Updates 2024/06/18

I always forget how to use numerals and units in academic writing - when I should spell out numbers and when I should not. There are general rules and many ‘exceptions’. Therefore, I decide to write a blog on this topic.

Note that I will follow the APA style in this blog. The rules of using numerals in Chicago style are quite different, for example, spell out numbers one through one hundred.

General rule

For values of one through nine, spell out numbers (i.e. use words); for values of 10 and higher, use Arabic numerals.

Examples:

The three sites—Taipei, Shanghai, and Bangkok—all experienced severe weather events in the time period studied.

Exception 0

Use numerals to express units of time, dates, ages, and numbers that denote a specific place in a numbered series.

Examples:

… 5 years…, NOT … five years …

Numerals are often preferable when a number is followed by a unit of measurement.

‘six centimetres’ can also be written as ‘6 cm’

Exception 1

Always spell out numbers at the beginning of a sentence, if the sentence cannot be rearranged to avoid starting with a number.

Examples:

Fifteen days previously… NOT 15 days previously…

Exception 2

For a mixture of numbers in one sentence, use a consistent number style.

Examples:

The sample included 34 men with type A blood, 15 with type B, and 3 with type AB.

Exception 3

Use different number styles when putting two numbers next to each other.

Examples:

… five 50-kg women…, NOT … 5 50-kg women…

Exception 4

Use numerals when the number follows the noun.

Examples:

page 3

act 7

grade 3

room 9

Others - Rates, proportions and fractions

  • Use a virgule (/) for proportions, and a colon (:) for ratios: About 1/3 of samples… The ratio was 3:4.5… The rate averaged 40/100,000 people…
  • Spell out fractions that modify nouns: Half the cases showed… A two-thirds majority…
  • When writing a range or series, give the unit after the final item:
    • BAD: 25 mg–30 mg
    • GOOD: 25–30 mg
  • Do not insert a space on either side of an em-dash (—):
    • BAD: The three sites — Taipei, Shanghai, and Bangkok — all experienced severe weather events in the time period studied.
    • GOOD: The three sites—Taipei, Shanghai, and Bangkok—all experienced severe weather events in the time period studied.

Reference

  • Numerals and units, Springer, from https://www.springer.com/gp/authors-editors/journal-author/numerals-and-units/1370#:~:text=Spell%20out%20numbers%20one%20through,avoid%20starting%20with%20a%20number. (This article relies heavily dependent on this reference)
  • https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Numbers.html
  • https://blog.apastyle.org/apastyle/2011/01/numbers-anyone.html