numbers: when to spell them out
Whether a number in running text takes words or figures.
Spell one through nine, and use figures for 10 and above: nine locks, 12 locks. Ordinal numbers follow the same division: the fifth row, the 15th row. Spell a number that opens a sentence, or recast the sentence: Forty-two members voted, or The vote drew 42 members. Use figures at every size for percentages, clock times with a.m. or p.m., page numbers, dates, addresses, and exact sums of money, and for any measurement with an abbreviated unit or symbol: 6 percent, page 8, $5, 5 km. A measurement written out in words follows the general rule: three miles, 12 miles.
When numbers in one sentence count things in the same category and any of them is 10 or more, use figures for all of them: The shelter took in 6 cats and 11 rabbits. For a round number of a million or more, pair a figure with the word: 4 million voters, $1.2 billion.
See also beginning sentence with numeral; numbers: cardinal / ordinal; ages: hyphenation and numerals; time of day: numerals, noon, and midnight.
Examples
Use The barge carried eight trucks.
Use They towed 11 trucks.
Avoid 11 trucks waited. (11 is the error: a sentence cannot begin with a figure. Write Eleven trucks waited.)
Avoid The fee rose to four percent. (Four is the error: a percentage takes a figure at every size. Write The fee rose to 4 percent.)
Many books and scholarly publications spell out whole numbers from one through one hundred and use figures above that: sixty-five pages.