When To Spell Out Numbers Using CMOS

A frequent question from writers is how to show numbers in their manuscript. Should the number be spelled out with letters or should digits be used? As with just about everything related to books, The eighteenth edition of The Chicago Manual of Style (CMOS) offers guidance on many different kinds of numbers. Other style manuals have their own guidelines, but this post will focus solely on CMOS.

The general rule is that numbers under 101 are written out, like twenty or seventy-four, while numbers greater than one hundred are shown in digits, like 122 or 748.

There are, however, many exceptions. Scientific, financial, and other documents containing data usually show their numbers in digits. Digits are also used with abbreviations of measurement, as in 3 cm or 8 lbs., and with symbols, such as 10% or $50. And digits are used in places you might expect, such as in floor numbers, telephone numbers, and complex numbers.

Round numbers in hundreds, thousands, millions, and billions are written out (e.g., eight hundred people, four million dollars), with the exception of scientific writing. When a sentence starts with a number, it is almost always written out.

Numbers that represent the time of day depend on how they are used and the level of specificity. For example, a time at a quarter hour is usually, but not always, represented in text while times that are between quarter hours are usually shown as digits. If a time is used with the word “o’clock,” the time is always written in text.

CMOS also advises consistency, yet flexibility, with how numbers are presented. If there are several types of numbers in the same sentence, you might consider recasting the sentence so they can all be written as text or all be shown as digits.

Almost every rule regarding numbers in CMOS comes with an exception, so if you have any numbers in your manuscript, you will want to review CMOS 9–9.69 to dive into the details.