Using numbers in written text confuses the ju-ju-bees out of a lot of people. Their use is made a bit more complicated by the fact that the different formalized writing styles have slightly varying rules about using numbers. For generations the rule was that any number less than 100 would be spelled out: seventeen, ninety-two, eight, etc. In very formal styles of English, all numbers are spelled out (like on fancy wedding invitations and such). However, someone decided that this rule wasn’t necessary for more informal types of writing, and instead the upper limit became ten. According to the Modern Language Association and the American Psychological Association writing style guidelines, numbers from one to ten are spelled out, while 11 and above can be numeric.
Of course, this is English. That means that we have all kinds of exceptions to this rule.
Exception number 1: If the number is the first word in a sentence, regardless of what the number is, it is spelled out: One hundred twelve participants attended the conference.
Exception number 2: A dollar sign followed by an amount can be written in either of the following two ways – I earned $15 babysitting. OR I earned fifteen dollars babysitting. The first example is considered informal, while the second example is slightly more formal. If you are writing your thesis for your Masters or your Doctoral degree, you would use the second style.
Exception number 3: If you are keeping a list within your text, you can use numbers. For example: Jennie gave me three reasons for breaking up: 1) Scott was a jerk; 2) Scott was a jerk; and, 3) Scott was a jerk.
Exception number 4: If you are writing a really big number (millions, billions, gobzillions), you can use a numeral as part of the expression along with the written term. For example: There must have been 5 million mosqitos at the lake. I heard that 2.3 billion people catch a cold each year.
Exception number 5: Addresses, phone numbers, or dates can all be represented with numerals. Be sure that you don’t add “th” or “rd” after a number in a date. For example: We agreed on the date for the party as November 12. OR We agreed on November 12 as the date for the party. (Not November 12th). If you really must have that “th” or “rd” then spell out the date: We agreed on November twelfth for the party.
There are many good reference books that help to clarify things like this. I’m fond of The Brief Handbook because it is really user-friendly. Strunk & White’s Elements of Style is another great source of information on things like this.
Hang in there – these really do get easier with time and practice.