How to Format Numbers in APA 7th Edition
The APA number formatting rule sounds straightforward until you actually try to apply it. Spell out numbers below 10, use numerals for 10 and above. Simple enough. Except there are exceptions for time, age, measurements, statistics, and numbers that begin sentences, and those exceptions catch nearly every graduate student off guard at least once.
The Basic Rule
Use numerals for numbers 10 and above. Spell out numbers nine and below. So "nine participants" is correct, and "15 participants" is correct, but "9 participants" and "fifteen participants" are both errors.
The Exceptions That Trip Everyone Up
The most important exception is for numbers connected to time, age, dates, and measurements. These always use numerals regardless of size. So "5 years," "3 months," "2 hours," and "4 weeks" are all correct even though they are below 10. The same applies to percentages, statistics, and scores. Write "5%" not "five percent," and "a score of 7" not "a score of seven."
Numbers that begin a sentence are always spelled out, regardless of any other rule. If you need to start a sentence with a large number, either spell it out or rewrite the sentence so the number does not come first.
Ordinal Numbers
Ordinal numbers follow the same rules. "First" and "second" are spelled out. "10th" and "15th" use numerals.
Why This Rule Feels Confusing
The reason this rule trips up graduate students is that it differs significantly from both Chicago style and everyday writing conventions. Most people learn to spell out numbers one through ten as a general rule, without the exceptions APA adds for time and measurement. If you learned a different style guide first, the APA exceptions feel arbitrary. They are not arbitrary, but they do require deliberate attention every time you write a number.
A Quick Check Method
Before submitting any paper, do a search for every number between one and nine written as a word. Check each one against the exceptions list. Then search for single-digit numerals and verify they fall into one of the exception categories. This takes about ten minutes on a standard research paper and catches the majority of number formatting errors.