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General: Spell out whole numbers zero through nine and the expressions a hundred, a thousand, a million, a billion; otherwise use figures: 10, 20, 35, 110, 6,000, 340,000.

The words million and billion are preferably spelled out: three million, 20 billion, 33 million, 1,500 billion.
Note that the term billion differs in the U.S. and British systems.

Use figures for all numbers that contain decimals: a 2.5-billion-dollar deficit (preferable to 2½ billion).

Spell out a number beginning a sentence. If this creates an awkward sentence, rephrase so that the sentence does not begin with a number. Follow the general rules above for numbers throughout a sentence or paragraph; it's fine to use figures for some and spelled out for others:
            Nineteen ninety-six was the year the thunder came.
            the five girls and 12 boys
            Of the 25 staff writers, fewer than nine may be in the office, while the other 16 or so are in the field.
            During the past five years 12 new 10-story office buildings have gone up between old structures of three or four stories.
            He stood 60 to 100 feet away.
Do not divide a figure at the end of a line. Recast the sentence if necessary.

1. Abbreviations: Use a figure when an abbreviation or a symbol is used for the unit of measurement:

30°C (86°F)
35-mm film

7.5 mm
10° 30' N
2. Ages of Persons, Animals, and Things: Spell out ages of persons, animals, and things from one through nine. Use figures for numbers 10 and larger and for fractions:

a six-month-old child
a three-year-old
He was six months old
flowers nine days old
10-day-old flowers
He looked sixtyish

in his 30s
twentysomething, also 20-something
3½-year-old goldfish
11-year-old structure
50-year-old boat
101-year-old building
3. Caliber: Use figures:

.50-caliber machine gun

4. Commas With Figures: In cardinal numbers use a comma in a figure of four digits or more. In a fraction, date, or temperature, use a comma in a figure that contains five digits or more:

1,750 nails
1250 B.C.

3,000 percent
25,000 B.C.
5. Compound Modifiers: Before a noun, an adjective that contains a number is hyphenated when the unit of measurement is expressed with the number:

a 4,000-year-old mummy
a mummy 4,000 years old

4,000 and some years ago
an eight-year-old building
6. Congress: Spell out through Ninth, then use figures:

First Congress

14th Congress
7. Constitutional Amendments:  Spell out through Ninth.

8. Dates: See main entry Dates.

9. Decimals: Use figures for all numbers that contain decimals: 3.4 inches of rain, 22.25 inches of snow, a 12.5-billion-dollar deficit. If the amount is less than one, the unit of measurement is singular: .33 inch (not inches) a day. If the figure is a one-digit decimal, use a zero before the point: 0.3 inch a day.

10. Dimensions: Generally express numbers one through nine in words rather than figures, which is more informal. The word by is usually preferable but, again, the symbol x may be used in a more informal usage.

a three-by-five card or a three-by-five
a four-by-six print, a 4 x 6 print
a four-by-four or 4WD or FWD or 4x4 are all acceptable for a four-wheel-drive motor vehicle

11. Dynasties: Use Arabic numerals for Egyptian dynasties:

18th dynasty

5th dynasty
12. Emperors, Kings, and Popes: Use Roman numerals:

Emperor Charles V
Queen Elizabeth II

King George VI
Pope Benedict XVI
13. Figures of Speech: Generally spell out; capitalization varies:

feel like a million dollars
thanks a million

Roaring Twenties
Gay Nineties

14. Fractions: State fractions in text in the simplest possible way, usually in words or decimals. Generally spell out half, third, quarter:

a half or one-half; two and a half
two and a half pounds, two-and-a-half-pound book
two and a half years ago
two and a half million acres, 2.5-million-acre reserve

  Spell out simple fractions under ten unless used in pairs or in dimensions, or unless they are cumbersome:

even-eighths, 4¾, 1/7000 (do not use th) of a pound, or 3½ by 4½ feet, 11½
a hundredth or one-hundredth
39 millionths of an inch

  Do not use commas in any part of a fraction with fewer than five digits.

15. Governments and Governing Bodies: Spell out First through Ninth:

Second Continental Congress
15th Party Congress

Third Reich
Fifth Republic
16. Highways, Roads, and Streets: Designate highways and roads with Arabic numerals:

Interstate 609, I-60
U.S. Route 29, U.S. 29

Maryland Route 579
Route 1
  Numbered streets through Ninth are spelled out:

1400 Second Avenue
32nd Place
51st Street
1403 35th Street
17. Hyphens: A compound modifier containing a number is hyphenated before a noun when it contains the unit of measurement. A hyphen means up to and including when used in a range of numbers:

an eight-foot pole
a 20-billion-dollar debt
an 8½-year-old boy
a 700-bottle shipment
a four-foot-nine-inch ladder
three- to five-day courses

a pole eight feet long
20 billion dollars of debt
8½ years old
700 bottles
8½-by-9-inch sheet
  When measurements before a noun consist of different elements, hyphenate within the elements and separate them with a comma: 19-inch, three-pound fish.

