If your writing contains numbers, the general rule is to spell out in letters all the numbers from zero to nine and use numerals for larger numbers; but there are exceptions. If what you’re writing is full of numbers and you’re doing math with them, stick with numerals. Approximations like “about thirty days ago” and catch-phrases like “his first thousand days” are spelled out. Large round numbers are often rendered thus: “50 billion sold.” With measurements, use numerals: “4 inches long.” Never start a sentence with a numeral. Either spell out the number involved or rearrange the sentence to move the number to a later position.

See also “50’s.”

List of errors