Writing with Numbers

The rules for expressing numbers in technical writing are relatively simple and straightforward. As a general rule numbers of ten or below should be written out with letters, not numerals. Numbers above ten should be written as numerals:

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

However, there are a few exceptions to these rules. 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 to the water at the time of testing was 16.16 feet.

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 very difficult year.
    • You may want to revise sentences like this so the number does not come first: “The year 1888 was quite difficult.”

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 imediately spot which category the numbers belong to.

Referring to Temperature Measurements

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 (i.e., 12 K is correct).