Rules About Treatment of Numbers
The basic rule about referring to numbers, according to The Chicago Manual of Style, is to spell them out when the total is one hundred or less and use numerals for larger numbers (the Associated Press Stylebook and some other style handbooks set the cut-off point after nine), but there are many exceptions. This post outlines those exceptions.
When referring to categorically similar totals, spell the pertinent numbers out if all totals are one hundred or less (for example, “sixty-five chairs arranged around twelve tables”) but use numerals if one or more totals are one hundred or less and one or more totals are more than one hundred (for example, “127 chairs arranged around 20 tables”). This rule applies only to two or more such numbers in proximity; previous or subsequent isolated numbers pertaining to the same category need not adhere. (Nor do unrelated numbers.) However, text with a concentration of statistics—whether an entire piece of content or one section—will likely benefit from the use of numerals in place of spelled-out numbers.
When totals appear in direct discourse (as when a speaker is quoted), spell out numbers, with the exception of years and elements of proper names; again, however, a concentration of numbers is perhaps best treated by using numerals. When reproducing quoted written material, however, do not alter number style.
Spell out large round numbers that include orders of magnitude (hundred, thousand, etc.).
Recast a sentence that begins with a numeral: For example, revise “2020 is the next leap year” to “The next leap year is 2020.” If the sentence must begin with a number, spell it out. In such cases, omit and in expressions such as “five thousand and three hundred.”
When a number consists of or includes a fraction, spell it out or use numerals according to the guidelines above, but numbers with decimals should be styled as numerals.
These rules also apply to quantities such as units of time or distance; exceptions can be made for such categories as temperature, clothing sizes, and miles per gallon.
Style quantities expressed with an abbreviation or a symbol in numeral form, and use numerals when a range is separated by an en dash (for example, “25–50 participants”). Refer to percentages with numerals. (However, spell out the word percent in nontechnical usage; use the symbol in statistical references.)
When referring to small amounts of money, spell out casual, isolated references but treat concentrations of such figures with the same guidelines as those for statistical materials. In addition, generally, express sums of more than one hundred dollars with numerals or with a combination of figures and words, such as in “$500 million budget”).
Related post: 10 Rules for Writing Numbers and Numerals
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