Insert commas into numerical values with four or more digits (those >1,000) unless the numbers represent the serial number of a product.

When to Write Out Numbers

Write out the number if it is a single-digit number (1-9) where there are no units or where the units are nonstandard (e.g., nine animals, five cells, or six-well plates).

Write out any numbers that begin a sentence (and the units of measure that follow) even if similar and coordinate numbers are written as numerals elsewhere in the same sentence. The units must be plural if values are >2. You can rearrange a sentence so that the numbers do not appear first or insert a semicolon to connect it to the preceding sentence. For example, you can change:

…for five ocelots. 100-ml aliquots were then added to their cages.


…for five ocelots. Aliquots (100 ml) were then added to their cages.


…for five ocelots; 100-ml aliquots were then added to their cages.

When Not to Write Out Numbers

Do not write out numbers ≥10. If double-digit numbers appear within the same sentence, use numerals for any single-digit numbers.

Do not write out decimals or other mixed-value numbers.

For values within parentheses, use numerals for both single- and double-digit numbers.

Use numerals for numbers implying arithmetical or statistical manipulation, e.g., a slope of 2 or r value of 1.

Notes Regarding Specific Mathematical Punctuation

Decimals. Some authors may use commas instead of decimal points (this is a European convention); change the commas to decimal points. By the same token, some authors may neglect the leading zero in front of decimal values; please insert it.

Multiplication signs. The multiplication symbol precedes the number to denote magnification ("×500").

Do not use a multiplication symbol in place of the word "times" (e.g., "at 10 times threshold").