Here are the complete rules of numbers:

Rules for Writing Numbers

Rule 1 The numbers one through nine should be spelled out; use figures for numbers 10 and above.
Examples I want five copies.
I want 10 copies.

Rule 2 With a group of related numbers where one number is above nine in a sentence, write the numbers all in figures. Use words if all related numbers are below 10.
Correct I asked for 5 pencils, not 50.
Incorrect I asked for five pencils, not 50.
Correct My 10 cats fought with their 2 cats.
My nine cats fought with their two cats.

Rule 3 If the numbers are unrelated, then you may use both figures and words. Again, one through nine should be spelled out.
Examples I asked for 30 pencils for my five employees.
My nine cavities are exceeded in number by my 14 teeth.
I have 10 toes but only one nose.

Rule 4 Always spell out simple fractions and use hyphens with them.
Examples One-half of the pies have been eaten.
A two-thirds majority is required for that bill to pass in Congress.

Rule 5 A mixed fraction can be expressed in figures unless it is the first word of a sentence.
Examples We expect a 5 1/2 percent wage increase.
Five and one-half percent was the maximum allowable interest.

Rule 6 The simplest way to express large numbers is best. Be careful to be consistent within a sentence.
Correct You can earn from one million to five million dollars.
Incorrect You can earn from one million to $5,000,000.
Correct You can earn from $500 to $5,000,000.
Incorrect You can earn from $500 to $5 million.
Correct You can earn from five hundred to five million dollars.
Incorrect You can earn from $500 to five million dollars.

Rule 7 Write decimals in figures. Put a zero in front of a decimal unless the decimal itself begins with a zero.
Examples The plant grew 0.79 of a foot in one year.
The plant grew only .07 of a foot this year because of the drought.

Rule 8 When writing out large numbers of five or more digits before the decimal point, use a comma where the comma would appear in the figure format. Use the word and only where the decimal point appears in the figure format.
Examples $15,768.13 (Fifteen thousand, seven hundred sixty-eight dollars and thirteen cents)
$1054.21 (One thousand fifty-four dollars and twenty-one cents)
NOTE: The comma is now commonly omitted in four-digit whole numbers.

Rule 9 The following examples apply when using dates:
Examples The meeting is scheduled for June 30.
The meeting is scheduled for the 30th of June.
We have had tricks played on us on April 1.
The 1st of April puts some people on edge.

Rule 10 When expressing decades, you may spell them out and lowercase them.
Example During the eighties and nineties, the United States economy grew.

Rule 11 If you wish to express decades using incomplete numerals, put an apostrophe before the incomplete numeral but not between the year and the s.
Correct During the '80s and '90s, the United States economy grew.
Incorrect During the '80's and '90's, the United States economy grew.

Rule 12 You may also express decades in complete numerals. Again, don't use an apostrophe between the year and the s.
Example During the 1980s and 1990s, the United States economy grew.

Rule 13 Normally, spell out the time of day in text even with half and quarter hours. With o'clock, the number is always spelled out.
Examples She gets up at four thirty before the baby wakes up.
The baby wakes up at five o'clock in the morning.

Rule 14 Use numerals with the time of day when exact times are being emphasized.
Examples Monib's flight leaves at 6:22 a.m.
Please arrive by 12:30 p.m. sharp.

Rule 15 Use noon and midnight rather than 12:00 a.m. or 12:00 p.m.

Rule 16 Hyphenate all compound numbers from twenty-one through ninety-nine.
Examples Forty-three people were injured in the train wreck.
Twenty-three of them were hospitalized.

Rule 17 Write out a number if it begins a sentence.
Example Twenty-nine people won an award for helping their communities.
That 29 people won an award for helping their communities was fantastic!