Spell out single-digit numbers. That means you write out the words for one, two, three, four, five, six, seven, eight and nine, and you use numerals for 10 and higher numbers.
Examples:
My dining room table seats eight people.
If only I had 100 wishes, I would wish for all the happiness in the world.
Defying the above rules, spell out a number if it starts a sentence. If this seems awkward, rewrite the sentence so that it does not begin with a number.
Correct: Fifteen years ago, this never would have been possible.
Correct: This never would have been possible 15 years ago.
Incorrect: 15 years ago, this never would have been possible.
The only time that this exception does not apply is when the sentence starts with a numeral that represents a calendar year. For example, "2008 was a historic election year."
If you mention two (or more) numbers in reference to items in the same category or group, be consistent. Either use numerals to describe all the quantities, or spell out all the quantities. Do this even if one number is below 10 and another is above 10.
Correct: My dining room table seats eight people, but if we really squeeze, we can fit ten.
Correct: If only I had 100 wishes instead of a mere 3.
Be careful: If you describe items that are not in the same category, use the original rules. Still stick to one format within each category.
Correct: It was a rough 15 minutes there; I glanced at the clock every 2 minutes, and I'm pretty sure four or five other people did the same.
(Quantities of minutes are in numerals; quantities of people are spelled out.)
This is a simple rule. Spell out numbers in expressions that are part of the vernacular, even if the numbers are above 10.
Examples:
Thanks a million.
A thousand times no.
Simple fractions should be spelled out, using a hyphen between terms. For example: one-half, three-quarters, two-thirds.
Mixed fractions (totaling more than one) can be written as numerals as long as they do not start a sentence.
Correct: It's been a good year, and so I expect an 8 1/2 percent increase in sales.
Correct: Eight and one-half percent is an impressive increase.
Incorrect: 8 1/2 percent is an impressive increase.
Spell out or use numerals according to an organization's own style. For example: 20th Century Fox and The ONE Campaign.
Use numerals to write ages of people and animals but not of inanimate objects.
Examples:
She was 8 years old.
My dog is almost 12 years old.
That car is a whopping twenty years old.
When ages are expressed as adjectives (either before a noun or instead of a noun), use hyphens to connect the numeral with the words "year old."
Correct: The 3-year-old girl absolutely loved to eat ice cream.
Correct: The 3-year-old was an ice cream fanatic.
Incorrect: The 3 year old had ice cream dripping down her chin.
There are several other situations in which numerals, rather than spelled-out words, should be used. These are: days of the month, degrees of temperature, dimensions, house numerals, percentages, proportions, scores, serial numbers, speeds, sums of money, time of day and votes.