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Is it July Fourth? Or is it July 4^th? Is it The Roaring Twenties? Or is it The Roaring 20s? Is your apartment on the first or the 1^st floor? Am I thirty-one or 31? When should I spell out a number and when should I use the numerals?

I am sure that questions such as these must often keep you up at night, and while I’d like to tell you I can put your mind at ease by explaining the different rules, I can’t. The rules governing how numbers should be formatted vary wildly and are based on discipline, intended audience, typographical appearance, and context.

The following will include some of the rules used to decide whether to spell out numbers or use the numerical equivalents. I tend to take my style advice from the Chicago Manual of Style, 16th ed. because I know it best and it fits well with 99% of what I write. I will, however, include alternate rules from other style guides to showcase notable discrepancies. The rules (and the theories behind those rules) differ from style guide to style guide. Unfortunately, I have no way of knowing all the permutations of these rules (and you wouldn’t want to read all that anyway.) This article should, at least, make you aware of how you format numbers in your writing, and allow you to make style decisions of your own that best fit what you are writing and for whom.

A General Rule (3 ways)

Numbers from zero (0) to one hundred (100) should be spelled out.

This is the Chicago Manual of Style’s preference. Considering the CMOS is geared mostly toward writers of the humanities, social sciences, literature, and scholarly works, the tendency is to use words instead of numerals. That said, the rule is meant to be practical, as numbers above one hundred tend to be long when you spell them out, and page space is always at a premium. Here are a few examples:

• My brother just found a 450-year-old spear head in the backyard.
• I’ll have one coffee and fourteen packets of sugar.

An alternative to the General Rule: Numbers from zero (0) to nine (9) should be spelled out.

While the CMOS tends to be word-happy, there are other style guides that insist that only numbers zero through nine should be spelled out while all numbers 10 and up should appear in numerical form. 

Not long ago, my dad turned me on to the Government Printing Office’s Style Guide. He stumbled on it himself while doing a little research about his dad (my grandfather) who worked as a typesetter in the Government Printing Office from 1938 to 1975. The GPO produces all material for the U.S. Government, and it’s all public domain and free. Because of this, they have a much different approach to style than the CMOS.

The GPO has the opposite style tendency than the CMOS. According to the first paragraph in the chapter on numerals, the GPO prefers numerals over spelled out numbers because “the reader comprehends numerals more readily than numerical word expressions.” 

The GPO Style Guide prefers to use numerals for numbers 10 and above with the exception of a few particular cases, many of them only based on typographical appearance.

The CMOS does list the zero-to-nine rule as an alternate baseline used by journalistic and scientific writing. It notes that “Most of the exceptions to the general rule also apply to this alternative rule.”

Yet another alternative to the General Rule: Numbers that only require a single word should be spelled out.

Another way you can decide between writing out a number and using the numeral is to only spell out numbers that require one word. Using this rule, numbers one through twenty would be spelled out, but 21 would require a numeral. Thirty, forty, fifty, sixty, seventy, eighty, and ninety would all be spelled out, but all the numbers in between those plus 100 would use numerals.

• I have thirty pairs of socks.
• I have 89 jelly beans.

This rule comes from the Little Brown Compact Handbook, Fifth edition which is directed toward college students. It also advises that students follow the style preferences of their field of study—thus English majors may write out all numbers under 101, while Statistics majors would tend to only write out numbers under 10. There were few other rules listed in the LBCH, so the assumption, it seemed, was that writers ought best to look to their disciplines for more exacting rules.

The Exceptions

No matter which general rule you follow, there will be exceptions. These are a few of those specific contexts in which a numeral or a spelled out number would be preferred. When possible, I have noted the difference between the preferences of the Chicago Manual of Style (CMOS) and the Government Printing Office (GPO) style guide.

When a number starts a sentence, spell it out.

Most style guides agree that numbers that begin a sentence should be spelled out.

• Twenty-five years ago, my parents moved to Alaska.
• Nine out of 10 people agree that the show Survivor has run its course and should be cancelled.

However, large numbers (such as years) can be spelled out, but the preference is to reword the sentence or add words like the and a to avoid starting with a large number written out.

You can say:

• Nineteen eighty-one marked both the birth of MTV and of me.

But it would be better to say:

• The year 1981 marked both the birth of MTV and of me.   – OR -  Both MTV and I were born in 1981.

You can say:

• Two hundred thirteen people attended the city council meeting.

But it would be better to say:

• In all, 213 people attended the city council meeting.

Round multiples of numbers above one hundred can be spelled out

When writing hundreds, thousands, or hundred thousands:

The CMOS allows for whole numbers in the hundreds, thousands, and hundred thousands to be "spelled out (except in the sciences)—whether used exactly or as approximations.”

