This page summarizes a bunch of rules that relate to the use of numbers in academic writing. In general we have followed the guidelines set out by the Modern Language Association (MLA).
The big question with numbers is whether to write them out or use numeric form:
Written out: twenty-two, nine, three million.
Numeric form: 2,000, $45, 99%.
To answer that question you should ask a few more specific questions:
As we answer these questions we’ll see that there are four criteria to consider: length, specificity, frequency, and clarity.
The basic rule is that if you can write out a number in one or two words then there’s no need to use numeric form:
By contrast, numbers that would take three or more words to write out should be written as numerals:
Note that hyphenated numbers between twenty-one and ninety-nine are treated as one word. Also, round numbers higher than a million can be written in a combination of words and numerals:
Dinosaurs are said to have gone extinct 65 million years ago.
You can break the previous rule if you’re using certain kinds of numbers. These are numbers that are used for specific kinds of measurements, dates, statistics, and so forth. Here are some examples:
Dates and Times:
19 times out of 20
26 Hammock Street
You might think of these numbers as more detailed or technical in character. In such cases you can usually use numerical form.
Again, there are always exceptions. For instance, if it’s easy to express a monetary amount in words you can do so (e.g., three cents, ninety dollars).
The last thing to consider is how many numbers you are using in a particular passage. If you are using quite a few numbers you might use numerals even though the numbers are not technical in nature:
Out of the 23 students, 15 returned the permission slip, 6 forgot and 2 refused to have their parents sign anything for them.
However, this is very much a judgment call. If it’s easy enough to spell out the numbers then do so.
Finally, you can sometimes mix written and numeric form to create clarity:
He made five 3-pointers in a row.
So to sum up what we’ve learned, spell out numbers of one or two words unless the numbers are more specific in nature, you’re using quite a few numbers in a row, or you need to provide clarity.
Don’t start a sentence with a numeral. Spell it out or change the word order:
Incorrect: 2,000 people rioted in the streets.
Correct: Two thousand people rioted in the streets.
Incorrect: 68% of the population voted in favour.
Correct: It appears that 68% of the population voted in favour.
If you’re dealing with a range of numbers, don’t shorten the second number for numbers under a hundred:
For a range of numbers higher than 100 you should provide at least the last two digits of the second number:
Of course it may often be necessary to provide more information:
You can add some clarity to large numbers by using a comma after every three digits:
However, this rule does not apply to specific numbers such as addresses or dates.
Roman numerals are not used as often as they used to be. For instance, in citing plays, references to act, scene, and line numbers are now generally given in arabic numerals (e.g., 2.3.15-16), though roman numerals are sometimes still allowed.
You will see roman numerals used for the pages of prefaces and prologues. They are also sometimes used to count the pages before proper pagination starts (e.g., pp. v-vii). When you’re dealing with such numerals, don’t change them to arabic form.
Roman numerals are also sometimes used in outlines (though this is not mandatory):
II. The History of Photography
A. The Daguerrotype
In addition, roman numerals are used in some names:
The MLA Handbook (8th ed.) recommends that in citations dates should be given in the format day-month-year:
20 Feb. 1988.
The rationale is that by not using commas you can maximize space. Who knew that one or two commas would be a big deal?
Elsewhere you can use a different format if you like. The main thing is to be consistent. A common way of writing dates is month-day-year:
August 3, 1766
Note that if your sentence keeps going after such a date you’ll have to provide another comma after the year.
The time of day can be given in different ways:
As long as you’re consistent, either of these formulations is acceptable.
Note that if you use the expression o’clock, the number is usually spelled out too (e.g., five o’clock).