Dictionary.com Word FAQs
When should numbers be hyphenated?
Whole numbers twenty-one through ninety-nine are hyphenated when they are written out, whether used alone or as part of a larger number, whether they are cardinal or ordinal, for example: twenty-one, twenty-first. You should also hyphenate numbers when they are part of larger numbers that are written out - but not other parts of large numbers, for example: 5,264 is written "five thousand two hundred sixty-four" and 523rd is written "five hundred twenty-third". The rule applies only to two-word numbers: 603 is written out, e.g., "six hundred three" (formal) or "six hundred and three" (informal). A whole number followed by hundred, thousand, etc., would be written as, for example, "one hundred," and not hyphenated. In a phrase like "one hundred and ten years," no hyphenation should be added. This will come in handy when you write personal checks!