Numbers

In general, Chicago likes to spell numbers out, and AP likes to use numerals.

The following is for nontechnical material, where numbers are not clustered in a paragraph or page, and are not tabular material (in a list, table, or chart). CMS allows numerals in those cases.

Quick glance

CMS: Spell out whole numbers zero through ninety-nine alone or followed by hundred, thousand, hundred thousand, million, billion, trillion.
AP: Spell out whole numbers one through nine unless followed by a unit of measure. Combo for million, billion, trillion.
Numerals for everything else, including decimals.

CMS AP
five five BUT 5 miles
twenty-three 23
fifty 50
one hundred 100
one thousand 1,000
thirty-two thousand 32,000
one million 1 million
thirty-two billion 32 billion
8.5 8.5
4.6 million 4.6 million

For money:
CMS: Spell out whole numbers zero through ninety-nine for money. Combo for million, billion, trillion. Numerals for other.
AP: Numerals for all money. Combo for million, billion, trillion.

CMS money
AP money
five cents 5 cents
five dollars $5
twenty-nine dollars $29
$100 $100
$2,000 $2,000
$2 million $2 million
$1.6 billion $1.6 billion

BOTH: 50 percent

BOTH: one-fourth

CMS: two and one-fourth (or 2¼ if numerals needed)
AP: 2 1/4

Deeper details

Under one hundred

CMS: Spell out zero through ninety-nine. Hyphenate twenty-one through ninety-nine. Same rule for money. (9.2)

AP: Spell out zero through nine, use numerals for all others. For money and units of measure,* numerals always.

CMS CMS $ AP AP measure AP $
four four dollars four 4 miles $4
thirty-two thirty-two dollars 32 32 years $32

One hundred+ and decimals

BOTH: Numerals for most 100+ and all decimals. Use a comma with 4+ digits. (9.2, 9.19)

101   650   1,500   2,635   10,010   650,000

2.5    68.7  100.9

Hundred, thousand, hundred thousand, million, billion, etc.

CMS: Spell out one through ninety-nine if followed by hundred, thousand, million, billion, trillion OR hundred thousand, hundred million, hundred billion, etc. (9.4)
But if money, use numerals. Use combo if money in millions, billions, etc. (9.8)
AP: Use numerals for numbers and money. Use combo if money in millions, billions, etc.

CMS AP Both $
one hundred 100 $100
thirty-two thousand 32,000 $32,000
four hundred thousand 400,000 $400,000
450,000 450,000 $450,000
three million 3 million $3 million
3.5 million 3.5 million $3.5 million
3,850,200 3,850,200 $3,850,200
forty-four million 44 million $44 million
one hundred million 100 million $100 million
120 million 120 million $120 million

Note, in American usage:
one billion = 1,000,000,000         = one thousand million
one trillion = 1,000,000,000,000 = one thousand billion  =  one million million
British English previously considered one billion = one million million (12 zeroes), and one trillion = one million million million (18 zeroes), but the system used for American English (actually created in France) came to predominate internationally. See Wikipedia: Billion.

At the beginning of a sentence, spell out the number.
CMS: Including year at beginning of sentence.  AP: But not if it’s a year.
CMS: Do not use “one hundred and.” One hundred seven (not and seven). AP: It is correct with or without and. Both “one hundred seven” and “one hundred and seven” are fine.
Source.

One hundred seven years passed. Six hundred fifty-five men and 750 women moved to the town.
CMS: Two thousand seventeen will be the best year yet.
AP: 2017 will be the best year yet.

*AP: Units of measure = ages, dimensions, distance, time, temperature, volume, speed, recipe measurements, sports scores, referring to math (multiply by 2), sequential numbering such as page, chapter, room, clothing size (size 2), rank (No. 2), highways and interstates, course numbers. NOT school grades (second grade, second-grader) or centuries (second century).

Special rules

Money

CMS: Spell out whole numbers zero through ninety-nine. (9.21, 9.25) Numerals for $100+ and decimals. (9.25, Q&A)
AP: Numerals for all money.
Both: Combo for $ in million, billion, trillion.

