Q. I am writing a fictional piece that includes a discussion of the Fujita scale of tornado intensities. I am trying to write “318 miles an hour, the top wind speed of an F5 tornado.” I know the Manual wants most numbers spelled out up through 999, but writing “three-hundred-eighteen miles an hour” just doesn’t look right.
A. Actually, the Manual spells out nonround numbers only through one hundred, so “318 miles an hour” is fine Chicago style. Please see CMOS 9.2–7 for the general rules on spelling out numbers.