8 | How to Express Numbers in Scientific Documents

Best Practices

Scientific documents are full of numbers. Many of these numbers are associated with units, whereas others are not. When writing about a number with a unit, for example, 70 kg, 1 mL, and 2 h, use a numeral. If the number is not associated with a unit, use numerals for numbers that are 10 or greater and consult the author instructions for the best approach to use for cardinal numbers less than 10 (five mice or 5 mice). The latest guidance from the American Medical Association (AMA) and Council of Science Editors (CSE) states that authors should largely use numerals for cardinal numbers less than 10 in scientific documents (occasionally, zero or one may need to be written out for clarity), but not all journals have adopted this new convention.

There are two important exceptions to the above general rule: lists and multiplication terms. Specifically, if you have a list of numbers in which one of the numbers is 10 or greater, then use numerals for all of the numbers, for example, “a total of 8, 12, and 5 replicates were used for experiments A, B, and C, respectively.” Similarly, if you are writing about a multiplication term, then always use a number, for example, 2 times, 4 fold.

In the rare instance that a sentence begins with a number, spell out the number regardless of whether it has a unit or not. Oftentimes, for brevity, you can just reword or rearrange the text. Examples of each approach are as follows:

• One milliliter of solution A was added to the beaker.
• Next, 1 mL of solution A was added to the beaker.

Another rare situation is when two numbers are written next to each other in a series. In these cases, it is acceptable to spell out one number and use a numeral for the other regardless of the above general rule. For example, one could write “twelve 3 cm tumors” for clarity. Hyphens are also helpful to employ in such situations (two 3-day conferences).

Lastly, spell out and hyphenate fractions that appear in the text. For example, “approximately two-thirds of the population were not aware that alcohol consumption is a risk factor for the development of cancer.”^[1]

Further readings and resources

• Livingstone, E.H. Numbers and Percentages, Chapter 18. In: AMA Manual of Style: A Guide for Authors and Editors, 11th Edition. American Medical Association (AMA), Oxford University Press, 2020.
• Lang, T.A., Ancker, J.S., and Lambert, R. (eds.) Numbers, Units, Mathematical Expressions, and Statistics, Chapter 12. In: The CSE Manual: Scientific Style and Format for Authors, Editors, and Publishers, Ninth Edition. Council of Science Editors (CSE), University of Chicago Press, 2024.