When does one write a number as words and when as digits?

I think I heard somewhere a rule that says "numbers up to twelve should be spelled out, numbers above can be written numerically" but not only do I not remember if that was exactly how the rule went, I'm fairly sure the source wasn't really reliable.

So, can you tell me whether - in a common text, not legalese, not calculations, nothing unusual - I should use digits or letters? Is it true you should write "eleven" but "13" in a literary text?

Also, I'm pretty sure that, even if it's true, there must be exceptions. If I'm right, can you name them?

For this question, I'd go to The Chicago Manual of Style. Their general rule is, for non-technical contexts, "spell[] out whole numbers from zero through one hundred and certain round multiples of those numbers."

Exceptions using digits:

• dates: 8 January 2012

• numbers referring to parts of a book: table 23 on page 4; chapter 11

• volumes, distances, other physical quantities: 15 kilometers; 6 cubic feet; 3 tonnes; 10 degrees of arc; ... (but if the context is non-technical, words can be used)

Exceptions using words:

• when the value is approximate: seventy-five million

• at the beginning of a sentence: Two hundred fifty visitors ... (but sometimes it's advisable to rephrase, such as if a year starts a sentence)

The actual list in Chicago is much longer and more complicated. Chicago also recognizes the practice of using digits rather than words for any value ten or greater. Usage is really rather flexible -- so consider the context you're working in, and try to be consistent.

• The British tabloid newspaper the Sun, owned by Rupert Murdoch, was famous (or notorious) at one time for its daily full-page pictures of topless young women. They always appeared at the same place in the paper, and they became known as 'page three girls'. Commented Jul 28, 2023 at 9:17

There is a nice article 10 Rules for Writing Numbers and Numerals

The main rules:

• Spell small numbers out. (My rule: Up to twelve - thirteen is a composed word and long)
• Don’t start a sentence with a numeral.
• Centuries and decades should be spelled out.
• If the number is rounded or estimated, spell it out

Form another source

• Be consistent within a category.
• Keep in mind this is only 1 of who knows how many sources. Commented Jan 24, 2013 at 22:22

I've heard different numbers from multiple sources on this matter. I've seen cutoffs of 10 and 20 and 100. I'm not sure if there is a fixed standard.