CMS: ninety-nine cents   twelve dollars   $100   $650   $3.99
AP:    99 cents                 $12                    $100   $650   $3.99

BOTH: $1 million   $65 million   $2.5 billion

Percents

BOTH: Use numerals, spell out percent. (9.18)

0 percent   5 percent   8.5 percent   50 percent

CMS: 75–80 percent
AP: 75 to 80 percent. Also, AP specifies using 0.6 percent, not .6 percent

verb choice: 50 percent of the class was, but 50 percent of the students were

Do not hyphenate adjective form: a 50 percent increase

Fractions

Simple fractions (less than one)

BOTH: Spell out and hyphenate (9.14) (AP & CMS agree.)

a two-thirds majority
one-fourth of the students

Compound fractions (more than one)

CMS: Spell out preferably. If it’s much more clear with numerals, try to use the fraction character with no space before: 6¾ rather than 6 3/4 when available. But consult your publisher for their preference. (9.15)
AP: Numerals preferred for compound fractions. Space between whole number and fraction. Prefer format as 2 1/4 but okay if system changes it to 2 ¼.

CMS: two and one-quarter miles    four and a half years   6¾ yards      11 feet 2¼ inches wide
AP:    2 1/4 miles                             4 1/2 years                 6 3/4 yards    11 feet 2 1/4 inches wide

Ages

CMS: Follow above rules for spelled-out or numeral form. (7.85 p. 375)
AP: Use numerals for all ages.

Hyphenate noun forms:
CMS: a six-month-old, a two-year-old, a ten-year-old, a thirty-five-year-old, a
103-year-old
AP: a 6-month-old, a 2-year-old, a 10-year-old, a 35-year-old, a 103-year-old

Hyphenate adjective forms before a noun:
CMS: a six-month-old girl, a two-year-old girl, a ten-year-old boy, a thirty-five-year-old man, a 103-year-old woman
AP: a 6-month-old girl, a 2-year-old girl, a 10-year-old boy, a 35-year-old man, a 103-year-old woman

Open adjective after a noun:
CMS: The girl is six months old. The girl is two years old. The boy is ten years old. The man is thirty-five years old. The woman is 103 years old.
AP: The girl is 6 months old. The girl is 2 years old. The boy is 10 years old. The man is 35 years old. The woman is 103 years old.

CMS also mentions twenty years of age.

Common news sentence structures with age:
AP: The boy, 10, was with his sister, 8. The woman, 33, has a son 6 months old.

Approximate age
CMS: The man is in his thirties.
AP: The man is in his 30s.

Plural (covered in CMS only)
a class of four- to six-year-olds. (Note the open hyphen after four.)

Same rules apply to things, not just people.

CMS: a forty-year-old book 
AP: a 40-year-old book

CMS: a 3.5-million-year-old fossil

Rule also applies to other “old” constructions:
a centuries-old tale BUT The tale was centuries old.

Streets

CMS spells out First through One Hundredth Street. (CMS 9.52–53)
AP spells out First through Ninth Street. (AP: addresses)

CMS: Fifth Avenue   Forty-Fifth Street   132nd Street
AP:    Fifth Avenue   45th Street              132nd Street

For more on street abbreviations and addresses, see Addresses post.

Highways and interstates

Use numerals. (9.51)

CMS: Interstate 80; I-80    US Route 71; US 71*  US Highway 40
AP:    Interstate 80; I-80*   U.S. Route 71              U.S. Highway 40

* CMS prefers US, but use U.S. if publisher prefers.
*AP says abbreviate as I-80 on second reference only.

Time

CMS: (9.38–39, 10.42)

Usually spell out even, half, and quarter hours in text. (9.38–39)

six o’clock in the morning    noon   midnight
eight thirty    half past five
a quarter to six    a quarter of six   (the “a” is optional)

But you may use numerals if exact times are emphasized.

