How to Say "One Time" in Japanese

一回

ikkai[ik̚ka̠i]

The English phrase "one time" is translated into Japanese as 一回 (ikkai).

一回 refers to a single occurrence of an event.

Is there a formal and an informal way to say it?

一回 has a more casual tone compared to 一度. In informal situations, you may also see the expression 一回だけ used to mean 'just once'.

ikkai

If you want to say 'one time' as in 'once upon a time', Japanese does not use 一回 but instead uses the phrase 昔々, which literally means 'long long ago'.
In Japanese, the character 一 (one) can be written in a more formal, 'daiji' style as 壱 when used in legal or financial documents to prevent tampering.
The counter 回 is used not only for frequency but also for counting things that rotate, such as the revolutions of a wheel or the turns of a screw.

ボストンには一回しか行ったことがない。
I've only been to Boston one time.
This sentence is useful for expressing how many times you have visited a specific place using 一回.
Tatoeba #12698943 by small_snow, translation #6252931 by CK
多くの学童は、少なくとも一回以上、ズル休みをしたことがあるだろう。
Every school kid has played hooky at one time or another.
This sentence uses 一回 to discuss the frequency of an event happening at one time or another.
Tatoeba #10559440 by h_aibe, translation #1188661 by darinmex
一回電話して
Call me once.
一度だけお願いします
Please do it only one time.
一回だけ会ってみましょう
Let's meet up just once and see how it goes.
一回だけ許してください
Please forgive me this one time.
一回だけ、教えてくれますか？
ikkai dake, oshiete kuremasu ka?
Could you show me just one time?

Yota and Yui are talking about a movie they saw.

陽太 Yota

あの映画、一回見ただけで内容がわかった？
ano eiga, ikkai mita dake de naiyou ga wakatta?
Did you understand the plot just by seeing that movie one time?
結衣 Yui

いいえ、全然わからなかったよ。
iie, zenzen wakaranakatta yo.
No, I didn't understand it at all.
陽太 Yota

やっぱり、もう一回見たほうがいいかな。
yappari, mou ikkai mita hou ga ii kana.
I guess it's better to watch it one more time.
結衣 Yui

そう思う。陽太くんも一緒に見る？
sou omou. Yota-kun mo issho ni miru?
I think so too. Do you want to watch it with me, Yota?

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