Generally speaking, spell out numbers one through nine, and use numerals for 10 and above. However, there are a number of exceptions to this general rule. The basic rules and exceptions are presented below, but many more details and examples are available at the AP Stylebook entry on numbers.

Spell out numbers one through nine:

• She earned three degrees from NC State.

There are a number of exceptions to this rule. Always use numerals in the following instances:

• Ages: a 6-year-old girl; the law is 8 years old; the 7-year-old house. 
• Decimals, percentages, fractions: 7.2 quake, 3½ laps, 3.7% interest. 
• Dimensions: She is 5 feet, 6 inches tall.
• Distances: He walked 4 miles. She missed a 3-foot putt. 
• Millions, billions, trillions: 1 million people; $2 billion.
• Money: 8 cents, $3
• Speed: 55 miles per hour 
• Temperature: 20 degrees

Use numerals for 10 and above:

• Kevin Howell is NC State’s 15th chancellor.

There are a few exceptions to this rule. Always spell out numbers in the following instances:

• At the start of a sentence — Twenty-five students registered for the course.
• In casual, colloquial turns of phrase — one in a million
• In literary or special uses — four score and twenty years ago