The flights left at 1:42, 3:30, 4:45, and 6:00.
The day begins at 8:30 a.m.
The conference call begins at 9:15 a.m. (EST).

CMS does not mention the common construction 8 a.m., so assume the preferred structure is eight o’clock in the morning or 8:00 a.m.

[Note, house styles vary. A press I work with prefers small caps for A.M., P.M.]

AP: (see under times, noon, and midnight)

8 a.m. to 5 p.m.
noon   midnight  (Do not put 12 in front of it.)
spelling out o’clock is acceptable, but a.m. and p.m. are preferred

Measurements

CMS: 7.85 p. 376 and 9.13. AP: numerals: dimensions and Q&A

CMS: a five-inch rainfall
AP: a 5-inch rainfall

CMS: The box was nine inches wide, six inches long, and two inches tall.
AP: The box was 9 inches wide, 6 inches long and 2 inches tall. (AP does not use the serial comma.)

CMS: a six-inch-tall notebook   the notebook was six inches tall (7.85)
AP: a 6-inch-tall notebook  the notebook was 6 inches tall

CMS: a two-and-a-half-inch border   the border was two and a half inches (7.85)
AP: a 2 1/2-inch border    the border was 2 1/2 inches (Q&A)

CMS: four-by-six-inch photographs (9.13)  the photographs were four by six inches (9.13)
AP: 4-by-6-inch photographs   the photographs were 4 inches by 6 inches  

CMS: eight-by-ten-foot rug   the rug was eight feet by ten feet   the rug was eight by ten feet
more informally: eight-by-ten rug    the rug was eight by ten
AP: 8-by-10-foot rug    the rug was 8 feet by 10 feet    the rug was 8 by 10 feet
more informally: 8-by-10 rug   the rug was 8 by 10  

Exception: Spell out the noun two-by-four (lumber cut in a two-by-four-inch piece) in both CMS and AP styles.

Height

CMS: (7.85 p. 376, and online Q&A)

he is six feet two inches    he is six feet two inches tall  (Q&A)

CMS considers the following abbreviated forms more informal:

he is six feet two (7.85)
a six-foot-two man  (7.85) a six-two man (Q&A)
He is six foot two. or He is six two.  (9.13)  also: six-two (Q&A)

AP: (under dimensions and numerals: dimensions)

he is 6 feet 2 inches   he is 6 feet 2 inches tall
a 6-foot-2-inch man    a 6-foot-2 man

An academic press and I agreed on the following rules for a book. Consult your publisher if it comes up often.

He is five feet nine; he is five feet, nine inches; but a five-foot-nine soldier (adjective before noun)
eight by ten feet, an eight-by-ten-foot canvas

With fractions, you can spell out easy ones, but they are often better expressed as numerals. Note, CMS prefers the fraction symbol with no space before:

6 feet 2¼ inches tall  (9.15)
8½″ × 11″ paper (9.15) (See 9.17 for repeated quantities rule.)

If you are using numerals (for clustered, tabular, or technical material)

CMS specifies using prime and double prime symbols, NOT apostrophe/quote. 6′2″
AP specifies using apostrophe and quote marks in this case. 6’2”

-odd

Follow basic number rules and hyphenate. (Only covered by CMS in 7.85 p. 381)

thirty-odd pills
a hundred-odd paintings
150-odd bottles

-fold

Generally, do not hyphenate. See this post for a little more information.

tenfold
hundredfold

Quotes and dialogue

Use your own editorial discretion. Just be consistent. (13.42)

Clustered numbers, tabular matter, or technical info

Use numerals for:

Numbers clustered in a paragraph/section where they need to be easily compared

Technical material with many numbers

Tabular material (tables, lists, charts, etc.)

 

Chicago Manual of Style 16 (books) and AP Stylebook 2017 (media) agree unless noted.
Chicago style dictionary is Merriam Webster’s Collegiate Dictionary, Eleventh Edition (MW11)
AP style dictionary is Webster’s New World Collegiate Dictionary, Fifth Edition (WNW5)