Numbers in Scientific Manuscripts: What Are the Rules?

Determining how to display numbers in scientific manuscripts can be challenging. When should numbers be spelled out? When should numerals be used? Should a sentence contain both spelled-out numbers and numerals, and if so, when? Below is a guide to help identify the best solution in these situations with ease.

First, and most important, be sure to consult your target journal to see if they have a preferred style. If they do, you must be sure that you adhere to all of their requirements and do so consistently.

Spelled-out numbers vs. numerals

You should avoid starting titles, headings, subheadings, sentences, or figure captions with a number. If possible, you should reword the text so that the number appears within it but not at the beginning or, if you absolutely cannot avoid it, you should spell out the number in full. For example, “22 samples were obtained from the patients” could be reworded as “We obtained 22 samples from the patients,” or you could simply spell out the number, “Twenty-two samples were obtained from the patients.”

Exception: If a chemical compound that has a number at the beginning of it must appear at the start of a sentence (for example, 2-butanol and 1,2-dimethylbenzene), you should not spell out the numerical portion.

In general, numbers up to nine are spelled out, while those that are greater are represented by numerals.

However, there are some exceptions. If you have a sentence that contains numbers that refer to the same grouping, they should appear in a consistent form. For example, “The study included 19 patients; 10 were female and nine were male” should be changed so that all of the numbers appear in numerical form because they all refer to patients: “The study included 19 patients; 10 were female and 9 were male.”

In addition, if a sentence contains both a number followed by a unit and one without a unit, the numbers should be represented differently. For example, “After 12 hours of fasting, the mice in the three groups were given different treatments.”

If numbers are followed by units of measurement, they should be represented by numerals, such as 12 mm, 3 years, 2:00 p.m., 5%, and 50ºC. Numbers should also be used for dates (January 1, 2022), weights (12 ounces), and money ($50).

Numbers that are presented in list form should all appear as numerals if one of them is greater than nine. “The infected trees were found in plots 3, 12, and 22.”

Frequently used fractions should be written out. “We used two-thirds of the solution.”

If you have two numbers in a sentence that are next to one another, try one of two strategies: either reword the sentence so the numbers are split up or spell out the number that is most easily put into words. “We removed 8 10-cm sections of pipe” could be changed to “We removed eight 10-cm sections of pipe” or “We removed eight sections of pipe that were 10 cm each.”

Ordinals

Single-digit ordinals are spelled out (such as first, fifth, and ninth), while those that are double-digit and above are represented by numerals (for example, 12th, 22nd, and 99th). If you have a mix of ordinal numbers, you should apply the numerical form to all of them. For example, “Data were obtained from the 1st, 15th, and 30th of June.”

Formatting numbers

A zero before decimals should either be consistently added or not. For example, either p > 0.05 or p > .05 is okay, but you cannot have both styles in one manuscript.

As for how many digits should be detailed, in general, most statistics are reported to two decimal places (p < 0.01); however, there are times when three decimal places should be reported (p < 0.001), but numbers should never exceed that. Too many digits can overwhelm the reader if you have numerous statistics in one paragraph and make it more difficult than necessary to understand the point you are trying to express. It may seem impressive to provide the precise number, but it could become a distraction and it may not be scientifically meaningful.

If your manuscript contains numbers that are larger than 1000, they should all have a consistent format. For example, 321000, 321,000, and 321 000 are all fine to use, but you must choose a consistent style to represent the large numbers (either no comma, a comma in between the six digits, or a space between the six digits).

Make sure numbers have appropriate spacing around them. You may have noticed earlier in the article that there was a space between the numeral and unit of measure listed (for example, 12 mm and 3 years), but please note that unless your target journal specifies otherwise, there should not be a space between numbers and degrees (50 ºC should be 50ºC) and numbers and percentages (22 % should be 22%). In addition, there should be a space before and after mathematical operators that typically appear with numbers, including =, >, and <.