Free The Blue Book of Grammar and Punctuation by Jane Straus, Lester Kaufman, Tom Stern

through ninety-nine and using figures thereafter.
    This is a complex topic, with many exceptions, and there is no consistency we can rely on among blogs, books, newspapers, and magazines. This chapter will confine itself to rules that all media seem to agree on.
    Rule 1 . Spell out all numbers beginning a sentence.
Examples :
Twenty-three hundred sixty-one victims were hospitalized .
Nineteen fifty-six was quite a year .
    Note : The Associated Press Stylebook makes an exception for years.
Example :
1956 was quite a year .
    Rule 2a . Hyphenate all compound numbers from twenty-one through ninety-nine.
Examples :
Forty-three people were injured in the train wreck .
Twenty-seven of them were hospitalized .
    Rule 2b . Hyphenate all written-out fractions.
Examples :
We recovered about two-thirds of the stolen cash .
One-half is slightly less than five-eighths .
    Rule 3a . With figures of four or more digits, use commas. Count three spaces to the left to place the first comma. Continue placing commas after every three digits. Important : do not include decimal points when doing the counting.
Examples :
1,054 people
$2,417,592.21
    Rule 3b . It is not necessary to use a decimal point or a dollar sign when writing out sums of less than a dollar.
Not advised :
He had only $0.60 .
Better :
He had only sixty cents .
OR
He had only 60 cents .
    Rule 4a . For clarity, use noon and midnight rather than 12:00 PM and 12:00 AM .
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    Note
    AM and PM are also written A.M . and P.M., a.m . and p.m ., and am and pm . Some put a space between the time and AM or PM .
Examples :
8 AM
3:09 P.M .
11:20 p.m .
    Others write times using no space before AM or PM .
Examples :
8AM
3:09P.M .
11:20p.m .
    For the top of the hour, some write 9:00 PM , whereas others drop the : 00 and write 9 PM (or 9 p.m., 9pm , etc.).
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    Rule 4b . Using numerals for the time of day has become widely accepted.
Examples :
The flight leaves at 6:22 a.m .
Please arrive by 12:30 sharp .
    However, some writers prefer to spell out the time, particularly when using o'clock .
Examples :
She takes the four thirty-five train .
The baby wakes up at five o'clock in the morning .
    Rule 5 . Mixed fractions are often expressed in figures unless they begin a sentence.
Examples :
We expect a 5½ percent wage increase .
Five and one-half percent was the expected wage increase .
    Rule 6 . The simplest way to express large numbers is usually best.
Example :
twenty-three hundred (simpler than two thousand three hundred )
    Large round numbers are often spelled out, but be consistent within a sentence.
Consistent :
You can earn from one million to five million dollars .
Inconsistent :
You can earn from one million dollars to 5 million dollars .
Inconsistent :
You can earn from $1 million to five million dollars .
    Rule 7 . Write decimals using figures. As a courtesy to readers, many writers put a zero in front of the decimal point.
Examples :
The plant grew 0.79 inches last year .
The plant grew only 0.07 inches this year .
    Rule 8 . When writing out a number of three or more digits, the word and is not necessary. However, use the word and to express any decimal points that may accompany these numbers.
Examples :
one thousand one hundred fifty-four dollars
one thousand one hundred fifty-four dollars and sixty-one cents
Simpler :
eleven hundred fifty-four dollars and sixty-one cents
    Rule 9 . The following examples are typical when using figures to express dates.
Examples :
the 30th of June, 1934
June 30, 1934 (no - th necessary)
    Rule 10 . When spelling out decades, do not capitalize them.
Example :
During the eighties and nineties, the U.S. economy grew .
    Rule 11 . When expressing decades using figures, it is simpler to put an apostrophe before the incomplete numeral and no apostrophe between the number and the s .
Example :
During the '80s and '90s, the U.S. economy grew .
    Some writers place an apostrophe after the number:
Example :
During the 80's and 90's, the U.S. economy grew .
Awkward