In general, use words to express numbers zero through nine, and use numerals to express numbers 10 and above.

there were five nurses on duty

the study had three conditions

students were in the third, sixth, eighth, 10th, and 12th grades

Numbers expressed in words are covered in Section 6.33 of the APA Publication Manual, Seventh Edition

However, there are exceptions to this general guideline for number usage. There are cases in which you should always use words to express numbers, even numbers 10 and above, and likewise, there are cases in which you should always use numerals to express numbers, even numbers zero through nine.

Cases in Which to Always Use Words for Numbers

Always use words to express numbers in the following cases, even numbers 10 and above:

Case

Example

Numbers that begin a sentence, title, or heading (when possible, reword the sentence to avoid beginning with a number)

Fifty percent of the students received the intervention, and the other 50% were part of a control condition.

Twenty people enrolled in the class, but 15 dropped out.

Common fractions

one fifth of the class

two-thirds majority

Certain universally accepted phrases

Twelve Apostles

Five Pillars of Islam