51. Writing numbers

The Chicago Manual of Style, our preferred guide, says that in nontechnical writing, we should spell out “whole numbers from one through one hundred, round numbers, and any number beginning a sentence” (380). Chicago provides an alternative for technical writers, which is to spell out “only single-digit numbers.” The authors point out, though, that such rules “should be used with flexibility so as to avoid such awkward locutions as ‘12 eggs, of which nine were laid yesterday’” (381).

Writers will want to avoid placing two numbers adjacent to one another to prevent a misreading. In such cases, it is helpful to spell out the smaller of the two numbers. For example, the phrase “10 9-inch nails” would be better written as “10 nine-inch nails.”

According to CMOS 9.4,“The whole numbers one through one hundred followed by hundred, thousand, or hundred thousand are usually spelled out.” The spelled-out form “fifteen hundred” qualifies. But the hybrid form “one thousand five hundred” does not. Paragraph 9.4 is intended to encourage spelling out round numbers like three hundred thousand, not awkward forms like “three hundred thousand six hundred”—or, for that matter, something like “thirty-three hundred thousand,” which would be better expressed as “3.3 million” (see CMOS 9.. So write “fifteen hundred” or “1,500,” depending on context. (For example, if numerals are otherwise rare in your text, opt for the former.)

Decimal quantities are considered to be plural; quantities expressed as fractions are considered to be singular. So write “0.8 miles” but “eight tenths of a mile.” For decimal forms, only the number one is singular: 1 mile. Once you add a decimal, even if it’s a zero, it becomes plural: 1.0 miles. See CMOS 9.19.

CMOS still recommends spelling out any number at the beginning of a sentence (see CMOS 9.5). If the result is awkward, as it often is with a year, the recommendation is to reword: The year 1980 was indeed a good one. This rule is an editorial nicety: a numeral isn’t as effective as a capital letter at signaling the start of a new sentence. In other words, it’s a rule you can break in all but the most polished, CMOS-approved prose—for example, you can ignore it in casual correspondence or where space is at a premium (or if you follow AP style!).