Style and Format from the Council of Science Editors (CSE style)4
AMA Manual of Style from the American Medical Association (AMA style)5
Style Guide Alert: APA Style
Although the Publication Manual of the American Psychological Association (APA style) recommends the metric system, it acknowledges that US customary units may be necessary under certain circumstances. In those situations, include the metric equivalent in parentheses.6
The test subjects walked at least 1 mi (1609.34 m) per day.
2. Use US customary units of measurement in nontechnical writing.
US customary units can be used in most general and business writing intended for a US audience. They can also be used in most academic writing in the humanities.
The average car weighs two tons.
The coffee table is forty-seven inches long.
As shown in the examples above, US customary units are normally spelled out rather than abbreviated.7 The choice to use numerals or spell out the numbers connected to the units of measurement will depend on your style guide.
In an upcoming post, we will look at when—and how—to abbreviate US customary units in specific situations.
3. Mix metric and US customary units of measurement when necessary.
Follow the advice of The Associated Press Stylebook (AP style) if you are writing nontechnical content for an international audience by including both US customary units and metric units.8
If your primary audience is in the United States, place the metric conversion in parentheses directly after the US customary unit:
The kitten weighs approximately 2 pounds (1 kilogram).
If your primary audience is outside of the United States, place the US customary unit conversion in parentheses, instead:
JoAnne used about 3.79 liters (1 gallon) of gas driving to the airport.
Both units of measurement are spelled out and rounded to avoid decimals or long number strings, when possible. Again, the choice to use numerals or spell out the numbers connected to the units of measurement will depend on your style guide.
You can find many conversion charts online, including this unit conversion web page provided by the National Institute of Standards and Technology.
1. Publication Manual of the American Psychological Association, 7th ed. (Washington, DC: American Psychological Association, 2020), 6.27.
2. “Imperial units,” Encyclopedia Britannica, accessed January 7, 2020.
3. Publication Manual of the American Psychological Association, 6.27.
4. Council of Science Editors, Scientific Style and Format, 8th ed. (Chicago: University of Chicago Press, 2014), 12.2.3.
5. American Medical Association, AMA Manual of Style, 10th ed. (New York: Oxford University Press, 2007), 18.5.1.
6. Publication Manual of the American Psychological Association, 6.27.
7. The Chicago Manual of Style, 17th ed. (Chicago: University of Chicago Press, 2017), 9.13.
8. The Associated Press Stylebook 2019 (New York: Associated Press, 2019), 183–84.