In general, use numerals to express numbers 10 and above, and use words to express numbers zero through nine.
there were 15 psychologists at the clinic
the study had 40 participants
students were in the third, sixth, eighth, 10th, and 12th grades
However, there are exceptions to this general guideline for number usage. There are cases in which you should always use numerals to express numbers, even numbers zero through nine, and likewise, there are cases in which you should always use words to express numbers, even numbers 10 and above.
Numbers expressed in numerals are covered in the seventh edition APA Style manuals in the Publication Manual 6.32 and the Concise Guide Section 6.1
This guidance has been expanded from the 6th edition.
Always use numerals to express numbers in the following cases, even numbers zero through nine:
Case
Example
Numbers that immediately precede a unit of measurement
Statistical or mathematical functions
multiplied by 2
Fractions or decimals (except common fractions)
the 5th percentile, the 95th percentile
the 3rd quartile
approximately 4 months
about 6 years ago
5 years old, 18 years old
5-year-old children, 18-year-old adults
Scores and points on a scale
scored 6 on a 7-point scale
Exact sums of money
$50 in U.S. dollars
Numerals as numerals
the numeral 2 on the keyboard
Also use numerals to write numbers that denote a specific place in a numbered series when the number comes after the noun (e.g., Step 1). The noun before the number is also capitalized. This guideline applies to parts of books and tables as well (e.g., Chapter 1).
However, when the number comes before the noun, the usual guidelines for number use apply, as in the following examples.
Number after a noun
Number before a noun
Grade 5, Grade 11
the fifth grade, the 11th grade
the fourth level
Question 2, Question 25
the second question, the 25th question
the second table, the fifth figure
the eighth column, the seventh row
Chapter 6, Chapter 14
the sixth chapter, the 14th chapter
Exceptions: Do not capitalize the abbreviations for page(s) or paragraph(s), even when they are followed by a numeral (e.g., p. 3, pp. 2–5, para. 9, paras. 1–4).