This is one of those areas where there is so much gray area that it is really almost pointless to try to define it. I’ve heard rules saying you should spell out all numbers under a hundred. I’ve also heard rules saying the same thing for twenty, fifteen, ten, fifty, and so on. Some say anything under twenty. Others say anything under ten. To make this simpler, I will show you the most common rule I’ve heard.
Twenty and Under Rule
This rule states that you should spell out every number that is twenty or lower. This rule also states that you should spell out large numbers that are whole. By whole, I mean you should spell out fifty, hundred, thousand, million, billion, and so on, but you would not spell out 57, 251, 1325, and so on. Simple enough, right? If you have trouble remembering it, a helpful rule to remember is to never spell out a word that contains a hyphen. When you write numbers, you always place a hyphen between words like twenty-one, fifty-nine, and so on. For example, you would spell 123 this way: one hundred twenty-three. You can also remember that you never spell out anything over one hundred unless it has lots of zeroes.
Yeah, it’s not necessarily precise to think of it that way, but it is accurate.
Personally, find a way that works and stick with it. Consistency in this case is much more important for messaging.