• The new musical drew a crowd of over three hundred people on its opening night.
• This temple was built more than two thousand years ago.
• The book sold five hundred thousand copies.

However, whole numbers over 101 that are not round are expressed with numerals.

• At the state fair, Sally guessed correctly that there were 3,345 jelly beans in the jar.
• The college enrolls 32,150 students.

And when writing millions, billions, trillions, etc.

CMOS suggests that the whole numbers that precede the words million, billion, trillion, etc. follow the same zero to one hundred (or zero to nine) general rule with the rest of the number expressed as a word. For example:

• The state had over one million voters last year.
• Scientists estimate the space rock is over 350 million years old.

CMOS also says that numbers that refer to fractional amounts in the millions, billions, trillions, etc. can be expressed with a mixture of numerals and words. Sometimes, though, a number that would generally follow the general rule is written as a numeral for the sake of consistency if both numbers are referring to the same thing.

To express a fractional number in the millions, use numerals:

• The city’s population is 2.3 million people.

To express a large number that falls under the general rule, use words:

• The city’s population is over two million people.

If two numbers appear in the sentence, and one is fractional, use numerals:

• The city’s population is 2 million people, and the state’s population is 3.1 million.

If there are two numbers and one is above 10 (or 100) use numerals for both…

For instance, if there are two numbers in a sentence but only one of them is 10 or up, both should be numerals. (Unless, of course, one of the numbers starts the sentence—as we learned already, that number should be spelled out.

• I have three siblings and 13 cousins. – INCORRECT
• I have 3 siblings and 13 cousins. – CORRECT
• I have three siblings and six cousins. - CORRECT

The “3” must be represented by a numeral because “13” is also in the sentence and must be represented by a numeral to allow for visual consistency.

…unless the second number is a unit of measurement, time, or money.

Time, money, and units of measurement (like miles, inches, millimeters, liters, etc.) are most often expressed as numerals, so you can mix the format of numbers in a single sentence if one of them is a time, money, or measurement.

• I have three ice cream cones and three dollars. ­- INCORRECT
• I have three ice cream cones and $3. – CORRECT

In fact, there a several types of measurements and time expressions that have specific rules about when to use numerals and when to use words.

Chicago says that in general (or nontechnical) contexts, "physical quantities such as distances, lengths, areas, and so on are usually treated according to the general rule."

So whether you follow the zero to one hundred or zero to nine rule, physical quantities such as heights, speeds, dimensions, degrees, etc. can be spelled out for numbers under 101 (or under 10).

• The blue car was only going five miles an hour on the freeway.
• Can you please make me four three-by-five copies of this photo?
• The temperature of the water dropped two degrees when we added the pasta.

CMOS adds that

It is occasionally acceptable to depart from the general rule for certain types of quantities that are commonly (or more conveniently) expressed as numerals; such a departure, subject to editorial discretion, must be consistently applied for like quantities across a work.

Here a few examples of such scenarios. Note that CMOS always uses numerals to express percentages.

• This lamp needs a 60-watt bulb.
• She wears a size 8 shoe.
• My old truck barely gets 8 miles per gallon.
• There is a 3 percent increase in our sales after lunch.

The GPO has some very specific rules regarding measurements, time, and quantities.

Measurements, percentages, proportions, game scores, market quotations, and serial numbers (such as page, chapter, line, phone, paragraph, and document numbers) are best expressed numerically (especially if they include decimals.)

• The property is 3 square acres.
• The recipe calls for 1 gallon of milk and two tablespoons of sugar.
• Each employee received a 3.4 percent salary increase.
• Our patrons seem to prefer the chocolate cake over the vanilla pudding at a rate of 2 to 1.
• The Mariners finally won the game with a score of 4 to 3.
• The news reported a Dow Jones average of 12005.6.
• Please read chapter 9, pages 24 to 30.
• To apply for the grant, you will need to fill out Form 45.

Degrees are expressed with numerals unless they are not referring to a specific unit of measurement:

So,

• The rocket shot up at an 80 degree angle.  – OR – The rocket shot up at an 80˚ angle.

But,

• When I got home late, my roommate gave me the third degree about my whereabouts.

Age:

The GPO expressly notes that age should be written in numerals, but I was not able to find a rule that specifically noted the same in the CMOS. However, ages under 101 expressed in other sections of the book were spelled out, so I assume they apply the general rule to ages also.

GPO:

• The 6-year-old rode his bike into the pool and emerged from the water smiling.

CMOS:

• The six-year-old rode his bike into the pool and emerged from the water smiling.

In the GPO, time (clock and otherwise), dates, and years are expressed with numerals generally. In the CMOS, they are expressed with words generally.

CLOCK TIME

GPO:

• The picnic starts at 4:30 p.m.
• The picnic starts at half past 4.
• The picnic starts at 4 o’clock. (Note: not 4 o’clock p.m.)

