Every now and then, I get one of those – ahem – marketing emails offering me a place in some seminar that promises to teach me good business writing. I love reading them to try and find not so obvious grammar errors (Grammar Nazi, moi?) and thereby vindicate my belief that such workshops are pretty worthless. Invariably, one of the bullet points of things you’d learn by attending the session is how to write numbers in text. “Learn one simple rule,” the email goes, “and never get it wrong again.” Or something along those lines.

It’s not a grammar issue but a style issue. And when it’s a style issue I reach for The Chicago Manual of Style. It is a marvelous tome full of good advice for writers.

I have the 15th edition (I haven’t sprung for the just-released 16th yet) and the relevant section is chapter 9, Numbers. Section 9.6 is probably the rule alluded to by the email:

An alternative rule. Many publications […] follow the simple rule of spelling out only single-digit numbers and using numerals for all others.

Sensibly, it goes on to say that this rule should be used with flexibility to avoid such disasters as “one of the 12 jurors would not budge.”

However, this is just the alternative rule. Chicago has a general rule, appearing in section 9.3, which seems eminently sensible to me. In general text, spell the following in words:

• whole numbers from one to one hundred (“ninety-nine red balloons”),
• round numbers, that is, hundreds, thousands, millions (“a picture is worth a thousand words”), and
• any number that starts a sentence.

Every other number should be written with numerals. Pretty simple, eh?

With regard to the second option, Chicago suggests that mixing numerals and words with large numbers is acceptable (“1.1 trillion dollars”, “a population of 55 million”, and so on).

Regarding the third option above, note that if the sentence looks awkward when spelling out the number (“Six hundred and sixty six was the number of the beast.”) then you should recast the sentence to avoid the clumsiness (“The number of the beast was 666.”).

It also makes the reasonable suggestion that above all you should be consistent without losing flexibility. The rules are not cast in stone. Notice that above, for example, I talked about Chapter 9 and not chapter nine, mainly because that’s how the book names its chapters. Ditto “15th edition” and not “fifteenth edition”: the former is how it’s referred to on the cover.

There you go, I just saved you that part of the seminar fee. You’re welcome.