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Source: https://writers.stackexchange.com/a/46267
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

Source: https://writers.stackexchange.com/a/46267
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

While you don't need to follow a style guide here (except your publisher's of course), it's helpful to look at them. The AP Style Guide (Associated Press) is a good one because it's for American newspapers. Newspapers work hard to bring in a large range of readers, so they [aim most of their articles at a high school](https://readable.com/blog/why-is-newspaper-readability-important/) (or even Jr. High school) reading level (teenagers and a few pre-teens).

[AP Style for numbers](http://www.gatehousenewsroom.com/2015/08/12/ap-style-numbers-age-score-year-ratios/):

- Spell out zero through nine, and use numerals for 10 and above (for both cardinal (one, 19) and ordinal (first, 19th).
- But use numerals when it's the name of a place or entity (1st Ward).
- Spell out numbers if they begin a sentence, except for years.
- Spell out casual use of numbers ("a thousand times").

Some style guides differ and you can make some decisions yourself. For example, [Chicago Manual of Style](https://www.chicagomanualofstyle.org/16/ch09/ch09_toc.html) says to spell out numbers under 100, with an alternate rule of doing so only for numbers under 10.

Your publisher will edit as needed. You may also wish to check out some books by the publishers you're likely to submit to and see how they do it.

General rules though are, always use numerals for the following:

- Dates (some formal documents like contracts and wedding invitations might spell them out, but it's hard to read). You can spell out months. So: June 26, 2019 or 19 Jan 1994.
- Phone numbers.
- Street addresses (both the house numbers and the number of the street if the name is a number) So: 345 17th Street.
- Zip codes/postal codes and anything else from an address.
- Social security and ID numbers. Credit card numbers. And so forth.
- Times. 7:26 AM will always be in numerals. But you can say things like: a quarter to three.
- Math and science formulas and notations.

You want to keep the line between overly formal and overly casual. A good way to do that is to follow the basic existing rules for popular writing.

Original score: 6