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How to Write Numbers

Last weekend, I was working on a book that contained a lot of numbers. I frequently found myself unsure as to whether to write numerals (e.g., “50”) or whether to spell the numbers out (e.g., “fifty”).

I realise, of course, that I may be the only one who finds this confusing, and that many of you may have no difficulty with it at all. If you are one of these people, you should probably not bother reading the article, but before you click away from this page, please consider the following introduction to the topic from the Chicago Manual of Style:

It is difficult if not impossible to be entirely consistent in the treatment of numbers in textual matter. As soon as one thinks one has arrived at a simple rule for handling some category of numbers, exceptions begin to appear, and one realizes that the rule has to be made more complicated.

For those of you who are not familiar with it, the Chicago Manual of Style, or “CMS,” as it is commonly known, is the main reference on matters of English style for writers, editors, and publishers around the world. Although it is published in America, it also covers the rules for British English. The CMS deals with a vast range of topics including the rules for using capital letters (another entry on that coming soon!), how to use the various types of punctuation, and, of course, how to write numbers in text. If you have a question about something related to style, google “[your question] + CMS” to get the Chicago rules.

The problem I have with books like the CMS is that I keep looking things up, finding the answer, and then promptly forgetting it. My primary motivation in writing this article is therefore to have a quick reference of the main points for myself. If others find it useful too, all the better.

Rule 1 (I am writing “1” instead of “one” because I am treating this as a numbered list.)
Whole numbers from one to ninety-nine are spelled out. Twenty-one to twenty-nine, thirty-one to thirty-nine, and so on are hyphenated, whether used alone or as part of a larger number, should the larger number for some reason be spelled out. (See Rule 5.)

one hundred (and) eighty-six.

Numbers of 100 and above are written in numerals.

2,670 pages
a length of 4,066 feet
space for 540 more cars

Rule 2
Rule 1 also applies to ordinals:

on the eight day
in twenty-first place
the 121st runner

Rule 3
Larger numbers may also be written with numerals followed by million, billion, etc.

2.3 million yen
4.5 billion years
3 billion stars
(or three billion stars)

Rule 4
Numbers that apply to the same category should be consistent, so if some require numerals because they are bigger than ninety-nine, use numerals for all.

There are 9 Korean students, 20 Chinese students, and 159 Japanese students in the department.

Note that numbers in the same context that apply to different categories may be treated differently.

In one block, a 103-story office building rises between two old apartment houses only 3 and 4 stories high.

Rule 5
If a number comes at the beginning of a sentence, it is always spelled out, even if this results in inconsistencies in the paragraph. If possible, reword the sentence so that it does not begin with a number.

Rule 6
In scientific texts, physical quantities such as heights, weights, pressures, etc. are expressed in numerals.

45 miles
240 volts
6.4 meters

In non-scientific texts, follow the normal rules.

twenty degrees
seventy-five miles per hour
more than two-thirds of registered voters

Rule 7
As far as I can make out, percentages are treated as an exception to the general rules and are always written in numerals. In scientific text, use “%” with no space after the numeral.

from 10% to 18%

In non-scientific text, use numerals with “percent.”

from 40 percent to 50 percent

If you know any other interesting rules or exceptions, if you think there are any mistakes in the above explanations, or if you think I have missed something important, please let me know and I will update the entry.


  1. Biwa on 2013年03月13日 at 13:22

    Hi David,

    Thank you for the useful list. Look forward to the next entry. I’d like to know the correct usage for “”, ”, :, ;, () and -aaa-.

  2. Fumie on 2013年03月14日 at 21:10

    I didn’t even know there are rules about writing numbers. I thought numerals are more casual than spelling out numbers. Thank you for the useful article.
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