Rules for Writing Numbers

Does your book or article contain numbers? Are you confused about when to use numerals and when to spell out numbers? The rules for writing numbers will vary depending on your purpose and the appropriate style guide used for your writing.

Today we’ll take a look at the best practices according to the Chicago Manual of Style (CMOS), the writing guide for most non-technical writing today.

We’ll focus on some of the basic rules and guidelines and a few exceptions so you can know when to spell out numbers, and when to write them as numerals. Following these simple rules will make your writing look consistent and polished.

Are you ready?
Then let’s begin.

General Rule for Writing Numbers

As a general rule, CMOS recommends spelling out all numbers from zero to one hundred, as well as multiples of hundreds, thousands, and hundred thousands. Hyphens are used for numbers twenty-one through ninety-nine.

Alternatively, you might choose to only spell out zero to nine, using numerals for all numbers consisting of two or more digits.

Consistency is key

Whichever guideline you choose—zero to one hundred or zero to nine—those same numbers would be spelled out, whether they are referring to distances, lengths, areas, centuries, decades, or street names. Even currency and time are spelled out with only a few little twists.

Below are some examples.

Distances

Example: three miles, twenty-two kilometers

Lengths

Example: four feet, twelve meters, seven inches

An exception to the above examples exists for things commonly expressed as numbers.

• If they are used as adjectives, they are hyphenated when the numeral comes first.

• • 60-watt lightbulb

• • 32-inch waistband

• They are not hyphenated if the numeral is after the first adjective.

• • Size 12 jeans

If measurement symbols or abbreviations are used, then use numerals with the symbol.

Example: 3 lbs., 2 L, 55 g, 4’6” (no space), 22.2°C, 72°F

Centuries

Example: Twenty-first century, but 1800s (the nineteenth century)

Note: No apostrophe in 1800s.

Decades

These can be numerals or spelled out.

Example: 1950s and 1960s, or 1950s and 60s, or the fifties and sixties

Note: Again, no apostrophe.

Currency

Whole numbers are spelled out when less than or equal to one hundred.

Example: seventy-five cents or fifteen dollars

Numbers are only used when a range includes numbers less than a whole dollar.

Example: Prices ranged from $0.89 to $10.00.

Time

Time is spelled out when referring to quarter-hour increments.

Example: five o’clock, three fifteen, six thirty, or ten forty-five

Specific minutes require numerals and part of day.

Example: 9:42 a.m. or 7:55 p.m.

To avoid any ambiguity, use noon and midnight when referring to twelve o’clock.

Ages

Numbers are spelled out and ages are hyphenated when used in both noun and adjective terms.

Example: a two-year-old, a fifty-seven-year-old woman, including nine-to-ten-year-olds, a group of five- and six-year-olds

Note: the last example is two specific ages so it uses the hanging hyphen, but the one right before “nine-to-ten” is inclusive, so it all is hyphenated together, just as the numerals would be written, “9–10.”

However, if simply writing someone’s age, you would not include hyphens, unless the number is between twenty-one and ninety-nine.

Example: She is four years old. He is sixty-five years of age.

Percents in Writing

One of the most common errors in writing occurs with percentages. Whenever a percentage is included, the number should be written as a numeral and the word “percent” is used instead of the symbol (%).

Example: 0.2 percent, 9 percent, 17 percent, or 115 percent

However, if you include a range of percents, the %  symbol is used and the range is separated by an en dash.

Example: 20%–25% of the retail price

Note: The en dash is longer than a simple hyphen, but not as long as an em dash. (Using an em dash is discussed here.) In Word, an en dash is formed using Ctrl+Num - (on the numeric keypad) or inserting a symbol/special character. In Google Docs, with many fonts, you can hit the hyphen twice to create the en dash, or go to the Insert tab, Special characters, type in “en dash” and select it.

Fractions in Writing

Fractions are also generally written out and are always hyphenated, whether they are used as a noun, adverb, or adjective.

Example: I live five-eighths of a mile down the road.

Of course, there is an exception here as well. If the fraction is combined with a whole number, it is only spelled out if it is a simple fraction.

Example: three and one-quarter hours

But if it’s a more complicated fraction, it is written as numerals.

Example: I need 6 7/8 yards of fabric.

And if whole and combination fractions are used in the same sentence, all are written as numerals.

Example: She is 4 feet 6 ¼ inches tall.

Note: Whole numbers would be written out, i.e., She is four feet six inches tall. Though, writing “four foot six” or “four six” could also be appropriate as long as the meaning of the numbers is clear in its context. Alternatively, you could write: She is 4’6” tall.

Decimals in Writing

Let’s take a quick look at decimals. Decimals less than one use numerals, include an opening zero, and are treated as plural.

Example: There were 0.43 black cats in the litter. (Note: “were” instead of “is”)

However, the zero is left off decimals when referring to gun calibers.

Example: .44 Magnum

Decimals larger than one use numerals.

Example: 3.4 million

Numbers Beginning a Sentence

Another basic rule for writing numbers at the beginning of a sentence is to always spell them out, whether they are years, quantities, or percentages.

Alternatively, you may choose to rearrange a sentence to avoid having one beginning with a number.

Example: One hundred twenty students were in her graduating class. (Note: No “and”—not one hundred and twenty.) OR Her graduating class included 120 students.

But not: 120 students were in her graduation class.

Plurals of Numbers

The last thing we’ll look at today regarding numbers is plurals. Anytime plurals are referenced, simply add an “s” or “es,” like in writing other words.

No apostrophes are used, since they are associated with ownership or contractions.

Example: We might refer to the terrible twos or people in their twenties.

Our only exception regarding using apostrophes would be if we shortened a specific year, like from 1976 to ’76. (Note the direction of the apostrophe is ’76 not ‘76.)

Don’t forget—the apostrophe is not needed when referring to a decade, like the 70s, but it is required if you are shortening the specific year, like ’76.

I hope you’ve had fun learning about some of the rules and exceptions when including numbers in your writing.

Remember, this is only a brief summary. CMOS includes well over twenty-five pages devoted to various uses and formats of numbers. Today I’ve only addressed some of the most common uses and the most common errors we see as editors.

If you follow these simple rules for writing numbers, you’ll find yourself more confident and breathing easier about using numbers in your writing. Before long, writing with numbers will be a piece of cake (or, perhaps a piece of pi?).

Of course, if you need help with your numbers (and with the words surrounding 'em), consider getting a copyedit! WWM editors will make sure all your numbers are written properly.

Happy writing!

(Love helping writers? Obsess over commas and character arcs? Our Editing 101 course teaches you how to edit both your own book and others. Master developmental, content, and copyediting, and even learn how to give professional feedback, build your editorial business, and work with real clients.)