I see number formatting errors and inconsistencies all the time in my work as a submissions reader. And not to throw shade on anyone, but most of these manuscripts have supposedly already been professionally edited, indicating that numbers are an issue that professional editors (myself included) struggle with. Because of this, I always recommend that authors make a special point of discussing number formatting with their editor. (And here I will throw some shade: if your editor tells you that she or he doesn’t use a style guide, fire them and hire someone else, because real editors are obsessive looker-uppers.)
For fiction writer—particularly novelists—I strongly recommend using The Chicago Manual of Style because it’s the most commonly used style guide in the U.S. fiction publishing industry. But whatever style guide you use, you must be methodical in applying it because readers are sensitive to inconsistencies.
Pro tip: I use Microsoft Word’s Ctrl+f feature to look at every numeral and spelled-out number in every manuscript I copy edit to make sure that the numbers are formatted correctly. It’s not as time-consuming as it sounds because you only have to Ctrl+f the Arabic numerals 0 through 9, the spelled-out numbers zero through nine to catch every case in the book. (Roman numerals are trickier to catch using Ctrl+F, but that’s a pro tip for another day.)
Anyway, here are the number-related issues I see most often:
Important: always spell out numbers when they’re the first word in a sentence.
Times of day
Depending on the context, there are several ways to format time of day:
11:00 a.m. This is the preferred format when to-the-minute precision is relevant to the story. If to-the-minute precision isn’t important to the context, then 11 a.m. is preferred. Note that a.m./p.m. is lower case and is separated from the numeral by a space.
Formatting the a.m./p.m. as am/pm, A.M./ P.M., and AM/PM is acceptable, but be consistent.
It’s acceptable to not use the a.m./p.m. in cases where morning versus evening has been established by other means. Example: “The flimsy blinds did little to keep the morning sun from shining in Ahmed’s face. He squinted at the alarm clock: 6:29. He still had another hour of tossing and turning before he had to get up for his shift.”
Eleven o’clock. Preferred format for full-, quarter-, and half-hour times, except in cases where the story context (say, a police report detailing a timeline of criminal activities) demands precision.
1100 (as in, “eleven hundred hours”). This comes up a lot in military contexts. Note that there is no punctuation between the hours and minutes.
Do not use ordinals (1st, 2nd, 3rd, 4th, etc.) in dates. So, “July 3rd, 2017” and “July 3rd” are incorrect and should be formatted “July 3, 2017” and “July 3” respectively.
Holidays and significant dates:
The Fourth of July is the proper name of a holiday, and so it should almost always be formatted in title case. In contrast, “December Twenty-Fifth” is a holiday, but it’s not the name of the holiday, and so it should be formatted “December 25”.
New Year’s Day and New Year’s are the correct formatting in the context of the holiday. However, in the generic context of “she hoped the new year would be better than the one that was rapidly ending”, “new year” is not treated as a proper noun. (The whole “Happy New Year” vs. “Happy New Year’s” debate is a matter I don’t have the strength to deal with today.)
“September 11” and “9/11” are correct, but “September Eleventh” and “Nine Eleven” are not.
It is usually appropriate to write years in numerical form: 2021. However:
On their own, spring, summer, fall, and winter are usually not treated as proper nouns.
When to use Arabic numerals versus spelling out
The CMoS method of using numbers in writing is to spell out whole numbers up to and including one hundred, and to use Arabic numerals for numbers above one hundred.
Some authors choose to spell out all of their numbers, which is acceptable so long as doing so doesn’t impede the ease of reading. This is not a practical method if your manuscript is number-heavy, though.
Whole numbers in the hundreds, thousands, hundred thousands, millions, billions, etc. are spelled out in most cases.
While it's ultimately the author’s choice, it’s best to avoid using Arabic numerals in dialogue. But if the numbers are too long, or otherwise excessively complicated to spell out, then don’t sweat the occasional Arabic numeral in dialogue.
Ordinal numbers (1st, 2nd, 3rd, etc.)
Simple fractions should be spelled out whenever practical, but use numerals in cases where spelling them out impedes the reading flow, or where needing to switch between numerals and spelled-out numbers several times in a short section of text is impractical:
“Back in 1911, I had to walk ten and a half miles through the snow to get to school.”
“Shaquila had eaten two-thirds of the sandwich before she realized that Frank’s secret ingredient was crab meat. She dropped the remainder on the plate and scrambled for her EpiPen.”
Compound fractions that don’t directly modify a noun only take a hyphen in the fraction: “She watched the same two and three-quarters movies during the flight that she always did every time she flew to St. Louis. Would she ever find out if Julia Roberts married Richard Gere? She decided she liked not knowing for sure.”
“Lisette remembered with clarity that the deck was 5 ¾′ by 11 ½′: after all, she was the one who’d proposed that the condo board require all decks to meet those exact dimensions. But it was also 9 feet above the ground, and even jumping from the seat of the lawn chair, at 5 foot 1, she wasn’t tall enough to grab hold of it.”
Note that the symbols denoting inches and feet are primes rather than apostrophes: ′ and ″, as opposed to ' and ”.
Note that there is a space between the whole number and the fraction.
Note that the fraction is formatted as a single character (i.e. ¾) rather than as separate characters (i.e. 3/4).