When you refer to a number in your writing, you often have to decide whether to use a numerical expression/figure or spell it out as a word. This article discusses numbers, how to write them correctly, and when to use numerical expressions instead.
General rules
Related numbers should be expressed consistently. For example, if you choose to use figures because one of the numbers is greater than nine, use figures for all of the numbers in that category. If you choose to spell out numbers because one of the numbers is a single digit, spell out all numbers in that category. For example:
Right: "two apples, six oranges, four pears, and three bananas"
Wrong: "two apples, 6 oranges, four pears, and 3 bananas"If a passage includes numbers that follow one another, one is spelled out and the other is represented with a figure.
Unclear: "The club celebrated the birthdays of 6 90-year-olds who were born in the city." (This may cause the reader to read '690' as one number.)
Clearer: "The club celebrated the birthdays of six 90-year-olds who were born in the city."
Rules for spelling out numbers
The rules of usage vary, but small numbers, such as whole numbers smaller than ten, should be spelled out. This is the one rule that is fairly uniform among all rulebooks. Most style guides also include all numbers that can be expressed in one or two words. Figures are recommended for the other cases. For example:
"Exactly three pounds"
"Two thousand dollars"
"About twenty-six years"
"Thirty-two people"
"Just 268 days until Christmas"
"Cost $46.15"
"1,238 tons"
"116.3 gallons"Another widely accepted rule is to write out a number if it begins a sentence. For example:
Right: "Six percent of the group failed."
Wrong: "6% of the group failed."You may need to reword sentences to avoid spelling out large numbers. For example:
Right: "Fans bought 400,000 copies of her new book on the first day."
Wrong: "400,000 copies of her book were sold on the first day."In English, the comma is used as a thousands separator and the period is used as a decimal separator to make large numbers easier to read. Thus, you should write the size of Alaska as:
Right: "571,951 square miles"
Wrong: "571951 square miles."If the number is rounded or estimated, spell it out. Rounded numbers over a million are written as a numeral plus a word:
Right: "About 400 million people speak Spanish natively,"
Wrong: "About 400,000,000 people speak Spanish natively."Hyphenate a number written as two words if it is under one hundred. For example:
"There are twenty-two students in my class."
Use a hyphen to separate the numerator from the denominator in written fractions. For example:
"Two-thirds of the class chose the correct answer."
Rules for using figures
The following should be expressed as figures.
Exact measurements followed by symbols or abbreviations: "55 m.p.h."
Exact amounts of money: "$110.79"
Decimals and fractions: "2.5, 3/5"
Percentages, scores, or statistics: "38%, score of 10-1"
Volume, chapter, and page numbers: "Vol. I, Chapter 12, page 87"
Act, scene, and line numbers: "Act III, Scene 2, lines 23-27"
Places
Figures are generally used for addresses. For example:
"16 Tenth Street"
"350 West 114 Street"
Time
Use numerals with the time of day when exact times are being emphasized or when using A.M. or P.M. For example:
"8:00 A.M."
However, use noon and midnight rather than 12:00 P.M. and 12:00 A.M.
Normally, spell out the time of day in text even with half and quarter hours. With o'clock, the number is always spelled out. For example:
"eight o'clock in the morning"
Dates
It is generally acceptable to express a decade by spelling it out or writing it as a figure. When expressing a decade as a figure, do not use an apostrophe between the year and the "s," i.e., 1980s. When spelling out a decade, use lower case letters, i.e., the eighties.
In general, do not use "st," "nd," "rd," and "th" after dates to indicate ordinals.
Right: "We will meet again on April 15."
Wrong: "We will meet again on April 15th."Here are a few more examples of dates:
"December 12, 1965 or 12 December 1965"
"A.D. 1066"
"in 1900"
"from 1971–72 or from 1971–1972"
Expressing numbers in mathematics and the sciences
Use an en dash, not a hyphen, to indicate a closed numeric range, i.e., 100–110.
Insert a space on either side of mathematical operators (-, +, =, etc.) or symbols unless they directly precede a number to indicate a value (<5).
There should not be a space between a number and a percentage sign, i.e., 100%.
Preferably abbreviate units of measurement when used with figures, but spell them out when not preceded by figures.
Write decimals in figures. Put a zero in front of a decimal unless the decimal itself begins with a zero.
Please keep in mind that this article discusses general rules regarding the expression of numbers in writing. In business and academia, you should always consult the applicable style guide. For example, the APA Publication Manual and the Chicago Manual of Style have extensive sections devoted to the use of numbers in technical papers.