Q. Hi, CMOS people, I can’t quite seem to figure out whether I should use spelled-out numbers or numerals with units of time—for example, seconds, minutes, hours, days, months, years. I am not sure whether it should be “2 to 4 weeks” or “two to four weeks”; “30 years” or “thirty years”; etc. I think for numbers over 99, numerals are used, for example “230 seconds.” I understand that numerals should be used with units of measure in general, like kg, cm, °C, and °F, etc. Thank you for your help.
A. For units of time (or any other measure) in nontechnical text, we like to spell out numbers up to and including one hundred: “The cake burned in forty-one minutes.” If, however, in a given paragraph the same time unit involves a mixture of numbers under and over one hundred, we style them all the same: “Ten runners clocked in at 94 minutes, and forty-three more finished in 101 minutes.” (Note that the numbers of runners are not changed to numerals because in that category there is no inconsistency in styling them according to the rule.)
Numerals are always used with abbreviated measures like the ones you list; and in technical or statistical texts, numerals are used even when measures are spelled out. Sometimes even nontechnical text will have a passage containing many numerical references, in which case the editor might decide to use numerals for all in order to save space and prevent what might seem to be inconsistencies. See CMOS 16, chapter 9, for a detailed discussion.