The sixth edition of the "Publication Manual of the American Psychological Association" establishes specific formatting and stylistic guidelines for writers in the sciences, social sciences, business and nursing disciplines to follow in order to create consistency and an easy-to-read style among publications. These APA standards include guidelines for when to write out or use numerals to represent numbers.
Typically, the number 10 and larger numbers are expressed by using numerals in APA format. This includes situations where you use 10 in measurements, such as a "10-mg dosage of medication," dates like "October 10," times like "10 a.m." and in sentences that use numbers in comparison such as "9 out of 10 dentists recommend it." When writing percentages and money references, use the numeral with the percent or dollar sign. The same rule applies when using 10 as an ordinal number: "10th graders take part."
In some instances, you should spell out the number 10 in APA style. Sentences must start with words rather than numbers, so you'd start a sentence with "Ten." You may need to spell the number out to avoid confusing your reader if you use multiple numbers in a phrase, such as "ten 8-point summaries." Spell out the number when giving an approximation of time rather than a specific amount. For example, you would write, "about ten days earlier." Another exception involves phrases where the number is commonly written out, such as "The Ten Commandments."