The General Principle

The basic guidelines for expressing numbers are relatively simple, but the details can get very complicated, and, as with many usage issues, opinions on how to express numbers will vary. If your instructor prefers a certain format, you should use their preferred format!

When you’re writing in a humanities subject (such subjects require relatively few numbers), numbers one hundred and below should be written out with letters, not numerals:

• There were sixty-three self-portraits in the first collection.
• The archeologists found 264 bracelets in one tomb.

One hundred is the magic number because over one hundred, the number cannot be spelled in one or two words. That’s why numbers such as sixteen hundred or four million are written out as words rather than as numerals; yes, four million is a bigger number than one hundred mathematically, but it takes fewer words to spell out than one hundred twenty-five, so we write “four million,” but “125.”

Note that you generally do not include page numbers in either written or numerical form in your sentences (or line numbers, if, for example, you are discussing a poem or play); when such numbers appear in citational parentheses, they take their numerical form.

• In Macroanalysis: Digital Methods and Literary History, Matthew Jockers argues, “Big data are fundamentally altering the way that much science and social science get done” (7).
• Discussions of John Milton’s readers frequently include his famous description of Paradise Lost‘s readership as “fit audience … though few” (7.31). [Here, 7 indicates Book Seven of Paradise Lost; 31 indicates line thirty-one of that book of that poem. It would be inelegant and unconventional to convey this source information in the main sentence, as in the following form: “Discussions of John Milton’s readers frequently include his famous description in line thirty-one of Book Seven of Paradise Lost of Paradise Lost‘s readership as “fit audience … though few.” Students sometimes write like this in an attempt to avoid mistakes in bibliographical formatting, or simply to take up more space as they work toward a page limit, but this habit causes more problems than it solves. If you’re still following these details, you might also note that Book Seven is represented in the citation as 7, not as VII (i.e., in Arabic numerals, not in Roman numerals). This is an example of a change in documentation fashion; academics used to use Roman numerals in such cases, but the current fashion, dictated by the leading bibliographical style guide in the humanities, is to use Arabic. So now you know.]

In STEM fields (science, technology, engineering, math), where numbers are a more common feature, the convention is to spell out numbers ten and below. Larger numbers should be written as numerals:

• This study is based on three different ideas
• In this treatment, the steel was heated 18 different times.

Other Guidelines

If a sentence begins with a number, the number should be written out:

• Fourteen of the participants could not tell the difference between samples A and B.
• Eighteen hundred and eighty-eight was a difficult year for Vincent van Gogh.
  • You may want to revise sentences like this so the number does not come first: “The year 1888 was difficult for Vincent van Gogh.”

You should treat similar numbers in grammatically connected groups alike:

• Two dramatic changes followed: four samples exploded, and thirteen lab technicians resigned.
• Sixteen people got 15 points on the test, thirty people got 10 points, and three people got 5 points.
  • In this sentence, there are two different “categories” of numbers: those that modify the noun people and those that modify the noun points. You can see that one category is spelled out (people) and the other is in numerals (points). This division helps the reader immediately spot which category the numbers belong to.

When you write a percentage the number should always be written numerically (even if it’s ten or under). If you’re writing in a technical field, you should use the percentage symbol (%):

• This procedure has a 7% failure rate.

If you’re writing in a nontechnical field, you should spell out the word percent:

• The judges have to give prizes to at least 25 percent of competitors.

All important measured quantities—particularly those involving decimal points, dimensions, degrees, distances, weights, measures, and sums of money—should be expressed in numeral form:

• The metal should then be submerged for precisely 1.3 seconds.
• On average, the procedure costs $25,000.
• The depth of the water at the time of testing was 16.16 feet.

In technical settings, degree measures of temperature are normally expressed with the ° symbol rather than by the written word, with a space after the number but not between the symbol and the temperature scale:

• The sample was heated to 80 °C.

Unlike the abbreviations for Fahrenheit and Celsius, the abbreviation for Kelvin (which refers to an absolute scale of temperature) is not preceded by the degree symbol (e.g., 12 K is correct).