18. Kings: See section 12 of this entry.

19. Latitude and Longitude (latitude is always given first):

latitude 72° 54' N, longitude 165° 53' W
72° 54' N, 165° 53' W
90th meridian of east longitude
81st parallel of north latitude
21° north (when spelled out); 21° N

20. Lens Aperture: f/16

21. Mathematical Expressions: Spell out one through nine but generally keep items consistent:

multiplied by four
x squared (note italic x)

four to one 11 to 1
22. Military Units: Spell out and capitalize military units through Ninth:

Second Battalion
Sixth Fleet

First Marine Division
474th Fighter Wing

  Use Roman numerals for corps: III Corps.

23. Money: Spell out one through nine and the expressions a hundred, a thousand, a million, a billion; otherwise use figures: nine dollars, $20, $35, a hundred dollars, $110, $6,000, $340,000.

If a number is spelled out, also spell out dollar; if a figure is used, then use the dollar symbol: nine dollars, a bill for three million dollars, $20 billion, a bill for $33 million, $13 million dam.

The words million and billion are preferably spelled out: $800 million, more than $30 billion, one million dollars.
Note that the term billion differs in the U.S. and British systems.

Use figures for all numbers that contain decimals: a $2.5 billion deficit.
Hyphenate compound modifiers that include spelled-out words: a two-dollar tie, but a $45 shirt.

Use an apostrophe in such expressions as 20 dollars' worth, a million dollars' worth.

Small sums: eight cents, 15 cents; not 15¢ or $0.15.

Dollars and cents: $2.98, $6.25, $625.40.

In general, spell out units of foreign currency and do not italicize them: two pounds 60 pence; 53 pesos, 74 euros. Give the nearest rounded U.S. equivalent at least once within parentheses: It cost a hundred kroner ($20), four francs (80 U.S. cents). When conversions are given within parentheses for sums of money employing the same unit of currency, generally follow these examples: In Canada the current quotation was $2.69 (U.S. $2.47) a box; the New Zealand dollar is worth around 55 U.S. cents; a 325-million-dollar (U.S.) resort development.

In British currency the pound (£) symbol may be used as one would use the dollar symbol. Note that the term billion differs in the U.S. and British systems.

24. Odd: Avoid expressions such as 25-odd. For odds see section 31 under this entry.

25. Olympics: Use Roman numerals: XXIV Olympiad, XXIV Olympic Games.

26. Ordinals: Spell out first through ninth; for ordinals greater than ninth use Arabic numerals with st, nd, rd, and th: 41st, 42nd, 43rd, 44th. Do not hyphenate ordinals with comparatives or superlatives: second largest producer, fourth most populous city, but first-grade potatoes, third-ranked city.

27. Percentages: Use figures, except for one percent. Do not hyphenate percentages:

a 60 percent increase

4 percent spoiled
28. Plurals: For plurals of figures add s:

in his 30s

Type 2s several C-54s
  Plurals of spelled-out numbers are formed regularly:

at sixes and sevens

hundreds of people
29. Political Divisions: Spell out ordinals First through Ninth:

Fifth Ward
10th Circuit

14th Precinct
Second Congressional District
30. Popes: See section 12 of this entry.

31. Proportions, Odds, and Ratios: Generally spell out one through nine and use figures thereafter: six parts steam, 11 parts sweat; six-to-four margin; one-in-20 chance; 50-50 chance. Write: two to one against; 42 to 36 in favor; not 2-1 against or 42-36 in favor. Generally spell out figures of speech such as fifty-fifty, unless the context makes numerals appropriate: Others thought he had a one-in-20 chance of winning, though he considered his odds were 50-50.

32. Roman Numerals: Use for rulers, popes, Egyptian dynasties, Olympiads, personal names, ships, and a few other special cases. For formation, see Number Table in Webster's.

33. Scientific notations are written as follows:
          4 x 1022
          x squared (note italic x)

34. Some: Use only with a round number. Use a hyphen when this suffix is used attributively: 80-some years.

35. Streets: See section 16 of this entry.

36. Temperatures: Generally expressed in figures. Use comma only with five digits or more.

minus 102°C
105-degree heat
0°C (32°F)

102° below zero Celsius
in the 90s
37. Time of Day: Use figures before a.m. and p.m. Spell out with o'clock:

7 p.m.
the 7 a.m. plane
a four o'clock snack
4:30 in the morning

eleven o'clock
noon (not 12 noon)
midnight (not 12 midnight)
11 in the morning
38. Year: Use a comma when figure consists of five or more digits:

1500 B.C.
15,000 B.C.

by the year 10,000
39. Miscellaneous:

figure eight
number 2 fuel oil
bull number 5

world's number one producer
number one hit
continue with stage two
class one soil
  Use proper name if known:

Number 4 oil well
Well No. 4
lock Number 7
Train Number 43