How to Say "Eighteen" in Japanese

十八

jūhachi[d͡ʑɨːha̠t͡ɕi]

The Japanese word for eighteen is 十八 (jūhachi).

The word is used as a cardinal number. In many informal contexts, it is common to simply use the Arabic numerals 18 to represent eighteen.

Meaning: cardinal number (num)

Which one should you use?

十八 is the standard form. 拾捌 is an alternative formal representation of eighteen.

jūhachi

While 十八 is the number eighteen, the expression 十八番 is so common that it is often written in katakana or hiragana, hiding its numerical origin from the casual reader.
The formal version 拾捌 uses complex characters called 'daiji'. These were historically used in legal and financial documents to prevent fraud, as simple characters like 十 are too easy to alter.
The visual components of the kanji for eight, 八, are actually the same as the character used to mean 'to divide' or 'to separate' in some contexts.

私は十八歳です。
I am eighteen years old.
This sentence is useful for stating your age.
Tatoeba #1037391 by bunbuku, translation #282225 by human600
日本では十八歳になると車を運転できます。
We can drive in Japan when we are eighteen years old.
This sentence is useful for noting driving laws regarding the age of eighteen in Japan.
Tatoeba #122806 by bunbuku, translation #281196 by CK
十八歳になったんだから、しっかりしなさい。
You should know better now you are eighteen.
This sentence is useful for advising someone to act maturely now that they are eighteen.
Tatoeba #1149059 by bunbuku, translation #70268 by CM
十八になる
I'll turn eighteen.
3 more sentences on Tatoeba

Yota and Misaki are counting items in a box.

陽太 Yota

全部でいくつあるかな？
Zenbu de ikutsu aru kana?
I wonder how many there are in total?
美咲 Misaki

ええと、全部で十八個あります。
Eeto, zenbu de jūhachiko arimasu.
Let's see, there are eighteen in total.
陽太 Yota

十八個か、意外に多いね。
Jūhachiko ka, igai ni ooi ne.
Eighteen, huh? That's surprisingly many.
美咲 Misaki

うん、十分足りると思うよ。
Un, jūbun tariru to omou yo.
Yeah, I think that's plenty.

Sources

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