Rule 1
Spell out single-digit whole numbers. Use numerals for numbers greater than two words. Prioritize this rule over all other rules.
Examples:
I want five copies.
I want one hundred copies. (two words)
I want 10,500 copies. (three words)
Rule 2
Be consistent within a category. For example, if you choose numerals because one of the numbers is greater than nine, use numerals for all numbers in that category. If you choose to spell out numbers because one of the numbers is a single digit, spell out all numbers in that category.
If you have numbers in different categories, use numerals for one category and spell out the other.
Correct:
My 10 cats fought with their 2 cats.
My ten cats fought with their two cats.
Given the budget constraints, if all 30 history students attend the four plays, then the 7 math students will be able to attend only two plays. (Students are represented with figures; plays are represented with words.)
Incorrect:
I asked for five pencils, not 50.
Rule 3
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 4
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 5
The simplest way to express large numbers is best. Round numbers are usually spelled out. 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 five hundred to five million dollars.
You can earn from $5 hundred to $5 million.
Incorrect:
You can earn from $500 to $5 million.
You can earn from $500 to five million dollars.
Rule 6
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 7
With numbers that have decimal points, use a comma only when the number has five or more digits before the decimal point. Place the comma in front of the third digit to the left of the decimal point. When writing out such numbers, use the comma where it would appear in the figure format. Use the word and 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:
If the number has no decimal point, authorities disagree on whether to begin using the comma with four-digit numbers or to begin using the comma with five-digit numbers. When writing out these numbers, I recommend using the comma where it appears in the numerical form.
Examples:
1,054 schools OR 1054 schools: one thousand, fifty-four schools OR one thousand fifty-four schools
12,154 schools: twelve thousand, one hundred fifty-four schools
Rule 8
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 9
When expressing decades, you may spell them out and lowercase them.
Example:
During the eighties and nineties, the U.S. economy grew.
Rule 10
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 U.S. economy grew.
Incorrect:
During the ’80’s and ’90’s, the U.S. economy grew.
Rule 11
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 U.S. economy grew.
Rule 12
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 13
Use numerals with the time of day when exact times are being emphasized or when using A.M. or P.M.
Examples:
Monib’s flight leaves at 6:22 A.M.
Please arrive by 12:30 sharp.
She had a 7:00 P.M. deadline.
Rule 14
Use noon and midnight rather than 12:00 P.M. and 12:00 A.M.
Rule 15
Hyphenate all compound numbers from twenty-one through ninety-nine.
Examples:
Forty-three persons were injured in the train wreck.
Twenty-three of them were hospitalized.
Rule 16
Write out a number if it begins a sentence.
Examples:
Twenty-nine people won an award for helping their communities.
That 29 people won an award for helping their communities was fantastic! OR
That twenty-nine people won an award for helping their communities was fantastic!
http://www.grammarbook.com/grammar_quiz/numbers_1.asp
http://www.grammarbook.com/grammar_quiz/punctuation_mastery.asp
https://www.grammarbook.com/members/PreviewQuiz.asp?quiz_id=194&PDF=true