When to spell out numbers

This week, a client of mine asked me to review some existing copy on his website – just to make sure he’d crossed his T’s and dotted his I’s before the site went live. As I was editing a rather number-heavy section of copy, it occurred to me that he might not be the only one in need of some guidelines on when to spell numbers out in full.

If there’s one guideline worth remembering, this is probably the one you’ll get the most mileage from: numbers up to and including ten are normally written out in full, while numbers 11 – 99 are typically expressed numerically. So, “we offer a four-week program and a 12-week program,” is perfectly correct.

In addition, the CP Stylebook suggests you spell numbers out:

• at the start of a sentence: Twenty to 30 children attended, as well as 40 adults;
• in informal or casual usage: She received hundreds of responses to her letter;
• in formal usage: In the year two thousand and four;
• in figures of speech and the like: Ten Commandments, Big Ten, a ten-gallon hat.

I hope that one day, some day, this entry comes in handy.