Write the Number when Writing:

1. A number or year of a date
2. Large numbers: $4 trillion
3. Page numbers: page 14
4. Time: 7:54 AM
5. Figures (more than 2 words): $25.24
6. Percentage: 75%
7. Addresses: 5400 Transylvania Road
8. Decimals: .54 ounces
9. Miscellaneous: Room 132

Spell the Number when Writing:

1. A number at the beginning of a sentence: "Twenty gallons of gas is required for the trip."
2. Figures (2-3 words): forty-seven cows
3. A century: sixteenth century
4. Any whole number b/w and including 1-9

Mix when Writing:

1. Two separate numbers, so as to avoid confusion: "That man bought six 49er hats for his children."

Sources: Rutgers Guide to Grammar and Style; Elements of Style; OWL