This is probably one of the lesser-known rules in writing. Mainly formal writing - news articles, journal articles, theses and essays. This is the “Number Rule”. Do you write the numbers as a word, or do you just put the digits down?
Well, the rule is actually very simple. Write single number like, “one”, “two”, etc, as words, all the way up to nine. Once you go into double digits, though, then it’s time to switch to digits, like “10”, “11”, etc. This is really important for reporting statistics, where you’re probably going to have percentages, means, modes, ranges and many other numerical findings.
That brings me to the exception of the rule. Usually, if you’re using statistics, chances are you won’t have any single numbers to write out. But if you do, you need to put consistency over correction and put those in digits, too. You can check with a supervisor, publisher or tutor if you’re not sure about this. But, when in doubt, make them all the same.
But, if you’re writing fiction, theses, essays or news, remember the double-digit rule. Singles are words, doubles are digits.