When do you spell out numbers?

Question

I have been learning English as a foreign language and my teacher told me instead of "I'm a 17-year-old girl" I should better write "I'm a seventeen-year-old girl". Why does it make any difference? In fact to me it just makes the sentence appear longer. When do you generally write out numbers and when is it better to stick to numerals?

Answer 1

There is no strict rule of English grammar on this. It is the sort of matter addressed by style guides. If you work for an organisation that publishes written material, or you are in education, you may be provided with a style guide.

Being clear is very important. When we write very long numbers in numerals they may be difficult for some to read as words, which is why we use commas to break them up make them more readable. But writing very long numbers in words can make it difficult to visualise and comprehend the number, even though it might be easier to read.

One style guide I found says:

As a rule of thumb, use written words if [the number] can be expressed in two words or less.

A requirement for all kinds of style guidance is that you write with consistency - so if you choose to write numbers in words, you should do so throughout. Following the guidance above may mean you should stick to writing with numerals.