When to Write Out Numbers

There are times when you should spell out a number. The general rule is that you write out the number if it can be spelled using one or two words. For longer words you can use the number. Here are a couple examples: When a Sentence Starts With a Number

If a sentence starts with a number you are supposed to spell out the word even if it takes more than two words to spell it. Here are a couple examples: The second sentence is a line from the "Seventy-six trombones" song in The Music Man. For a long number you might to re-order your sentence so the number is not first.

When Numbers are Next to Each Other in a Sentence

If two different numbers are next to each other you should spell out one of the numbers to make it more legible. Grammar Girl says this is because it makes it easier to read. For example, "The job required fifteen 6-foot long boards."

Don't Spell Out Numbers in Dates, Addresses, Page Numbers, etc.

You don't have to spell out numbers when they are part of dates, addresses, time of day, percentages and page and chapter numbers. You can find a longer list of exceptions on the Purdue OWL.

APA and MLA Style on Spelling Out Numbers

A useful entry on the APA Style blog explains the different between APA and MLA style when it comes to numbers.
The two styles have very different rules for when to write numbers as words or numerals. MLA Style spells out numbers that can be written in one or two words (three, fifteen, seventy-six, one thousand, twelve billion) and to use numerals for other numbers (2¾; 584; 1,001; 25,000,000). APA Style, on the other hand, generally uses words for numbers below 10 and numerals for numbers 10 and above.
If you are following APA or MLA Style for a research paper you will want to consult the latest edition of the guides.

More Information About Writing Out Numbers

Here is an instructional video with additional information on when to write out numbers.