The “rules” about how to style numbers still trip up veteran writers (and editors). There are a lot of rules, and numbers is one of the topics that style guides often have very different ways of handling, which adds to the confusion.


In the US book-publishing world, we use the Chicago Manual of Style to determine how to treat numbers in text, whether fiction or nonfiction.

Here are the basics, although inevitably, there will be an exception. Also remember, sometimes there are nuances to the rules that may not be mentioned here, and a few items not covered, such as addresses and percentages.

The Basics:

This is usually referred to as the “general rule.” Write numbers zero through one hundred as words. Numbers twenty-one to ninety-nine should be hyphenated. Numbers larger than one hundred are written as numerals.

I walked seven miles on the trail, although it felt more like ninety-two. Maybe even 192.

     The exception: If you have two numbers that are typically treated differently in the same sentence, choose one style for both.

I walked seven miles on the trail, although it felt more like a hundred ninety-two.
or
I walked 7 miles on the trail, although it felt more like 192.


Larger Numbers:

Whole numbers followed by hundred, thousand, or hundred thousand are spelled out. Whole numbers, followed by million and billion, are also spelled out.

At least two hundred thousand people protested worldwide. Some say that number is much too low, though, and the real count was closer to 2 million.

Dates:

Years are written as numerals. Dates are also written as numerals unless in dialogue.

I was born May 14, 1977. 
“Are we celebrating my birthday on May fourteenth this year, or will we wait for the weekend?” 


Time:

Time of day is usually spelled out unless an exact time is being emphasized, in which case use numerals. Lowercase a.m. and p.m.

My day started at six. I had hoped to sleep in until seven thirty. No such luck.
Actually, it was 6:08 a.m. before I crawled out of bed.


Money (US currency):

Follow the general rule. Amounts of one hundred or less are spelled out. But for amounts greater than one hundred dollars, write as numerals. For amounts of a million or more, write as a mixture of numerals and spelled out numbers.

I have thirty-nine dollars in spare change.
They made $142 at the yard sale.
I heard tonight’s lottery was up to $5 million. Better buy a ticket!


Measurements:

Measurements usually follow the general rule, although there is more leeway here to use numbers rather than words with the unit of measure.

The water’s temperature was a chilly sixty-five degrees.
The Appalachian Trail is 2,190 miles long.


In dialogue:

Most numbers should be spelled out in dialogue.

     The exceptions: Phone numbers, including emergency numbers, like 911, and addresses are written in numeral form, even in dialogue. And most everyone agrees that 24/7 should be styled as numerals, too.

“He collected seven hundred twelve sea shells on our last trip.”
“My numbers 867-5309. Give me a call.”


Consistency:

The most important thing to remember is consistency. Whatever style choices you make with numbers, follow them all the way through your manuscript.

CMOS Suggestion:

Avoid starting sentences with numbers when possible.

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