Select a system and stick to it. One book in the For Dummies series has “10” and “ten” in the same paragraph!
An article in the Wall Street Journal says: "And the bombs blew up within 3 miles of six level-one trauma units."
- One of my personal rules is to use the same style when numbers are nearby: “eight to twelve” or “8 to 12”—not “eight to 12.”
- However, to avoid confusion and misreading, I write “four 10-lb bags, not “4 10-lb bags.”
- I don’t spell out numbers in addresses or prices, except for low numbers like “One Main Street” or “five bucks.”
- When numbers are approximate and used to present a mood rather than data, I usually spell the number, as in: “The chairman was surprised when more than fifty people showed up for the meeting.”
- "A million years ago" is assumed to be an approximation. If you type "1,000,000 years," people will slow down and notice the digits and may assume that the number is precise.
- Never say something like "about 486 people were arrested." If you have an approximate number, round it off and say something like "nearly 500 people . . . . "
(from my upcoming No More Ugly Books!: design help for writers who don't hire artists)