Arabic numbers: half-width or full-width?

Question

Using my computer, I type a lot of numbers in Japanese text, and I am not sure when to use half-width or full-width.

Are there rules?

Right now I only use half-width, is it fine?

Off-topic: Here is a picture of a book showing a happy mix of half-width and full-width.

Answer 1

It's perfectly fine to use only half-width arabic numbers.

• 2009年6月30日

However, there are other rules in operation, coming from various time in the history of writing and printing:

• A. Don't use arabic numbers at all - maybe seen in formal documents:
  • 二千九年六月三十日
• B. Half-width for two-digit numbers, otherwise full-width - mostly in printed materials:
  • ２００９年６月30日
• C. Full-width for a digit, half-width for more than two digits - sometimes used in electronic texts and printed materials:
  • 2009年６月30日

If you're writing some serious documents, check if there is any style guide provided that specifies which to use when. Or you can adopt one of the publicly available style guides. For example,

• Microsoft's Japanese style guide (chm): 3.2.1 算用数字と漢数字 (Arabic and Chinese Numerals)
• Sun's Japanese style guide (pdf) p.40 半角文字の使い方 (How to use half-width characters)
• Wikipedia's style guide: Typography conventions, part of Manual of Style

All of the three recommends to use half-width numbers in general. So there you have a justification to use half-width numbers only.

[Note: my knowledge is biased towards software documentations]

Historically:

Traditional Japanese typesetting faced the problem of how to arrange multilingual text that include Japanese and Roman characters in an aesthetically pleasing way, when Roman characters were introduced to Japan.

Take vertical typesetting. When you're writing vertically, there is no way for a word with more than two half-width letters to fit in a single cell without losing readability. They'll have to occupy individual cells vertically next to each other. In fact, Rule B is actually optimized for vertical writing. ex:

|２|
|０|
|０|
|９|
|月|
|６|
|月|
|30|
|日|

When digital publishing entered the scene, these conventions have been carried over to electronic text composition. Consequently, some people use different standards for full-width/half-width numerals as we've seen above.

Side note: Another practice in writing electronic text tries to replicate the aesthetics of horizontal typesetting. That is, inserting a half-width space whenever there's a pair of half-width letter and full-width letter: 空白を 1 つ - 空白を1つ. This is a workaround around the fact that modern consumer softwares don't layout Japanese texts properly. Probably except TeX. Whether this practice is acceptable or not is a favorite topic of Japanese blogosphere that pops up every so often.

Further reading:

• 数字の扱い方 (how to handle numerals) from a company specializing in desktop publishing. Covers historical backgrounds.
• More ways of mixing different character types are covered in W3C's Requirements for Japanese Text Layout, which is based on a standard established by Japanese Industrial Standards Committee.

Answer 2

Although it looks odd if you ｍｉｘ it， but I don't think there is rules for that.

But some web forms only allow 半角 on phone numbers, and some only allow 全角 for Ａｄｄｒｅｓｓ, so you may need to force one on those places.

Answer 3

I am not sure if there is any rule either (sounds like too recent a problem to have a strong tradition attached to it), but I thought this comment made (on the original Stack Overflow) by a native Japanese was interesting:

I'm a Japanese, and I loathe full width numerals! Please just use the half-width numerals within Japanese sentences. All you need is to pad them with spaces. ここに10個林檎があります looks bad, but ここに 10 個林檎があります looks completely OK! ここに１０個林檎があります looks horrible to me

(funnily enough, as a non-native, my visual preference would be the exact opposite...)

Answer 4

I don't really think it matters. And even in your image, the full-width numeral is used only to take up the same amount of space as the pair of half-width numerals next to it.

Answer 5

I always use half width for numbers and letters for purposes of compatibility. Not everything can handle double-byte characters.

As for a matter of style, I like to use something crazy like the numbers in circles (①, ②, ③...) or the old style kanji (壱、弐、参...) for effect (if I'm trying to draw attention, like on ニコニコ動画 or something).

Answer 6

You should use only the standard digits "0123456789" (which you call "half-width"), not the full-width forms.

(The digits "0123456789" are not necessarily half the width of a Chinese character, though they typically are in Japanese fonts.)

The full-width variants are considered compatibility characters in Unicode:

Compatibility Character. A character that would not have been encoded except for compatibility and round-trip convertibility with other standards.

Basically, these exist only for compatibility with legacy software, and their use is discouraged. But since they remain on Japanese keyboard layouts, people keep using them.