Style and context matter when you're using numbers in a sentence.
Since this is my 100th episode, it seems like a fitting time to talk about how to use numbers in sentences.
[Note: There are many exceptions to the rules about how to write numbers. These tips will point you in the right direction, but if you are serious about understanding all the rules, you need to buy a style guide such as The Chicago Manual of Style or The Associated Press Stylebook.]
Whether to use a numeral or to spell out a number as a word is a matter of style. For general writing, most guides agree that you should use words for the numbers one through nine, but for larger numbers the rules vary wildly from style guide to style guide. Some say to use words for the numbers one to one hundred, one to ten, any word that can be written with one or two words, and so on. Typically, people who write business or technical documents are more likely to use numerals liberally, whereas people who write less technical documents are more likely to write out the words for numbers. If someone handles numbers a different way than you do, they're probably using a different style guide, so the best advice I can give you is to pick a style and stick with it when it makes sense. (Since I used to be a technical writer, I write out the words for numbers one through nine, and use numerals for most other numbers.)
Fortunately, some rules about writing numbers are more universally agreed upon than the general rules I just told you about.
Let's say you’re writing about snail development--a technical subject--and you've decided on a style that says you use words for the numbers one through nine and numerals for anything bigger. If you come upon a case where you have two related numbers in the same sentence, you should write them both as numerals if you would write one as a numeral. The idea is to write them the same way when they are in the same sentence. So even though you would normally write out the word "one" if you were writing,
"The snail advanced one inch,"
if you added a number over nine to that sentence, then you would use numerals instead of words when you write,
"The snail advanced 1 inch on the first day and 12 inches on the second day."
(You'd write both 1 and 12 as a numeral.) Most style guides agree that you should break your general rule in cases like that, when doing so would make your document more internally consistent.
Web Bonus: Normalization
If you have a third number that would normally be written as a word in the example sentence above, and if it isn't referring to inches, you would still write it out as a word. You only normalize to numerals if the numbers are referring to the same thing:
The five researchers noted that the snail advanced 1 inch on the first day and 12 inches on the second day.
Next: When Numbers Are Next to Each Other