Arabic numerals are used for numbers above nine, and also for ages ("7 years old," "a 9-year-old child"—note two hyphens), very large numbers (i.e., a million or larger: "7.5 billion."), monetary figures ("$5," "$15.80," "$150 million"), and percentages ("8 percent"—spell out "percent"). A hyphen is not necessary in a compound adjective that includes arabic numerals to represent dollars: "an $18 million building."
The numbers one through nine (and zero) should be spelled out, unless used as above. When any number is the first word of a sentence, it should be spelled out. However, the sentence can usually be reworked to avoid having to spell out a large or hyphenated number, a monetary figure, or a percentage.
Use the same style to express related numbers above and below 10; if any of the numbers is above 10, express all of them in numerals: "The quartet has two violins, one viola, and one cello"; but, "the orchestra has 18 violins, 8 violas, and 6 cellos."
In isolated cases, spelling out a number above 10 can serve to make it seem indefinite or a casual expression: "I have a hundred things to do today," "I can give you a thousand and one reasons why it won’t work."
Use an "en" dash between numerals rather than a hyphen to indicate intervals: "1939–1945," rather than "1939-1945"; "8–10 p.m.," rather than "8-10 p.m." (It’s a special symbol in word processing programs.)
Spell out fractional amounts that are less than one in text copy, using hyphens between the words: "one-half," "three-sixteenths." Use figures for all fractions larger than one, employing "case" fractions (i.e., single units) wherever possible. Most type fonts include only a few common fractions as single units; other fractions need to be constructed with a slash: "3½," "2¼," "but 1-3/16." (Note the hyphen in the last example, to avoid any possible confusion of the integer as part of the numerator in the fraction—"1 3/16"; this is an exception to AP style.) Instead of trying to remember all of this, convert to decimals whenever practical.
When listing telephone numbers, don't enclose area codes in parentheses (it's awkward to use them that way in parenthetical material); instead, use hyphens throughout (avoid periods in running type, although they may be appropriate in display usage) or "slash and dash": 507-933-7650, 507/933-7650.
