The advice proffered here is meant primarily for standard academic prose. Business and technical writing sometimes goes by a different set of standards, and writers of those kinds of text should consult a manual dedicated to those standards. (The APA Publication Manual has an extensive section devoted to the use of numbers in technical papers. The Chicago Manual of Style [chapter 13] addresses just about every issue that might come up in a technical or mathematical text.)

Write out numbers that require no more than two words :

Remembering that a hyphenated number between twenty-one and ninety-nine counts as one word. Some writing manuals will suggest that whole numbers from zero through nine should be written as words, and numbers from ten on up should be written as numerals, especially when the word modifies a noun as in five students or two professors.

Use numerals, however, when the number modifies a unit of measure, time, proportion, etc.: 2 inches, 5-minute delay, 65 mph, 23 years old, page 23, 2 percent.

Use numerals for decimals and fractions: 0.75, 3.45, 1/4 oz, 7/8 in. (Notice that abbreviations are always written in the singular form whether they would be expressed as plurals or not: 14 oz, 12 in. The period can be omitted from such abbreviated measurements unless confusion would result [after in., for example]).

Use numerals for any number greater than nine: 237 lb, 32 players. (But this may be determined by context and how exact the numbers are. In business and technical writing, yes, all such numbers would be written as numerals; in other kinds of text, you might see something like six million victims, four thousand volunteers.

Approximate figures — fractional or otherwise — may be written out as words: one half the students, a quarter cup of sugar, a third of the time, four times as often.

Place a hyphen after a unit of measure when the unit modifies a noun: 10-foot pole, 6-inch rule, 3-year-old horse. (The unit of measure in such expressions is, for some reason, always singular.)

When many numbers are involved, use all numerals unless all the numbers are whole numbers less than nine.

When fractional or decimal expression are 1 or less, the word they modify should be singular: 0.7 meter, 0.22 cubic foot, 0.78 kilometer. Precede decimal fractions with a value less than one with a leading zero before the decimal point.

Percentage expressions should be written out as words: Last semester, 78 percent of the first-year students passed English Composition. (as opposed to 78%)

Avoid using ordinals when writing dates: February 14, not 14th.

There are twenty-six students in my wife's third-grade class.

Juan is over 183 centimeters tall.

Hartford has over ninety-three thousand citizens.
(Some people would argue that all such statistical information should be expressed in numerals; when rounded off, however, spelled-out words are appropriate.)

Hartford has 97,500 citizens.

Consistency is important here!

Juan is about 183 centimeters tall, which means that he is just over 6 feet tall.

To avoid confusion by running numbers together, combine words and numerals when one number follows another. Generally, write out the shorter number.

My wife teaches 26 third-grade students.

There were 10 four-foot boards on the trucks.

The lab has 24 seventeen-inch monitors.

We need six 50-watt bulbs for this apartment.

Avoid beginning a sentence with a number that is not written out.

Seventy-two inches equals approximately 1.83 meters.
An exception: you can begin a sentence with a date:

1997 was a very good year for owls.

Use figures instead of words for :

Dates and years: December 18, 1997. Avoid using ordinals when writing dates: Her birthday is on April 4th.

Decimals, percentages, and fractions: 235.485, 55%, 14 1/4

Scores: The Bulls won the final game by a score of 114 to 106.

Addresses: 1032 Maple Avenue. Sometimes, though, an address is part of a building's name, and then you'll want to spell it out: One Corporate Plaza. Unless space is at a premium, write out numerical street names (of one hundred or less): 1032 Fifth Avenue.

Political and military units (for numbers of one hundred or less): Seventh Precinct, Fourteenth Congressional District, Fifty-third Regiment, Third Batallion, 112d Artillery

Finances: Tickets cost $35.50 apiece. The city spent $1.1 million for snow removal last year. (Or use $1,100,000.) You can leave the comma out of figures in the thousands: They spent $7500 on that car before junking it. Also, leave the comma out of addresses and year-dates: In 1998, they moved to NE 12887 53rd Avenue.

Ranges: Between 18 and 25 bald eagles have been counted near the Connecticut River this spring.

Time: 9:15 a.m. If you use the word o'clock, however, for rounded off times, spell out the number in words: We left at seven o'clock. Use a.m. and p.m., not AM and PM.