Basics

Zero to Nine - Spell out all single-digit numbers from zero to nine (e.g., ‘two’ instead of ‘2’).

10 or Above - For a text that has more numerical data, express number(s) that are 10 or above in a paper as numerals (e.g., 10) (Turabian Chapter 23, 23.1.1, p. 318-319).

Zero to 100 - For a text that has less numerical data spell out whole numbers (e.g., ‘one or twenty’ instead of ‘1 or 20’) from zero to one hundred (Turabian Chapter 23, 23.1.1, p. 318-319).

Beginning a Sentence - The word format of a number (e.g., 'Ten' instead of '10') should be used with any numbers that begin a sentence or title/heading (Chapter 23, 23.1.2.1, p. 319).

Expressions

Mathematical Expression - When a number is cited in statistical/mathematical format as a percentage or a fraction, it should be expressed as a numeral (e.g., 16%) (Turabian Chapter 23, 23.1.3, p. 320-321).

Measurement Expression - When a number is stated directly before a unit of measurement it should be expressed as a numeral (e.g., 10mg) (Chapter 23, 23.1.1, p. 319).

Time, Date, Age

Time, Date, and Age - When a number is expressed as a time or date, then it is typically written as a numeral (e.g., 20 years) (Turabian Chapter 23, 23.1.5, p. 322; Turabian Chapter 23, 23.3.1, p. 326).

Time Words - When time is being presented in the increment of an hour, a half-hour, or quarter-hour, present the time in the word format with appropriate hyphenation (e.g., three-thirty) (Turabian Chapter 23, 23.1.5, p. 322).

Punctuation & Roman Numerals

Plural (No Apostrophe) - Whenever numbers are written in plural form they should not contain an apostrophe (e.g., 50s, sixties) (Chapter 23, 23.2.1, p. 324).

Comma Use - Commas are typically inserted for every three digits in a number unless commas do not commonly break up the number, such as in the instance of page numbers, temperature, serial numbers, etc. (Chapter 23, 23.2.2, p. 324-325).

Roman Numerals - If it is the norm to use Roman Numerals within a specific frame (e.g., Super Bowl), then it is best to not change to Arabic Numerals. If, however, it is an instance of seriation used throughout the paper, then Arabic Numerals are more commonly used.

Decimals - Decimals should have a zero before the decimal point if the number is less than one (Chapter 23, 23.1.3, p. 320-321).

Common, Ordinal, & Cardinal

Common and Accepted Numbers - When expressing common fractions and commonly recognized numbers write out the number as a word (e.g., one-sixth, or Ten Commandments) (Chapter 23, 23.1.3, p. 320-321).

Ordinal and Cardinal - Ordinal numbers should be treated the same as cardinal (or standard) numbers (e.g., ordinal - sixth grade, cardinal- six grades) (Chapter 23, 23.1.1, p. 319).