When to use numerals and when to spell out numbers varies according to style, genre, and your professor.  Some guides say that anything under ten should be spelled out, while ten and over should be written as numerals (this is AP Style, always true for journalistic writing).  Other guides recommend that you spell out numbers that can be expressed in one or two words (such as ten and twenty-two), but use numerals for numbers that require more than two words to spell out (such as 145).  And in some technical and business writing, numerals are preferred even when spelling would be brief. 

The best thing you can do is be aware of the conventions for the style or genre you’re writing in, and, when in doubt, check with your professor and see what he or she recommends.

There are certain cases when it’s almost always best to use numerals, no matter what the style you’re writing in.  These include:

  • Dates – July 4, 1776
  • Addresses – 519 W. 42nd St.
  • Percentages – 75%
  • Fractions – ¾
  • Decimals – 5.4
  • Scores – 7 to 3
  • Statistics – average age 37
  • Surveys – 4 out of 5
  • Money – $11.24
  • Divisions of books – Chapter 7
  • Divisions of plays – Act I
  • Identification numbers – serial no. 1098
  • Time of day – 4:00 p.m.

Exception: In all cases, if a sentence begins with a number, spell out the number or rephrase the sentence.

For example:
One hundred fifty-three children in our program need expensive dental treatment.

Hacker, Diana. A Pocket Style Manual. Boston: Bedford/St. Martin’s, 2000.