Q. I’m editing a nontechnical text, but I’ll occasionally come across a technical unit of measurement such as °F or m². What is the rule regarding when to use the symbol versus writing out the measurement?
A. Except in tables and similar contexts where abbreviations are the norm (and where they’re often required to make things fit), it’s usually best to spell out abbreviations and symbols for units of measurement and the like in nontechnical prose (e.g., “water freezes at 32 degrees Fahrenheit”; “more than ten square meters”).
See chapter 10 in CMOS for more details and examples (start with the overview in paragraphs 10.1–12). For the use of numerals for exact quantities (as in “32 degrees Fahrenheit”), see paragraph 9.14.