The General Rule

The rules for expressing numbers are relatively simple and straightforward. When you’re writing in a nontechnical subject (like English or art), numbers ninety-nine and below should be written out with letters, not numerals:

• There were sixty dogs in the competition.
• I don’t think it’s possible to get 264 bracelets made in one week.

In technical fields (like math or science), you spell out numbers ten and below. Numbers above this should be written as numerals:

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

Other Rules

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 immediately spot which category the numbers belong to.

When you write a percentage the number should always be written numerically (even if its 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 to 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 (i.e., 12 K is correct).