Write numbers from one to ten as words and from 11 upwards as figures. Same goes for first, second, third, and so on, up to tenth. But if you’re writing (for example) 9–11, don’t mix it up.
And don’t use figures at the start of a sentence. ‘Twelve days after we ordered them, the new sofas finally arrived.’
If you’re spelling out a number over 20 at the start of a sentence use a hyphen – thirty-one.
Put a comma in numbers over 1,000. And remember that every three zeros get a comma – ‘Anya told us the budget was £4,750,350’ (we wish).
Use hyphens in phrases where a number’s part of the adjective, like four-hour session or five-year-old child. And in fractions: two-thirds majority.
Spell out million and billion, like ‘12 million people use them’ or ‘The company spent £2 million’.
This all goes out the window if you’re writing a quote involving rates. For table-type stuff involving hours and money use figures (like ‘1 day with 2 trainers’ or ½ a day). It makes it easier for the reader to add it all up.
Ditto when the number comes with a unit of measurement. Then it’s figures all the way. ‘Three trainers walked for 3km to get to their workshop’, or ‘We split the bill for after-work drinks between six of us and it came to £8 each’.
Same for page numbers, such as ‘have a look at page 8’.
‘To’ or a dash?
If you’re writing a span, like ‘three to four’ or ‘120–160’, use ‘to’ when you’re writing words and an en dash (not a hyphen) when you’re writing figures. Simple.