Additional Resources

Number Expressions

Technical documents often contain quantitative measurements. In order to reflect the precision of the data, numbers must be presented in a way that ensures reader accuracy. NOTE: While the guidelines presented below generally apply to number expression in engineering, different disciplines may have different rules. Consult a discipline-specific style guide for information on expressing numbers and units.

• Spell out single-digit numbers zero through nine. Numbers >10 are expressed numerically. Zero is usually written as a numeral. Numerals are used when a number >10 appears in a range or list with higher numbers.

  The collision damaged seven vehicles and injured four passengers.

  It will take between 9 and 12 months to finish the project.

  The x and y axes intersect at 0.

• Approximate numbers follow the same rules as exact numbers.
  • About 50,000 discrete events were recorded.
  • Approximately 8 samples in 20 had to be discarded.
• Avoid starting sentences with numbers; either spell out the number or if the number is tedious, revise the sentence.

  Incorrect:

  25 engineers were involved in the project.

  Correct:

  Twenty-five engineers were involved in the project. OR The project involved 25 engineers.

• Hyphenate compound numbers 21 through 99 when written as words.

  Forty-nine measurements were taken.

• With two adjacent numbers, the first number is typically spelled out unless the second number is shorter when spelled out.

  The technician ordered twelve 26-in. pieces.

Note: A modern and consistent approach is to retain the number that goes with the unit and spell out the other number.

The solution was divided into four 250-mL flasks.

• Measured quantities involving decimal points, dimensions, degrees, distances, weights, measures, time, and sums of money should be expressed in numerical form, even if they are less than 10.
  • 8 cm
  • 3 percent
  • Factor of 2
  • Chapter 2, Vol. 3, No. 7, Figure 4
  • 7 seconds, 3 minutes 2.5 hours
  • $0.26 per unit, $1.1 million, $45

• If an exact number is followed by a unit of measurement, abbreviate the unit. A space is usually placed between a number and a unit of measure. If the number is an approximation, spell out the unit. The period is often included after the abbreviation for ínch (in.) to differentiate from the preposition in.

  The heat transfer surface of the cylinder was covered with 25 mm of insulation.

  A single lightning bolt contains over a billion watts.

• Symbols for units are written in the singular form.

  There are 1000 mm in 1 L. NOT There are 1000 mms in 1 L.

  
  
• If the unit of measurement is named for a person, the unit is capitalized. NOTE: Some exceptions include °C, °F, K, L).

  The strength of an average solar sunspot is equal to 0.3 T.

• Write secondary units in parentheses after the primary units.

  Install a 2-in. (5.08-cm) diameter rod.

• Place a hyphen between the number and the unit of measure when they modify a noun.

  The 25-µm beam provides optimal resolution.

• Spell out the unit when using it in a non-numerical context; all units have a lower-case first letter when written out (even if they are named for a person).

  A tesla is a unit of measurement of the strength of a magnetic field.

• Isolated fractions in the text should be spelled out and hyphenated unless the fractions are long and complicated.

  The formula called for one-half of the amount.

  Only 18/25 of the mixture was used.

• If the number is one or a fraction of one, the unit is written in the singular.

  0.8 ton of steel NOT 0.8 tons of steel

• When the precise value of a fraction is required, use the decimal form.

  The technician added 0.5 L to the mixture.

• For numbers less than 1.0, insert a zero to the left of the decimal point. NOT .47

• Use scientific notation for very large or very small numbers.

For the number 5,000,000, write 5 X 10⁶.

For the number 0.000005, write 5 X 10^-6.

• Avoid using the words million, billion, trillion, with engineering units.

Correct:

3.2 X 10⁶ km

Incorrect:

3.2 million km

• By SI convention, commas are omitted from numbers greater than three digits (e.g., 1000). Numbers of at least five digits on either side of the decimal point may have a nonbreaking space inserted between groups of three digits. Ideally, narrow or half spaces should be used.

Equations are part of the text in a technical document and should not be referred to as figures. The following guidelines represent typical formatting considerations; a discipline-specific style guide for should be consulted for rules specific to one’s major or organization.

Equations should be:

• Separated by a blank line above and below the equation
• Indented from the left margin (some style guides suggest centering the equation)
• Numbered sequentially with the equation number right aligned
• Referred to in the text by the equation number
• Followed by a definition of the variables*
• Written using an equation editor
• Separated by a blank line when two equations appear together

Example:

One well-established relationship is the stress-strain relationship for a linear-elastic member in uniaxial tension can be defined as

σ = F (L - x) c / I (1)

Where

• F = Force at end of beam
• L = Length of beam
• I = 2nd Area moment of inertia
• x = Distance from the base of the beam to the point of interest
• t = Thickness of beam

*Defined variables can either be listed vertical form or horizontal form separated with commas.

If the document contains a nomenclature, it is not necessary to provide the names and units of the variables. A nomenclature provides a list of all symbols and abbreviations that appear in the document (excluding common abbreviations regularly used in writing). This list usually appears after the table of contents and the list of figures and tables.