The accepted formatting for writing numbers is to write numbers below twenty as single words and to hyphenate the numbers from twenty-one to ninety-nine, with one hundred written as two words. Depending on the writing style used, certain numbers always are written out, while others are expressed only as numerals.
Written numbers follow the same general rules for hyphenating, with compound words such as fifty-five and ninety-nine always hyphenated. However, the Associated Press Stylebook and the Chicago Manual of Style have different rules for which numbers are written as words and which are expressed as numerals, for purposes of specific documents or publications. The AP Stylebook dictates only spelling out the numbers from zero through nine, with some exceptions, while the Chicago Manual of Style recommends writing out zero through ninety-nine and using numerals for one hundred and above.
When fractions are written out, they are hyphenated. Two-thirds, three-fifths and one-half are examples of acceptable written fractions. Each writing style has its own conventions for which numbers are written or expressed with numerals. When not adhering to a specific style guide, which numbers are written is a matter of personal preference, although whatever format is adopted should remain consistent throughout the article or document.