The more you get right in your manuscript before submitting to editors, the less work they'll have to do on mechanical issues ---- and the more time they can spend on making your writing shine. Plus, learning the conventions of fiction style* can save you money.
*This is Articulate Editing house style, which is based on generally accepted fiction conventions (in the US) and a pinch of personal opinion. Most of what is written here will apply across the board. If a guideline has wiggle room, I'll let you know. And if you're working with a publisher, make sure you follow their specifications, as their house style may differ slightly. (This style guide also applies to creative nonfiction.)
The general rule
This is the general rule for non--technical writing, which includes fiction, of course. Remember that consistency is more important than slavish adherence to these guidelines—and readability is even more important than consistency—so be flexible and use editorial judgement. (For more discussion on numbers in fiction, Carol Saller wrote a great article at CMOS Shop Talk called "Numbers in Creative Writing.")
Spell out numbers zero through one hundred.
Whether you're nine or ninety-nine, you'll enjoy this game.
Hyphenate compound numbers from twenty-one to ninety-nine. All others are open. (Note that numbers over one hundred are usually spelled out in dialogue.)
"Three hundred forty-five people attended my seminar."
"Well, three hundred and fifty people attended mine."*
*While "and" is discouraged in formal writing (preferring "three hundred fifty"), in fiction do what sounds natural, including writing "a hundred" instead of "one hundred."
Use numerals for numbers above one hundred, except round numbers that are followed by hundred, thousand, hundred thousand, million, or billion.
He just won two million smackeroos.
If only I made three hundred thousand dollars a year.
Special cases and exceptions
With fractional quantities over a million, use a mixture of numerals and spelled out numbers.
2.3 billion people
Ordinals follow the general rule (spell them out to one hundred). The letters in non-spelled-out ordinals shouldn't appear as superscripts.
This is the thirty-second time I brought the donuts this year!
Don't start a sentence with a numeral. When a number starts a sentence, spell it out.
Or better yet, recast. Especially if it's a complicated number to spell out. With dates, adding "the year" before the date is an easy fix.
1969 was a tumultuous year. X
The year 2000 was a turning point for his business. ✓
But ... there are times in fiction dialogue when this rule can be ignored.
"3D movies cost so much more than regular movies."
"7-Eleven's my go-to convenience store."
Maintain consistency in categories of numbers. (Except for numbers at the beginning of a sentence.) It's okay to have both numerals and spelled-out numbers in the same sentence/paragraph as long as categories are consistent. (And it's okay to treat numbers differently in dialogue and narrative.)
Forty-three members tweeted a total of 5,324 times.
One town had a population of 3,500, and the other two towns had a combined population of only 800.
Avoid dense clusters of spelled-out numbers. Exceptions to the general rule can be made for the sake of readability.
With side-by-side numbers where both would normally be spelled out, spell out one and write the other as a numeral.
They needed four 8-foot pieces of trim.
"Pick up 3 two-by-fours, will you?"
You can depart from the general rule for numbers that are commonly expressed as numerals. This requires judgement. If something feels more natural expressed in numerals, it's likely the best approach. (This is less true for dialogue.)
A 60-watt lightbulb is too bright for this room.
She tried on the size 8 dress, but it was too small.
His new car gets 40 miles per gallon.
I was assigned to room 29
Channel 5 news
I live at 236 East Sixtieth Street
Spell out numbers in dialogue unless it would be awkward to do so, even with numbers that would normally be expressed as numerals in the narrative. Spell out abbreviations and symbols as well. For more on numbers in dialogue, including the exceptions to this rule, see my Fiction Style Guide: Dialogue.
Physical dimensions such as distance or length are usually treated according to the general rule.
I printed an eight-by-twelve photo.
It was 383 centimeters, to be exact.
Cautious, she stuck to fifty-five miles an hour.
Heights are spelled out a number of different ways.
five foot two
five feet two inches
she was five two
In general, use words rather than symbols or abbreviations in narrative and dialogue.
Spell out degree, percent, and number instead of using the symbols. Use the word "dollar" instead of the dollar sign ($), unless using very specific amounts ($105.73).
Spell out simple fractions and hyphenate them in noun, adjective, and adverb form.
a two-thirds vote
over by three-eighths of an inch
Express percentages in numerals with the word "percent" rather than the percent symbol (%).
The jeans were 25 percent off.
Use an en dash, not a hyphen, for number ranges.
Here are the records for 1995–2005.
Don't use the en dash for a range that follows "from" or "between." Instead, use "to," "through," or "until" with "from" and "and" with "between."
I worked there from 1995 to 2005.
I worked there between 1995 and 2005.
Times of day in even, half, and quarter hours are usually spelled out.
I left for work at quarter to six, but I didn't arrive until seven thirty.
Use numerals when emphasizing a specific time (except in dialogue).
He informed us that he expected our arrival at 11:05 on the dot.
At precisely 8:37 that evening, her world fell apart.
When you want to include a.m. or p.m., use numerals. Use lowercase letters with periods or small caps without periods for a.m. and p.m. Don't add a period if a.m. or p.m. falls at the end of a sentence, but following a.m. and p.m. with other punctuation is fine.
I can't believe I need to wake up at 5 a.m.
Wake at 5 a.m.? Are you kidding me?
I'd love to start at 5 a.m., but it's a little early for Joe.
When you use "o'clock," you must spell out the time.
At nine o'clock all hell broke loose.
Don't use hyphens to connect hours and minutes.
We need to leave by three-fifteen. X
At eleven forty-five, the fun begins. ✓
Spell out (and lowercase) centuries.
the eighteenth century
Decades can either be spelled out (and lowercased) or expressed in numerals.
Make sure the apostrophe before the decade is facing left when using "smart" quotes (Word will automatically make it face right, thinking it's an opening single quotation mark, so you can cut and paste one from elsewhere).
If the year is included, dates should be written in numerals.
September 23, 1989
But dates without years can be written a number of ways.
the twenty-third of September
Don't use ordinals, either spelled out or in numerals, with months and years. The reader may think "September twenty-third" when they see "September 23," but it must still be written as "September 23."
September 23rd, 1989 X
September twenty-third X
An exception is made for dialogue.
"Is the party on September twenty-third?" ✓
Money tends to follow the general rule, but as with all numbers in fiction, do whatever is the most readable and makes the most sense for your book.
If the amount is straightforward, spell it out.
That guy owes me two dollars and fifty cents.
If the amount is specific and complicated, use numerals and the dollar sign.
It cost $2,367.43 to be exact.
Don't hyphenate dollar amounts except for compound numbers from twenty-one to ninety-nine.
one thousand twenty dollars
one thousand twenty-two dollars
Write the names of guns the way the manufacturer would. Capitalize the names and use numerals.
9mm Winchester Magnum
In dialogue, spell out millimeter. Spelling out numbers is okay for casual references.
"Grab that twenty-two, would you?"
"I use a nine millimeter."
The zero before a decimal point is omitted when referring to guns.