The rules regarding numbers in writing vary among academic disciplines. The conventions within engineering, for example, may differ from those within economics. In this article, we present 10 rules or guidelines to spell out numbers. Note that these rules are appropriate within the context of formal writing.

Rule #1

Spell out numbers from one to nine for both ordinal (first, third, tenth, etc.) and cardinal numbers (1, 3, 10, etc).

Example 1

Avoid: He served 9 years in prison following his 1954 break.

Use: He served nine years in prison following his 1954 break.

Example 2

Avoid: If shadowing were a factor, it would have affected the 7th and 8th stages.

Use: If shadowing were a factor, it would have affected the seventh and eighth stages.

Rule #2

Spell out any number that begins a sentence.

Example

Avoid: 1000’s of Human Genes are MicroRNA Targets.

Use: Thousands of Human Genes are MicroRNA Targets.

Rule #3

Use a combination of figures and words for numbers when such a combination will keep your writing clear. For example, spell out any number that precedes another number expressed in numerals.

Example

Avoid: The difference from baseline stimulus sensitivity scores for each condition during the 3 20-min periods following baseline.

Use: The difference from baseline stimulus sensitivity scores for each condition during the three 20-min periods following baseline.

Rule #4

Spell out indefinite numbers and amounts, including large approximations.

Example

Avoid: Our survey indicates that 560 villages are arsenic-affected and more than 1,000,000 people are drinking arsenic contaminated water

Use: Our survey indicates that 560 villages are arsenic-affected and more than a million people are drinking arsenic contaminated water

Rule #5

Spell out fractions that are less than one, but use numerals when the wording becomes awkward. That is, spell out common fractions like one-fifth, two-thirds, etc,

Example 1

Avoid: Morson estimated that up to 2/3 of all colorectal carcinomas arise from adenomatous polyps.

Use: Morson estimated that up to two-thirds of all colorectal carcinomas arise from adenomatous polyps.

Example 2

Avoid: The real price of cigarettes rose by three seven-eights between 1980 and 2001.

Use: The real price of cigarettes rose by 3 7/8 between 1980 and 2001.

We hope these rules were helpful and easy to understand. Check out our next post for the remaining 5 rules!