Style guidelines from the American Psychological Association help writers maintain a uniform way of writing. Because APA style is used for scientific and clinical writing, numbers are often included. Readers can more easily understand information when it is consistently communicated in one one way. Many rules also help save space.

The most common rule

The most common and frequently used rule for writing numbers in APA style is determined by whether the number is under 10. All those numbers should be written out in word form, such as four or seven. A number 10 and above is written as a numeral, such as 10, 17, 233. One of the exceptions to this rule is when a sentence starts with a number. In this case, the number is always written out as a word. For example, "Fifteen students were sick; two teachers went home." If possible, avoid starting a sentence with a number.

Other exceptions to the most common rule

Another exception to writing out numbers under 10 is when writing a specific measurement, amount, date, age or money. In this case, the number should be written as a numeral: 8 miles, 4 cents, a 7-year-old boy, 6 weeks ago, 3 hours before dinner, $2 for the fine. And finally, keep things consistent, even if it means bending the rule. For example, write "6 out of 10 dentists preferred the last treatment" instead of "six out of 10."

Other commonly used rules

A commonly used number rule in APA style is to write out fractions as words. For example, write "half" instead of 1/2. But, if a fraction is unusual, such as 3/16, then use the numeral form. For decimal usage, always place a "0" before the decimal: 0.91, for example. And finally, when writing about percentages, always use a numeral, even if the number is less than 10. For example: 7% of people surveyed did not respond to the question.