1. Spell out whole numbers from one through ninety-nine. Use hyphens where necessary.
There are forty-three ways to leave your employer.
2. Spell out the words million, billion, etc. to express very large numbers.
On Tuesday the ODLT had 6.5 billion hits.
3. Spell out numbers that are followed by hundred, thousand, hundred thousand, million, etc.
The amphitheatre seats two hundred.
4. Spell out numbers that begin sentences.
Two hundred and ninety-six bottles on the wall and they're all mine.
5. Don't hyphenate the part of the number that is 100 or greater.
Nineteen ninety-six was a very good year for callouses.
6. For brevity's sake, use numerals if the sentence or paragraph contains lots of numbers.
The exam scores were: 77 (Bob), 88 (Karen), 91 (Kenji), 99 (Dale), and 96 (Brenda).
7. If you spell out a currency amount, also spell out the currency unit; otherwise, use numbers and symbols.
8. If the number is between one thousand and ten thousand, express it in hundreds.
I wrote 4500 rhyming couplets while working at Starbucks.
9. Pluralize numbers by adding s; don't add 's
He gave me two 20s. (good)
I gave him two 20's. (bad)
10. Pluralize spelled-out numbers the same way that you would pluralize any other word.
The prisoners were in their twenties, thirties, and forties.
11. Use numbers as ordinals when referring to rankings.
Canada placed 2nd, 4th, and 12th in cooking.
12. Spell out the word percent when using percentages, but use the % symbol if several percentages are being listed or if you're using a table or a graph.
His intelligence increased by 7.1 percent in one year.
In 1973 intelligence increased by 17% in Burnaby, but it decreased by 14% in New Westminster and a whopping 24% in Surrey (for the fifth year in a row!).