TITLE: Formal writing: numbers under 10
QUESTION [8 upvotes]: I've been tasked with proofreading an Engineering/Mathematics thesis paper. I was always told that numbers under 10 should be spelled out (one, two, three, ...) but I was wondering if this rule holds in math and science fields as well. This paper understandably uses a lot of numbers in it and switching back and forth between numbers written out in words and numerals seems inconsistent at best and somewhat confusing at worst.

REPLY [2 votes]: I believe that the rule here is to spell out numbers used for counting (unless they require three or more words to be spelled out), and use digits otherwise. Just check the examples in the answers above to see that they all fit this rule. As one more example: "There are five theorems in the manuscript labeled Theorem 5".