CMOS:

• The picnic starts at four thirty in the afternoon.
• The picnic starts at half past four.
• The picnic starts at four o’clock.

Exact times, however, are expressed with numerals:

• The flight leaves at 8:14 a.m.
• Please attend the meeting on June 1 at 5:30 p.m. (PST).
• Susan ran the race in 2 hours, 57 minutes, and 2.3 seconds.

TIME

GPO:

• You have 5 minutes to put down that toy and get over here.
• It’s been 8 days since I last saw Fluffy.

GPO allows words for expressions that are not exact time frames.

• It’s been four decades since I visited my hometown.

CMOS:

• You have five minutes to put down that toy and get over here.
• It’s been eight days since I last saw Fluffy.
• It’s been four decades since I visited my hometown.

DATES

Both GPO and CMOS advise that dates are written with numerals.

• My grandmother was born May 15, 1920.
• We are having a party for her on May 1^st. (if the date is specific)
• We are having a party for her in the first part of May. (if the date is not specific or not set.)
• Happy Fourth of July! (the holiday)
• Your library books are due on the 4^th of July. (referring to the date)

CMOS, however, does allow dates that are not mentioned with the month or year to be spelled out.

• I wanted to mail this to you by mid-month, but it’s the thirtieth and it’s still sitting on my table.

YEARS

Both the GPO and CMOS say that years should be expressed as numerals.

• The electric trolley ran from 1901 to 1938.
• The mummy dates to 650 B.C. (after for B.C./B.C.E./C.E.)
• This house was built in A.D. 1880.  (Before the year for A.D.)

Additionally, the CMOS prefers that centuries are spelled out.

• This fresco was painted in the sixteenth century.
• I am really glad I was not born in the seventeen hundreds.

When referring to decades, there are options. You can spell it out, or write it as a numeral made plural with an s at the end. Note the use of the apostrophe on the last example to show the omission of the “19.”

• I love the nineteen nineties.
• I love the nineties.
•  I love the 1990s.
• I love the ‘90s.

In both the GPO and CMOS style guides, money is usually written with numerals unless it refers to a physical amount of money item(like a coin or bill). See the third and sixth examples.

• I have 5 cents.
• I have $0.05.
• I have five pennies.
• I have 10 dollars.
• I have $10.
• I have two fives.

A few other things to remember

Ordinals

Ordinal numbers are numbers that express order. First (1^st), second (2^nd), etc. In general, ordinals follows the rules set forth by the general rule (either the under 101 rule or the under 10 rule), and they also follow most of the exceptions talked about above.

Plurals

CMOS offers this rule for writing numbers as plurals. When spelled out, form plurals just like other nouns:

• Most engineers at the company were in their sixties.

When using numerals, add an s to the end, but don’t use an apostrophe:

• Plaid shirts were ubiquitous in the alternative music community of the 1990s.
• I bowled two 185s and three 300s.

Those are a lot of rules. How do I remember them all?

You may have noticed that many of the exceptions allowed, regardless of the style guide, were made in the name of appearance, consistency, and/or clarity. Spelling out a number at the beginning of a sentence looks nice, but spelling out a large three-, four-, five-, six- digit or higher number would be hard to read, so numerals are used. When many numbers appear in a sentence or paragraph, format like numbers alike to maintain consistency. Here is an explanation from the Chicago Manual of Style, 16^th ed.

If according to rule you must use numerals for one of the numbers in a given category, use them for all in that category. In the same sentence or paragraph, however, items in one category may be given as numerals and items in another spelled out. In the first example, the numerals 50, 3, and 4 would normally be spelled out (see [section] 9.2); in the second and third examples, 30,000 and 2,000, respectively, would normally be spelled out (see [section] 9.4; see also [section] 9.8). For numerals in direct discourse, see [section] 13.42.

A mixture of buildings—one of 103 stories, five of more than 50, and a dozen of only 3 or 4—has been suggested for the area.

In the second half of the nineteenth century, Chicago’s population exploded, from just under 30,000 in 1850 to nearly 1.7 million by 1900.

Between 1,950 and 2,000 people attended the concert.

To avoid a thickly clustered group of spelled-out numbers, numerals may be used instead in exception to the general rule.

When in doubt…

Look it up. As you can see, rules vary depending on which style guide you are following. And if you are not following a particular guide (say while writing a short story), create a sort of style guide for yourself so that you can maintain consistency throughout your writing. If you spell out numbers under 101, under 21, or under 10, make sure to be consistent. I also was not able to get into all the nit-picky details of formatting every kind of number under the sun, so you should be aware there are yet rules on formatting mathematical expressions, pope numbers, divisions of the army and navy, government groups, etc. etc. etc. Good luck, and if you want to share what you know, write it in the comments.

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