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?
Best Answer
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.