When to Spell out a Number

There are a ton of different style guides, so know that what I'm referring to here are the APA style rules found on p. 111 (sixth edition). If you're following another style guide, you might want to double check to see if it follows the same rules. If you're not following a particular style guide, then jump on board with APA! Why not, right?

Spell out all numbers nine and lower.
(One, two, three, four, five, six, seven, eight, nine)

Use figures for all numbers 10 and higher.
10, 11, 12, 13......100...550....1,450...5,000

Once you hit something like 1 million, you can do a blend.

There are a ton of exceptions, though. For example, increments of time, percentages, and numbers that represent a place in a numbered series will all be figures.

5-years-old
2 hours ago
9%
Grade 3 (but note: third grade)

Likewise, all numbers that are the first thing in a sentence will be spelled out.

Twelve years ago I was in Kansas.

See APA pp. 111-114 for all the dirt!