When to Write out Numbers—Using The Chicago Manual of Style—As Explained by a Copyeditor
Trying to figure out when to spell out numbers vs. when to use numerals can give any nonfiction author a headache, am I right?
If you’re not confused, you’re probably clueless.
And you’ve probably introduced inconsistency and amateurism into the manuscript you have such high hopes for.
If you’re wondering about the rules for spelling out numbers, and you’re an American nonfiction author writing in American English, read ahead.
As an experienced copyeditor for hire, each year I help dozens of authors polish up their book manuscripts and get them ready for publishing. I help authors put their best work out there.
One of the things I do all the time is encounter (and resolve) has to do with spelling out numbers in writing.
Let’s talk about numerals vs. words.
Here’s what The Chicago Manual of Style (and a nonfiction copyeditor) has to say.
The Chicago Manual of Style is the industry standard for most nonfiction authors. I’ll quote below from the 17th edition—from the section “Numerals versus Words: General Principles,” in chapter 9, “Numbers.”
You can get the new 18th edition of The Chicago Manual here; its stipulations on numbers are essentially unchanged.
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Rules for Spelling out Numbers
Chicago’s general rule: “Chicago advises spelling out whole numbers from zero through one hundred and certain round multiples of those numbers.” Here’s an example: Twelve houses sold in my neighborhood last year.
Any numbers followed by hundred, thousand, or hundred thousand are usually spelled out: a period of one thousand years. Some forty-seven thousand people attended the fair.
Chicago’s alternate rule calls for “spelling out only single-digit numbers and using numerals for all others” (with exceptions, below). This is more often applied “in scientific or journalistic contexts.” I recommend the alternate rule only to authors who have many numbers in their books, who are or have been journalists, and who are writing books in science, sports, or personal finance and investing. Particular centuries are always spelled out and lowercased, even if you’re using the alternate rule: the nineteenth-century.
Never begin a sentence with a non-spelled-out number. Always write out the number or recast the sentence: Some 1,300 children visited the zoo yesterday. While you’re at it, avoid beginning a sentence with a year.
Chicago style says to “avoid a thickly clustered group of spelled-out numbers. . . . If, for example, a book includes many mentions of ages, all ages might be given as numerals.” My advice: if you have three or more numbers in close proximity, go with the digits—the numerals, not the spelled-out version.
“If . . . you must use numbers for one of the numbers in a given category, use them for all in that category.” Here’s an example: Between 350 and 400 people attended the event. (Ordinarily four hundred would be spelled out.) Here’s another example: At 1 hour and 29 minutes, this movie is much shorter than other Tom Hanks films.” (Notice hours and minutes are two different categories, and the sentence doesn’t begin with a numeral.)
“In the same sentence or paragraph, however, items in one category may be given as numbers and items in another spelled out.” Example: He biked 3 miles, 17 miles, and 26 miles in three-, four-, and five-hour sessions.
“If, for example, a book includes many mentions of ages, all ages might be given as numerals.” (This might make sense for a family history or genealogy book.) Key word: might, not must.
“The letters in ordinal numbers should not appear as superscripts” (e.g., 122nd, written like this. The nd is not small and up higher. It's the 33rd Congress, and the 101st Airborne. You can, as I do, turn off the feature in Microsoft Word that automatically superscripts your ordinals.
In a phrase like “to the nth degree,” n is italicized. And of course, th is not typed in superscript.
Some bonus tips:
Convert Roman numerals (like the ones here) to Arabic numerals, regular digits—except when the preface or front matter of a book is involved. In other words, write volume numbers in digits.
Don’t spell out numbers (and do spell out “percent”) when expressing a percentage: More than 38 percent of VIP guests spend more than $5,000 annually. You wouldn’t write “38%” or “thirty-eight percent” in a sentence.
“Decades are either expressed in numerals or spelled out (as long as the century is clear) and lower cased. Chicago calls for no apostrophe to appear between the year and the s.” Both of the following are correct: the 1960s and 1970s and the ’60s and ’70s. Make sure to use a “right apostrophe” (opening away from the number)! Also correct: the sixties and seventies. My advice on this? Pick a convention and stick to it. I prefer the first option.
Large numbers with “millions” or “billions in them are written out based on the above rules, i.e., fourteen million, 5.6 billion, 143 trillion.
Now that you know the “General Principles” for spelling out vs. using numerals, according to The Chicago Manual of Style . . .
you’ll be able to apply what you’ve learned to your general nonfiction and scholarly works in the humanities and the social sciences, or even in scientific and more number-heavy writing.
And hey, it wouldn’t hurt to buy a copy of The Chicago Manual, 18th Edition!
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Happy writing,