Using AP-Style (Associated Press Style)
Spell out numbers one through nine. Use figures for numbers 10 and higher.
Begin a sentence with a spelled out number, never a figure. For example, "Twenty of our classmates are going on the trip."
Use a hyphen only for compound numbers ending in "y" to connect them to the next words. For example, twenty-two or one hundred thirty-four.
Keep proper names as they are commonly written. For example, 20th Century Fox. Always spell out numbers in catch phrases or expressions. For example, "Thanks a million."
Using CMS (Chicago Manual of Style)
Type numbers 0-99 as spelled out words, then use figures for 100 and up in the CMS style of writing.
Start with a spelled out number if it is the first word in the sentence, just like in AP style. For example, "One hundred fruit flies swarmed around the rotten strawberries."
Avoid mixing numbers and figures when talking about the same thing. Example: "I have 2 cats and Mary has 22 cats." Since the sentence is talking about cats only, use the figure "2" to match with "22." When you refer to different things, it is OK to mix figures with words. For example, "She gave me seven dollars, but I still owe her 20 cookies."
Add hyphens to compound numbers ranging from twenty-one to ninety-nine.
Refer to centuries by writing out the time period before it. For example, "He lived in the seventeenth century, but she lived in the twentieth century."
Be consistent. To avoid confusion, follow the same rules every time you write numbers. Even if you are unsure about a specific rule, make a commitment to doing it one way.
Use figures to express dates. "We will meet on the 2nd of November, even though the meeting was originally scheduled for November 6."
Choose from several methods for writing decades. You may spell them out completely (forties, sixties) or use figures (1990s, 2000s). Never use an apostrophe after the year; however, you can abbreviate a decade by putting an apostrophe before the year ('70s, '20s). All three of these styles are acceptable in number writing.
Write out fractions as full words if they stand alone. Use figures if it is a number plus a fraction. Example: "I'll give you one-half the chocolate bar if you give me one-third of the popcorn." Or, "She got an 8 1/2 percent raise yesterday."
Simplify large numbers by keeping them consistent. You can spell them out or use a figure, but don't do both. Example: "In a savings account, you can earn from one million to five million dollars." Or, "The stocks can rise from $1,000 to $1,000,000."