When writing is it correct to use numerals (e.g. 1,2,3,) or spell out the number (e.g. one,two,three)?

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    There is no general rule in English. This is governed by style (essentially typography), not orthography. The only time you have to be careful about it is formal publication (i.e. not a blog but a periodical or book), and in that case, you should investigate, determine, and adhere to whatever style guidelines govern that publication (eg the Chicago Manual of Style, AP, etc). The editors will help you ensure you're playing by the rules. – Dan Bron Mar 6 '16 at 0:01
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    To reiterate what @DanBron writes, it's not even that there are fixed formal rules that you need not apply in the vast majority of circumstances: even when there are rules they will entirely differ from place to place, and only really exist for the sake of consistency within a publication and to avoid arguments. – Dan Sheppard Mar 6 '16 at 0:58
  • My own custom is to spell out one-word numbers, which are easier to parse mentally, and to use the numerals for two-word and larger numbers; i.e., to spell one to twenty, thirty, forty, and so on, but to use numerals for 21, 22, 23, 57, 101, and so on. – Anonym Mar 6 '16 at 4:33

While there are indeed no concrete rules for general usage, I would argue that the general rule is to spell out a numeral by default. Consider the following sentence as an example of sloppy usage:

He had 3 dollars.

It might be technically correct, but it looks incredibly amateurish.

Though some may disagree, I think one might argue that the importance of spelling out a numeral is roughly correlated with a numeral's size. Consider this sentence:

He had 113 dollars.

That doesn't look quite as bad as the first sentence, and who wants to write "one hundred thirteen dollars," anyway?

On the other hand, when writing something that includes a lot of numerals (especially if they're big), written words can become confusing. For example, let's say you're making comparisons between the number of species in various animal families. Consider the following:

There are thirty-six species of felids, thirty-seven species of canids and one hundred seventeen species of mustelids.

This is much easier to read:

There are 36 species of felids, 37 species of canids and 117 species of mustelids.

However, the first sentence isn't too bad if it's accompanied by tabular data:

Felidae | Canidae | Mustelidae

36 | 37 | 117

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In my long ago technical writing class I was taught to spell out one to nine and use numerals for 10 and above. I don't remember how we were to treat zero, but I'd guess we were supposed to spell it out.

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