Tips for Memorizing the Number Rule:

1. If the number is smaller than 10, write it out.
Example: I write for five websites, but I still prefer the pen and paper.

2. If another number in the sentence is spelled out, you must also spell out that number. This applies to all numbers, even those above 10.
Example: Put it up to eleven.

3. Always write out a time followed by o’clock–a numeral never precedes this.
Example: I began writing my story at six o’clock.

4. If a number begins the sentence, spell it out instead. This also overrides the “numbers smaller than 10″ rule.

5. If you’re stating a time without the A.M or P.M., don’t make it numerical. It needs to be spelled out.
Example: I’m going to go to sleep at ten.

6. Write out large, round numbers (hundred, thousand, million, billion, etc.). This also makes it easier to read.
Example: It felt like the weight of a thousand men fell on my shoulders.

7. Common fractions or expressions are written out.
Example: One-third of the site’s visitors were male.

8. Ordinal numbers (“rd”, “th”) follow the “numbers small than 10 rule”. Spell out seventh, third or second, but don’t spell out 16th or 32nd.

9. If a numeral is listed after a number, the number needs to be written. Fourteen 13 year olds is good, but 5 100-meter races is not.

AP style and Chicago style can also affect these rules–for example, Chicago style doesn’t follow the “numbers above 10″ rule. Most websites use AP style, however. If in doubt, check with the website or client.

• Rules for Writing Numbers (GrammarBook.com)
• More Rules for Writing Numbers (YourDictionary.com)