Age

For ages, always use figures (cardinal numbers). If the age is used as an adjective or as a substitute for a noun, then it should be hyphenated. Don’t use apostrophes when describing an age range:

A 21-year-old student.
The student is 21 years old.
The girl, 8, has a brother, 11.
The contest is for 18-year-olds.
He is in his 20s.

Centuries, Decades, and Eras

Use ordinal numbers to express all but the first through ninth centuries. Use a hyphen when the century is used as an adjective:

21st century
fifth century
20th-century history
18th-century science

Formal style for decades is to use all four digits of the year, followed by an “s.” Informal use permits substitution of an apostrophe (not a single quotation mark or a tick mark) for the first two digits:

the 1960s
the ’60s

For eras, use the secular form—C.E. (Common Era) and B.C.E. (Before the Common Era) rather than A.D. and B.C. Set the letters in all caps, and use periods.

Numbers

Use words for Arabic and ordinal numbers one through nine, nonspecific (or casual) references to numbers, numbers in quoted speech, and numbers beginning a sentence. Use numerals (figures) for 10 and above:

There are seven key points in the argument.
There are a million reasons to disagree on the issue.
Forty-five students have signed up for the class.
The 45 students in the class are all traveling to Europe for spring term.
“The Outing Club will offer three new events this spring,” says President Gutmann.
Exception: when two or more numbers apply to the same category in a paragraph or series.
There are 20 sections to Dante’s work: 15 are set in the first sequence and 5 in the latter.

Use a combination of numerals and words when expressing very large numbers.

Penn received a $10 million gift.
It is a country of 15 million people.

Use figures where ordinals indicate a sequence that has been assigned in forming names (usually with geographic, military, and political designations):

1st Ward
1st Sergeant or Sgt.
7th Fleet

Time of Day

Use numerals to express time of day. Use lowercase letters and periods for a.m. and p.m.

7 a.m.
6:30 p.m.
Noon (12 p.m.)
Midnight (12 a.m.)

In quoted text, use of “o’clock” is acceptable:

“Students must be in their dorms by 11 o’clock every night this week,” says the director of student safety.

Telephone Numbers

Put the area code in parentheses, and use a hyphen between the three-digit and four-digit numbers that follow:

(603) 646-1110

Temperature

For whole numbers, spell out “degrees” rather than use the degree symbol (°), except when used in tables or in reference to scientific measurements. Exception: Zero degrees. Use Fahrenheit or Celsius only when it would be confusing not to:

It is 86 degrees outside today.
Her fever is 101.5°F.
The office temperature is generally maintained in the high 60s.

Units of Measure

Use a combination of numerals and words to express units of measure:

A MacBook Air weighs 3 pounds.
The new facility will include 10-by-12-foot storage closets.
5 feet 3 inches (note no intervening comma, no abbreviations)