Writing Numbers, Units of Measure, and Equations

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WRITING NUMBERS, UNITS OF MEASURE, AND EQUATIONS

Adapted from The Elements of Technical Writing
by Gary Blake and Robert W. Bly1

Because numbers are used so frequently in technical writing, the rules for writing them are designed for clarity and consistency. Some rules are hard and fast, and others will vary depending upon the style manual consulted. We will focus on a few rules that will help you write in EG.

Numbers

  1. All numbers below 10 (including zero) should be written out in words, with the following exceptions: age, time, dates, page numbers, percentages, money, proportions and units of measure. Numbers greater than nine are written in numerals.
  2. Rule examples:

    • one robot
    • two containers
    • three team members
    • eight workstations
    • zero chance


    Exceptions:

    • 2 seconds
    • $3
    • 4:00AM
    • 8 percent
    • 9 years old


  3. If two or more numbers are presented together, write them all as numbers. This makes your work consistent, neat and easy to understand. However, if all the numbers in the section are below 10, write them all as words, e.g., We used 4 worm gears, 15 pulleys, and 3 motors. Or, We used four worm gears, two pulleys, and three motors.
  4. Hyphenate a number and a unit of measure when they modify a noun, e.g., a 4-year-old boy, a 12-inch-long pipe, or an 8-pound baby.
  5. Write fractions and decimals as numerals, not words. So it is 0.25, not zero point two five.
  6. Be consistent when you write decimals. Always begin with a zero if the number is less than one (0.25). This guarantees consistency when you write them in columns with other numbers. You may align these numbers in a column by the decimal point:

    20.23
    1.25
    0.33
    10.232

    Or flush left:

    20.23
    1.25
    0.33
    10.232

    Or flush right:

    20.23
    1.25
    0.33
    10.232

  7. If a number is an approximation, write it out, like twice as much and half finished.
  8. You may not begin a sentence with a numeral. Write the number out in words or rephrase the sentence, not, 16 years after the project began, funding was cut. But, Funding was cut 16 years after the project began.

Units of Measure

  1. When writing units of measure, be consistent. If you are measuring temperature as degrees Fahrenheit, do not suddenly switch to degrees Celsius.
  2. Units of measure can be written as symbols, words, or abbreviations. For basic units of measurement, use words: 25 pounds, 12 inches. For derived units of measure — ones formed using a calculation — use symbols: 38mph, 27ft/s2. Some derived units of measure have two symbols: one that represents the derivation and one that represents the word. In this case, use the one that represents the word because it will be more familiar to your reader. Use F for Farad, Hz for Hertz, and V for Volt.
  3. To indicate multiplication, use a raised dot ( ∙ ). To indicate division, use a slash (/). If you are writing out the unit in words, use a hyphen for multiplication, and the word per for division: The force was 22 kilogram-meters squared, The speed was 50 miles per hour.
  4. If you want to add a secondary unit of measure after your primary one, put it in parentheses following the primary unit of measure: 10°C (50°F).

Equations

  1. Do not use too many equations; it is easy to make a mistake and they are cumbersome in the text. If your readers are not technical, try not to use them at all.
  2. Put your equations on a separate line, center them, and number them:
  3. (1) y = ax2 + bx + c

  4. When you introduce your equations with words, punctuate the introductory remarks normally. To do this, substitute the phrase this equation for the actual equation and add the correct punctuation. For example, The current in the wire is calculated using E=IR. Not The current in the wire is calculated using: E=IR. Note the period following the equation. If the sentence were, The current in the wire is calculated using E=IR where E is voltage, I is current, and R is resistance., then the period after the equation is not necessary and appears at the end of the sentence instead.

This is by no means a comprehensive list of the rules for writing numbers, symbols and equations. For a complete list consult an appropriate style guide.

Footnotes

1 Blake, G., and Bly, R.W., The Elements of Technical Writing. New York: Longman, 1993

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