How to write numbers in Japanese – Vertical writing and horizontal writing

First of all, the way of writing numbers is different in vertical writing and horizontal writing.
When you write Japanese horizontally, basically, Arabic numerals are used.
Kanji for numbers (漢数字, Kansūji) is basically for vertical writing.

Kansūji chart 

Rules of writing numbers in horizontal writing

First, I will explain the basic rules in vertical writing because English is written only horizontally. Then I will show how the numbers are written in horizontal writing.

Situations using 単位数字 (tan’i sūji), such as 十(ten) or 百(hundred)

When writing the amount of money, or 和暦(Wareki, Japanese imperial year), you use 単位数字.
Examples
三十円(30 yen)…①
平成三十一年(The 31st year of Heisei)…②

If you want to express decimal, the point is written as “・.”
Example
五・五(5.5)…③

When writing fractions, you will use “XX分のX.”
Example
二分の一 (1/2)…④

Round figures are expressed with “、.”
Example
二、三人 (two or three people)…⑤
“人” (nin) is a quantifier when counting people.

Situations not using 単位数字

If you are writing a year in the western calendar, phone number, zip code, or address you will not use 単位数字. Instead, you will use 〇 for zero.

Examples
a) Year:
二〇二〇年 (2020)…⑥

b) Phone number:
〇三―五二五三―二一一一 (03-5253-2111, Prime Minister’s office)…⑦

c) Zip code and address:
〒一〇〇―八九六八
東京都千代田区永田町一―六―一 (Nagatacho 1-6-1, Chiyoda-ku, Tokyo, 100-8968)…⑧

“〒” is a symbol for zip code and pronounced as 郵便番号yūbinbangō.

By writing horizontally, you may find it hard to distinguish one（一）and hyphen(－), However, there is no need to confuse since hyphen is written vertically.

Situations using Arabic numerals

When the number is combined with alphabets in a word or is a part of a name, numbers are written in Arabic numerals.

Example
A4版(ē yon ban, A4-sized)
ビタミンB1(Bitamin bī wan, Vitamin B1)
ボーイング787（Bōing nanahachinana, Boeing 787）…⑨

Let’s see how they look like in vertical writing.

Rules of writing numbers in horizontal writing

Basically, Arabic numerals are used in horizontal writing. However, there are some exceptions to use Kanji.

1. Japanese origin words
   Examples
   一つ(hitotsu, the word meaning “one” used when counting something)
   一人(hitori, one person)

2. Proper nouns
   Examples
   四国(Shikoku, the name of an island in Japan), 第一次世界大戦(Dai ichiji sekai taisen, World War I)
3. Kanji compounds or idiomatic phrase
   Examples
   一般に(ippannni, generally), 一生懸命(isshōkenmei, with all one’s might), 四捨五入(shishagonyū, rounding numbers), 一石二鳥(issekinichō, kill two birds with one stone).

Are you finding more complex Kanji? Maybe that is 大字(Daiji), another system to write numbers

The Kanji characters explained above are the ones used in daily lives.

However, there are more complex Kanji characters to write numbers. These are called 大字(Daiji) and are used in documents such as 領収書(receipt written by hand), or loan agreements.

These are for avoiding falsification since Kanji characters such as 一・二・十 could be altered by adding some strokes.

Probably the most common situation Japanese use these characters is when writing the amount of money on the envelope of a wedding money gift.

You write your name, address, and the amount of money you give on the inner envelope. You have to write the amount of money by 大字, for example, 金参萬円(money 30,000 yen).

Daiji chart

*¹ 新字体(Shinjitai) and *² 旧字体(Kyūjitai) : 新字体, “new character form” is the simplified form of Kanji used nowadays. It is made by reducing strokes of 旧字体, ”old character form”, which is more complex.
*³ 俗字(Zokuji) : non-standard characters

Although Daiji was initially made to avoid falsification, these Kanji characters somehow have some traditional, retro, or nostalgic images. Therefore, it may be used as a way of expressing those kinds of impressions.